Hi! I was recently asked to build a line stage and power amp for someone. The request was for single chassis designs at reasonable cost which would still offer outstanding sound quality. The natural choice for that is a design which does not use any of the overly hyped and expensive tubes, so I proposed a SE 6CB5A amp. The 6CB5A still is among the most underrated tubes. I chose a similar circuit as already discussded in this single ended amplifier concept article. With some minor changes. Instead of the Lundahl LL1663 or LL1664 I decided to use the LL1682/70ma as output transformer. This transformer has a slightly higher primary impedance and a 5 Ohm secondary. This makes the amplifier usable with a wider range of speakers. It has a better damping factor for good low frequency control at a minor loss in maximum output power. As in the original design the LL1660 interstage transformer is used. Here is the complete schematic of the new design, showing both channels which share a common power supply: There are also some changes to the original power supply design. As in almost all my recent power suppies, I opted for a full wave Graetz bridge rectifier scheme. Since everything is supposed to fit into a single chassis, the available space for tube sockets is limited. Therefor the decision was to go for a hybrid bridge. Two 6AX4 TV damper tubes for the upper part of the bridge supplying the B+ and UF1007 low noise silicon diodes for the ground legs. Separate filter chokes provide isolation between the channels to avoid any interaction through the common supply. As in all my designs, a ground lift switch is used for convenience. This offers the possibility to break ground loops in case the signal ground is connected to earth in several places in the system. Of course all metal parts are always connected to safety earth, no matter if signal ground is lifted or not. The amp will be constrcted in my 'classic' style, everything mounted on a metal plate which will be inserted into a wooden frame. The metal plate is already manufactured: The back side of the plate: The detailed construction will be covered in a second article. A 6AH4 line stage is the perfect preamp for the SE 6CB5A. Circuit and assembly of the linestage will also be shown in separate posts. Stay tuned! Best regards Thomas

Hi! It seems that filament bias became very popular recently. There is a long thread on the DIYaudio discussion forum about preamps with the 26 triode and filament bias seems to be used a lot there. So I thought it would be a good idea to write a few articles about this biasing method. This one starts with the basic introduction of the concept. I came up with the idea more than 12 years ago when I spent a lot of effort to minimize the capacitors in the signal path. I do not claim that this is an original idea. I have seen similar schemes in old textbooks especially with battery DHTs. The most conventional and in my opinion also one of the best biasing methods is good old cathode bias. In this scheme the plate current is returned to ground through a dropping resistor which is placed between cathode and ground. In case of directly heated triodes, cathode and filament are the same electrode. A point needs to be chosen as the cathode connection. In case of AC heated filaments this can be the center tap of the filament transformer secondary or the wiper of a hum pot between the filament terminals. Filament bias is only relevant for DC heated triodes. In this case many people still put a hum pot across the filament and use the wiper as connection point. I'm of the opinion that in case you heat with DC, it should be as clean as possible so that no hum bucking is necessary and no ugly pot distrubs the signal path. I commonly choose the negative filament as the cathode point. From there you connect your cathode resistor to ground. If the full amplification shall be utilized or if the triode is transformer coupled, it is advisable to bypass that cathode resistor with a capacitor so that the cathode has a low AC impedance to ground. Otherwise the internal resistance of the tube would be increased by roughly the cathode resistor value multiplied by the amplification factor of the tube. This would move the internal resistance into a region which is not usable with transformer coupling in most cases. In case of directly heated filaments the bypass cap will also shunt any noise voltage which might be present between the B+ and filament supplies. Such noise voltage can build up if transformers without electrostatic shields are used. Obviously this cap is in the signal path and has a great influence on the sound of a gain stage. So how can we eliminate it? In the case of cathode bias, the plate current through the cathode resistor generates a voltage drop which elevates the cathode to a positive potential, which in turn translates to the grid being negative with respect to the cathode. The beauty of this scheme is that it lets the tube find it's own operating point. As the tube ages and emission drops, the bias voltage decreases which counteracts the aging effect somewhat. Also in case of some fault the cathode resistor acts as a safety mechanism. In order to eliminate the cthode bypass cap we also have to eliminate the cathode resistor or reduce it's resistance to a very low value compared to the plate resistance of the tube. One way to do that is fixed bias which feeds the grid with a bias voltage from a separate supply. Obviously that additional supply is in the signal path, but we wanted to minimze that. Filament bias utilizes the filament current in addition to the plate current to generate the bias voltage through a cathode resistor which now can be much smaller. The picture above shows conventional bias vs. cathode bias. The difference is minor, the negative end of the filament supply moves to ground rather than the negative filament terminal of the tube. Now the filament current flows through the cathode resistor. With filament currents typically in the range of one to several amperes, this means we can develop 10s of volts for the bias with resistances in the 10s of Ohms. This is about a factor of 100 below the typical plate resistances and won't hurt much if left unbypassed. Now the filament and B+ supply are referenced to the same ground, no danger that any electrostatic noise voltage builds up between the two. Of course the filament supply needs to be very clean and well filtered since everything present on top of the filament voltage will be amplified. In the schematics above the filament voltages are supplied through chokes to the triodes. This is to isolate the filaments from the supply and capacitance therein. Instead of the choke the filament bias voltage can also be supplied through a constant current source which seems quite popular lately. I still favour the passive way with good iron though. This scheme works nicely and is proven in various configurations. It simplifies the signal path and ties filament and b+ supply nicely together. Of course the self biasing and safety aspect of cathode bias are lost. This basically acts like a fixed bias stage now. There is another disadvantage: The cathode resistor needs to dissipate a lot of power. This calculates as filament current times bias voltage. So in case of a 26 DHT we will dissipate about 10-15W in each filament bias resistor, depending on the operating point which is chosen. Worse in a 801A or 10Y: Here we get 20-50W depending on the operating point. This is some hefty power dissipation which requires serious resistors with proper heatsinking. I still use filament bias, especially with triodes like the 26 or the UX201A which only needs 0.25A filament current. With these the heat dissipation is managable. With other triodes I mostly use a scheme called ultrapath which is another way to get the cathode bypass out of the signal path. In some cases I even combine them both. I will go a bit more into deatils in future articles and also show some actual implementation in a new preamp which I plan to build. There will also be an article about a further development beyond filament bias which I named DirectPath. This eliminates the last remaining cap in such a stage, the last B+ filter cap. Stay tuned! Best regards Thomas

Hi! In previous posts schematic and assembly steps of transformer coupled mono blocks with the 6GE5 as output tube have been shown. As promised a cost optimised version will also be presented in form of a stereo amplifier. Going stereo instead of two mono blocks is already a considerable cost saving measure with minimal impact on sound quality. To reduce cost further we need to let go of the interstage transformer and use RC coupling in the driver stage instead. Next is the reduction of the number of chokes which requires to go for a cap input filter instead of choke input. And lastly the exchange of oil caps with electrolytics. Here is the schematic: I wanted to keep the ultra path connection in the output stage, hence two separate chokes for each channel to isolate them. All other capacitors are electrolytic. The first cap after the rectifier sees the highest voltage peaks, hence two 22uF/350V caps in series. This results in 11uF capacitance which is still reasonably low and does not stress the rectifiers too much. The two 150uF caps after the chokes can be implemented with 3 47uF in parallel as I did. These caps can also be increased but the 150uF proved sufficient for quiet operation. The bypass caps at the cathode resistors of the 6GE5 are needed to suppress any residual ripple which would otherwise be coupled from the high voltage to the cathode. The driver stage is a standard RC coupled stage. A rather simple circuit which is easy to build. I will show the completed amp in the second part and will also offer this version as a kit. Stay tuned! Best regards Thomas

Hi! In part 1 of the series about Shellack reproduction, I covered the basics about the different EQ curves used. In this post I will show the circuit of a variable EQ preamplifier for Shellac playback. In the first part the faceplate of the preamp was already shown with 12 settings each for the turnover frequency and 10kHz roll off. Turnover and 10kHz roll off shall be implemented passively and separately so that they do not interact with each other. This requires a 3 stage design. A first stage which drives the turnover EQ network and a second stage to drive the high frequency EQ. Followed by a first stage which should provide low output impedance. Although I am in favour of LCR EQ networks, implementing a total of 24 would require a lot of space and become impractical. Therefor the EQs will be implemented with RC networks. The bass EQ will require 3 passive components. An input resistor followed by a RC segment to ground which will determine at which frequency roll off will start and stop. A fixed 100k resistor can be chosen as input feed and a selectable RC segment based on each setting. The high frequency EQ only requires a capacitor after the input resistor. The same 100k is chosen followed by selectable capacitance to ground. The flat (0dB) position will not require a capacity so that position of the switch will be left open. For the equalisation to remain fairly constant with ageing of the driving tubes, it is desirable that the tubes chosen have a plate resistance which is substantially lower than the input resistance of the networks. Ideally less than one tenth. The output resistance of the driving stage is in series with the EQ input resistor. This way a 10% change in plate resistance would translate to less than 1% change of the EQ input resistance. So we would want tubes with plate resistances well below 10kOhms. Another factor which determines the tube choice is the overall gain needed. MC cartridges for Shellac records usually have a rather high output voltage compared to regular cartridges. I have chosen the Ortofon SPU MONO CG 65 which has an output voltage of 1.5mV at an internal resistance of 6 Ohms. So with a step up transformer this will deliver 10-20mV depending on the step up ratio used. The loss in the EQ networks will vary depending on the EQ setting. This can be anywhere from around 20dB to a bit over 32dB loss. The preamp shall be used with an active line stage, so a sensible raw gain (excluding step up transformer) would be in the range 60-70dB which would leave 30-50dB after the EQ, depending on the EQ setting. Adjustment of the gain could still be done with choice of MC step up transformer. In the last tube of the month article I already presented the 6SN7, which seems perfect for the job. It has a mu of 20 and a plate resistance of 7kOhm. With resistive plate load this would give a gain of about 15 at an output impedance of around 5kohms. The two 6SN7 stages (one triode system per stage) would result in a raw gain of 225. To bring that up to the desired total gain we need another factor of 5 or 6. A 6AH4 seems perfect for the job with it's mu of 8 and plate resistance of 1780 Ohms. This is a sketch of the circuit: The values of the resistors and capacitors in the EQ networks might need some tuning. The prototype build of this circuit will be shown in part 3. Stay tuned! Best regards Thomas

Hi! So far 38 different tubes have been presented in the Tube of the Month series. 4 types have been revisited for a total of 42 posts. I like to show tubes outside of the usual mainstream. Tubes widely ignored by amplifier designers yet very suitable for audio applications. While I was contemplating about which tube to pick for this month, I thought why not something very unusual. A tube type which probably many never have seen or even heard about. When rummaging through my tube stock I came across some acorn types and thought these are definitely worth writing about. There are about a dozen different acorn tubes. Among those some interesting triodes, like the 955. But while we are at strange tubes, lets pick a directly heated one. There are two interesting DHTs among the acorns, the 957 and 958A. I chose the latter one for this post. Here is the 958A. Acorn tubes got their name from their small size and shape which resembles that of the acorn. These tubes got introduced in the middle of the 1930ies and were already obsolete about decade later, superseded by miniature type tubes. Acorn tubes were developed for high frequency applications. The usable frequency range of a tube is limited by several factors. Among them the size of the electrode elements and their connections which determines inductances and capacitances. Acorns had an extremely small size and the connections were brought out at the side of the glass for very short signal paths and extremely low inductances and capacitances. Some of them also had connections coming out of the top and bottom of the glass. The pinout on the left shows plate and grid are coming out of the glass on one side, the filament connections on the other. The negative end of the filament is connected to two separate pins. One for connecting to the filament supply and the other to serve as cathode connection. The way the pins are brought out is probably the reason why the acorn type disappeared very quickly. By inserting the tube into it's socket, force is applied to the pins like to a lever, this could easily lead to breaking of the glass. Tubes with their pins coming out at the bottom are much less critical in this regard. But if handled with care this should not be an issue in audio applications. The drawings on the right show the tiny size of these tubes. The data sheet of the 958-A also shows the typical values for the use as audio frequency amplifier. With an amplification factor of 12 combined with a plate resistance of 10kOhm this tube could serve in a line preamplifier or as input tube of a 3 stage power amplifier. Filament voltage is only 1.25V and filament current is a very low 100mA which opens up some interesting possibilities. More about that later, lets first have a look at the plate curves: These look quite promising, let's see how an actual tube looks on the curve tracer: That's even better looking than the data sheet curves and promises excellent linearity. Although acorn tubes have only been produced over a short period they can still be easily found at low prices. Here some made by RCA: They came in beautiful small boxes with the RCA 'meatball' logo: The tubes are held in place with an internal piece of cardboard: The RCA 958A: The exhaust tip at the bottom: A 958A without manufacturer marking: The packaging is less fancy than the RCA: Probably from ex-military stock since the tube has the military designation VT-212. An acorn tube in comparison to a 6SN7: While the tubes are still easy to find, the sockets are a different matter. If you find them they will most likely be quite oxidised. A tube placed in the socket: Despite the low filament power, it lights up nicely: So how could this tube be used in an audio circuit? As mentioned above, the very low filament current opens up some interesting possibilities. I already wrote a post about the filament bias concept. This could be nicely applied to the 958A without the need to dissipate a lot of power in the filament bias resistor. We could even go a step further and use my DirectPath circuit which was presented in the article about the UX201A Sound Processor. Here is that circuit adapted for the 958A: The 100k grid to ground resistor determines the input impedance of the circuit. A quarter Watt type can be used here. The 68 Ohm filament resistor will dissipate less than 1W. A 5W type would be recommended. About 10W are converted to heat in the 1k resistor so best would be to use a 50W type for long term reliability. The output transformer can be chosen according to the gain needed and desired output impedance. For example a Lundahl LL1660 would do here, wired in 4.5:1. The PP type could be used as this can handle enough DC current. The 958A draws about 3mA. For lower output impedance LL2745 or LL1689 could be used as well. A common supply for a stereo gain stage like this would need 200mA. This is easily obtainable from a pair of TV dampers like the 25AX4 which got covered last month. This is a circuit which is truly without capacitors in the signal path. Of course the 958A could also be used with more conventional circuits. For example it could serve as input tube in an all DHT power amp. I usually use step up input transformers in such amps to be able to stay with just 2 stages and still have enough gain. The 958A could provide that gain instead, for example to drive a 26 which in turn drives a 45. I have never build the circuit above. B+ voltage and the resistors might need some tweaking to get exact filament voltage and operating point. But that would be easy to do and as usual with such circuits, things don't need to be trimmed to super exact values. I never tried this tube so I have no experience about it's micro phonics in audio applications. If you are going to try the 958A in this circuit or any other, I would be curious about the results and would be happy to report your findings on this blog. I have too many projects on going and will probably never get around to give this a try. It would be great if someone breadboards this and reports back. Best regards Thomas

Hi! Probably not every visitor of my blog is interested in technical details and schematics. Don't worry if you found my last post too dry and boring. I will also write about some other stuff, like favorite music and occasionally also about topics I find interesting but which are not related to audio. And of course I will also entertain you with some tube amplifier porn. So here is the first installment of the gallery section in which I will show photos of some amps and preamps I built. First I'd like to thank Holger Barske who was so kind to send me photos which he took of equipment which I brought to the European Triode Festival in 2009. First photo is a close up of a single ended 801A amplifier: This is a Stereo Amp with external PSU. 6N7 Driver stage, transformer coupled to 801A. All Tango transformers. This is another version of SE 801A amp. Same configuration as the other, but with a mix of Lundahl interstage and Tango output transformers. This one is also powered from an external PSU. One of the PSUs is shown in the next picture. It can be configured for various voltages so it is usable with different amplifiers. The PSU has different rectifier sockets which are wired in parallel. It can use either of these: 5R4, 5U4, 5X4, 83, 5Z3 or a pair of TV dampers like 6AX4, 6AU4. There is a separate rectifier for the driver, a 6BY5. The filter can be configured for either choke or capacitor input. Next is a picture of different amps. The 801A again in the front row left and a single ended 6CB5A amp besides it. In the second row is a mono 211 SE amp with it's PSU. In the back on the right side you see a power supply of a preamplifier. And finally a SE 300B in the left corner in the back: And here the preamps, A 801A linestage and EC8020 LCR EQ phono stage: Close up of the EC8020 phono: Another univeral power supply with 866A mercury vapour rectifiers: And here a picture of the complete system which was presented at the ETF09, besides the right speaker in the fron there is a SE 45 amp with it's power supply besides it: Best regards Thomas

Hi! This months tube is a very special type with some extraordinary properties, the 6HS5. This is a quite 'modern' tube, developed in the 1960ies. It's application was high voltage regulation for the acceleration voltage of color TV picture tubes. It was intended as a pulse type high voltage regulator. This tube is a beam triode. This means it has another electrode between grid and plate. Similar to beam power tetrodes, it has a beam forming plate. What makes this tube so unusual are it's extremely high amplification factor of 300 together with a transconductance of 65.000 micromhos! This results in a plate resistance of about 5kOhm which makes the tube usable for LC or transformer coupling. Thus it's full amplification can be utilized. The 6HS5 has a 12 pin compactron base. The compactron socket was introduced by General Electric. The purpose of the many pins was the possibility to integrate multiple systems within a single envelope. The 6HS5 only uses part of the available 12 pins. Control grid and beam forming plate are brought out to two pins. Only one of them needs to be connected. The beam forming plate can be connected to the cathode at the socket. There is a family of similar tube types which all share almost the same electrical parameters and have the same basing diagram: 6HS5, 6HV5A, 6HZ5, 6JD5. They only differ in the heater current and some have 35W plate dissipation instead of 30. I work mostly with the 6HS5 due to it's rather modest heater current of 1.5A at 6.3V. 30W seems ideal for the usage as an output tube. In single ended confuguration this would yield about 7W. More like 8W with the 6HV5As 35W max plate dissipation. These tubes are available in abundance from almost each reputable tube dealer at ridiculously low prices. Why are they so overlooked by the audio crowd? Well, nothing comes for free. While the tube has exceptional transconductance paired with a very high mu, it requires a rather high plate voltage to operate. In single ended Class A, over 1000V B+ is needed to achieve sensible results. This is probably the main reason why it is not so commonly used, besides the 'TV tube stigma' which causes many very interesting tubes to be ignored by audio designers. An important factor for audio use is the linearity. When I first discovered this tube in the tube manuals, I thought there must be some disadvantage why it is not used. It's probably not very linear. Back then I did not have any plate curves. So I ordered a few 6HS5 and tried them in a test set up. Linearity was exceptional given the other parameters of the tube. In the meantime I also found datasheets with plate curves. See picture on the right. The curves in the datasheet compare well with my measured results. The picture below shows the plate curves of a actual tube, taken with a curve tracer. This is only a small section of the curves since the tracer only reaches about 350V. As mentioned above plate voltages beyond 1000V are needed for reasonable results. Still this small part of the curves shows that it is very well usable for audio. Plate voltages of 1200-1300V are still quite reasonable and DIYers who are used to build with transmitting tubes like 211 or 845 can handle this. Another difficulty however is the highish plate resistance. Measurements showed plate resistances above the 5kOhms mentioned above. So an output transformer with high primary impedance would be needed. Very few such transformers exist. Suitable would be the Tango FW20-14S with 14k primary impedance, or the Lundahl LL2735B with 16k primary impedance. The schematic on the left shows the circuit of a power amp with the 6HS5. Due to the high amplification factor, no driver or input tube is needed. This makes a so called spud amp possible with this tube. The awkward term 'spud amp' has a somewhat funny history. An amp with one tube per channel is a 'one tuber'. Tuber is also an expression for a potato. That's why these amps are also called potato amps sometimes. And spud is just another synonym for potato. So all which is needed is the output transformer, a cathode resistor with appropriate bypass cap and a high voltage supply. The schematic also shows a decoupling choke and the decoupling cap is connected in ultrapath fashion, my favorite topology for transformer coupled stages. The tube operates at about -3 to -3.5V grid bias. So up to 7V peak to peak input voltage is needed for full power out. That's about 2.5V RMS. A bit above the typical line level, so it is preferred to drive such an amp from a pre amp with some gain. Below is a photo of a single ended power amp using one 6HS5 per channel: This circuit can be easily scaled up to achieve more power by paralleling tubes. A Parallel Single Ended verison with two tubes per channel delivers twice the power: This scheme can be even scaled further. Below a photo of Quad Parallel Single Ended mono blocks. These deliver solid 30W single ended pure Class A: Close up photos, showing the glow of the tubes. The blue glow is normal and does not indicate bad tubes: I will cover details about this family of amplifiers in an upcoming series of posts. But the 6HS5 is not only suitable as output tube. I already introduced it as a driver tube in my recent 211 amp. Here the high B+ voltage is already available in the amplifier, since the 211 has a similar B+, so the plate voltage for the driver could be derived form the output stage through a separate choke and decoupling capacitor. Other applications are possible as well. I have successfully used the 6HS5 even in phonostages. Both in single ended and fully differential configurations. Some care is needed to control microphony in a phonostage and it needs well filtered B+ and DC heater supplies in this application. But it is managable. A very versatile tube which provides many possibilities of use for creative DIYers! But how does it sound, you might want to know. The sound of an amp is not determined by the tube alone, but the whole concept. A simple circuit like this has much less parts in the signal path which can alter the signal. This gives the chance to build a very direct sounding amp with high resolution. Kind of like a single chassis speaker without crossover. The 6HS5 does not have the refinement and ultimate musicality of a directly heated triode. But it compensates a lot by the complete lack of a driver stage. Since it uses so few parts, better parts can be chosen and in fact this is mandatory to achieve a good result. The sound of the amp is ultimately determined by the tube, the output transformer and the ultrapath cap. Due to the low part count excellent results can be achieved with a moderate budget. An amplifier with directly heated output tube and the same power rating would require to spend at least 3 times as much on parts to surpass the sound level of such a 6HS5 amp. Since it is indirectly heated no elaborate filament supply is needed. It can be AC heated without any hum. In single ended configuration a well executed 6HS5 with a top quality output transformer like the mentioned Lundahl and paper in oil capacitors gives a very nice mellow tone with little coloration not very different from a 45, but with more power. This tube would actually also be quite suitable for a multi channel amp or for an active system. Multiple 6HS5 amps which share the same PSU can be easily integrated into a single chassis. I did build something like that for the European Triode Festival 2004, 4 stereo amps (8 channels) in a single chassis for a 4 way active speaker. This consisted of 4 SE amps using one 6HS5 each, two PSE amps with 2 tubes each and 2 parallel push pull amps with 4 tubes each. Even the linestage was integrated using yet another 6HS5 per channel. So 18 6HS5 tubes in a single chassis. The photo on the left shows an impression how that looked like. If you find this tube interesting and plan to build an amp with it, you should be aware of the dangers of the high voltage this tube requires. Only attempt to build such circuits if you have experience with high voltages. It is important to ensure proper creepage and clearance distances. Voltages over 1kV can 'jump' over small distances of air. For this reason I do not mount the sockets for the 6HS5 directly to metal plates. They are recessed below the plate. In addition I always drill the unused pins out of the socket. These are the pins 5,6, 8 and 9 which are on both sides of the plate pin 7. Also the internal wiring of such an amp needs to be carefully isolated. Addtional insulations sleeves over all wires carrying high voltage are recommended. Keep in mind that the plate swings far above the B+ voltage coming from the PSU when the tube is driven to full power. If the necessary precautions are taken, the 6HS5 will deliver excellent performance. It is very tough and will last long even when operated at it's plate dissipation limit. The tube seems also suitable for Class A2 operation with the grid being driven into the positive region. This way it can also be used with lower plate voltages. However I have not explored this myself yet. Stay tuned for upcoming articles around this tube! Best regards Thomas

Hi! The first shipment of ELROG tubes arrived today. The tubes are nicely packaged as matched pairs. A measurement certificate signed by Dr. Schaffernicht comes with each pair: Each tube has it's individual serial number, stamped on a metal plate inside the glass: Craftsmanship is exceptional. The plates always have the same alignment with regard to the pins, which is not always the case with NOS tubes. A NOS GE211 / VT4C in comparison to a ER211: They are about the same height. The ELROGs have a slightly larger diameter. Earlier ELROGs had a smaller diameter glass. This was changed for the final series production to give better heat dissipation. The most apparent difference is the location of the electrode system. While the system is mounted in the upper half of the tube in the GE. ELROG mounts the system close to the base, which gives much shorter wire lengths between bins and electrodes. I ran some measurements on a curve tracer. Comparing a NOS GE211 with ELROG. Unfortunately my tracer only allows plate voltages up to 400V. Here is the set of plate curves of the GE: On the X axis the scale is 50V per division. On the Y axis we have 2.5mA per division. The grid steps is 5V, starting with 0V with the left most grid line. The ER211 in comparison: Remember this is only up to 400V and 20mA. These curves show why these DHTs are so great. They are exceptionally linear. The ELROG is remarkably close to the GE the grid lines are a little bit steeper with the ER211 indicating slightly better transconductance. The tubes will be tried in various amplifiers and I will provide feedback on the sound. Stay tuned! Send an email to thomas -at- vinylsavor -dot- de for inquiries about these tubes. Best regards Thomas

Hi! In the poll most visitors voted for single ended amplifiers. Most of them for a low budget amp, but many also for a cost no object SE amplifier. So let's see how we can cover both with a single concept which can be adapted to various budgets and will deliver exceptional sound quality within a given cost range. 'Low budget' has quite a different meaning for different people. So let's discuss this first. In a DIY project there is a wide range of possibilites to influence the budget. The more time you are willing to put in to search for cheap parts, the lower you can go. Since this concept needs to be reproducable we need to resort to readily available parts. The foundation of a good amplifier is the iron: output transformers, interstage transformer (if any), power supply transformer and power supply chokes. As I mentioned already in the post about the 6CB5A, the idea for this concept was born on the german tube forum Röhren und Hören. There was a thread about a DIY amplifier concept which should be fairly easy to build, affordable and provide excellent and hum free sound. The desired cost range was determined through a poll. The result was a budget of 1000 Euros for all electronic parts, including tubes but without chassis material. This should be for a stereo amplifier. Of course there was a wide spread in the votes for the budget, so some flexibility was desirable to be able to scale the cost down to 500 Euros by selection of cheaper parts or a simplified concept and also to scale it up by using better parts, building mono blocks, external power supply, etc. But the requirement was that the base concept for the cost of 1000 Euros should deliver exceptional sound quality and come with very good parts. There was a clear preference for a single ended amplifier concept. But not too low in power output, something which gets closer to 10W than to 1W. Many people would have liked the 300B tube but that would have taken out a big portion of the available budget. Especially if there is the desire for some spare tubes. And no way to even think about original manufacture Western Electrics with the given budget. And in my opinion: If you want 300B sound go for original WE 300Bs (not the reissues of the 90ies) but that will be covered in a later post. Most people are too focussed on just the output tube anyways. It is the whole concept which determines the sound quality. Driver stage at least as much as the output stage. And of course the power supply. Most designs have 4-5 parts in the signal path per stage. Well optimized designs get it down to three. All of them have an equal influence on the sound quality. This is a very simplified view, but if you look at it this way, the output tube maybe contributes a third to the overall quality of the output stage. Power amps have 2-3 stages, so best case, the output tube makes up one sixth of the overall sound. And this is not even counting the power supply! Therefor equal effort was spent in this concept to get all parts on a comparable level. I'd rather listen to a well designed amplifier with a lesser tube than the 300B but with solid iron and capacitors, than a 300B amp with cheap output transformers, electrolytic caps and a marginal design. There was another reason not to go for a directly heated triode like the 300B. The amplifier should be as hum free as possible even on sensitive speakers. A directly heated triode would have required DC filament supplies, except maybe the 45 or 2A3 which run on 2,5V filaments. But these were ruled out due to their low output power. A DC filament supply would have added cost and complexity. An indirectly heated triode would be as simple to use as it can get with regard to heating. How that lead to the choice of the 6CB5A was described in the tube of the month post from last week. Besides exceptionally low cost, the 6CB5A has another advantage. It's operating points and requirements to the output transformer are very close to that of the 300B. So the same concept could be very easily changed to the 300B output triode and a comparison between the too would be very easy, even allowing the comparison in the same amp, with the same parts, except for an additional filament supply for the 300Bs. With the tube cost beeing so low, that left almost the entire budget to spend on high quality parts, especially the iron. But before we come to the choice of parts, the basic architecture needs to be defined. The number of stages in an amplifier has a major impact to the complexity (and also cost). This is also dependent on the gain requirements. If you read my post about gain, headroom and power, you'll remember that my philosophy is to use only as much gain as is necessary, with as much headroom as possible. This lead to the choice of a two stage concept (driver and output stage). To keep complexity low, the driver should be supplied from the output stage B+ via a separate decoupling circuit (RC or LC). Interstage transformer coupling would yield good headroom from a given B+, better than RC coupling. Also transformer coupling was not very widely used ta that time in Germany, so such a solution would bring some new concepts into the scene. Of course also because I always got the best sonic results from transformer coupling. The requirement for the concept to be fairly easy to build naturally leads to cathode bias as the method to maintain the operating points. This avoids additional supplies for bias, and any complexity to ensure the right sequencing order of the supplies during turn on. Just a single B+ supply for both channels. Tube rectification will take care of delayed and slowly rising high voltage. In order to use something better than average, a nice, classic choke input filter approach was selected for the HV supply. Again to keep it simple also for beginners to build, no regulation in the pwer supply just good solid passive filtering. So the basic architecture was defined. A two stage transformer coupled concept, using indirectly heated tubes, cathode biased with a single tube rectified and choke input filtered B+ supply and AC heating throughout. No silicon at all in the entire amplifier. Where does this leave us with the budget? Here is a raw calculation: A good choice for excellent sounding transformers at moderate cost is Lundahl . They have a wide range of suitable tarnsformers. For the primary impedance requirement (3-5k) of the output transformer the LL1663 or LL1664 would be suitable. That is about 250 Euros the pair. The LL1660 interstage tarnsformer is about 180 the pair. A heavy duty power transformer for the PSU would be around 100-120. Chokes come at 50-75 each, depening on supplier. At least 3 chokes would be required. One for each channel for decoupling. One in the common PSU, better two since additional smoothing might be required due to choke input. That is 200-300 Euros for the chokes. This sums up to 750 Euros max. For the iron which leaves 250 Euros for the rest. Since tube cost is low, this allows even for some nice oil caps. Here is a sketch of the schematic of the concept so far: Straight forward circuit, a separate choke in each channel which allows the use of a common supply with minimal interaction between the channels. The driver stage B+ is derived from the same supply via it's own RC filter segment. In a more elaborate implementation this could be upgraded to LC, but since we are on a moderate budget, let's stick with RC here. But wait, there is one unusual aspect which is not commonly seen: The capacitors from B+ to cathode in both output and driver stage. This is the so called 'ultrapath' concept. The origins of this approach go back to the engineers from Western Electric. Lynn Olson covered this on his website in an article Western Electric - Rosetta Stone for Triodes. As far as I'm aware the first person to mentioned this approach again in 'modern' times and who re-introduced it to vacuum tube audio is Jack Elliano of Electra Print. He is also the one who named it 'ultrapath' in an article in the magazine Vacuum Tube Valley. What ultrapath basically does is to provide a 'shortcut' for the signal path. Normally the signal would traverse from the tubes plate through the primary winding of the coupling transformer to B+. From there through the power supply (usually the last cap in the filter chain) to ground. From ground through the cathode resistor and/or the cathode bypass cap (which usually is an electrolytic) to the cathode of the tube. The ultrapath cap is usually fairly low in value and a high quality cap can be chosen. It bypasses the cathode circuit with it's electrolytic alltogether. Depending on the circuit, tube and output transformer, often the cathode bypass cap can be left out with the ultrapath connection. For clarity it is left in the above scheme. There is quite a lot of misunderstanding out there about the purpose of the ultrapath cap and it also got some bad press recently. In my opinion it is a very effective and cheap way to boost the performance of any transformer coupling stage by reducing the components in the signal path. It can also be used to reduce powers supply rejection, since it couples residual ripple from B+ to the cathode. The ratio of ultrapath and cathode cap can be chosen such that ripple is cancelled out. But this is not the purpose of this approach here. We only use it to control the signal path. Especially if the cathode cap is omitted, ultrapath will require a very well filtered and hum free B+ supply, since ripple is coupled to the cathode. Hence the provision for the second choke in the PSU, which will be 3 LC stages if the separate decoupling chokes per channel are counted. If the available budget is much lower, the circuit can be significantly reduced, by abandoning the ultrapath concept and changing the interstage coupling to RC. For further cost reduction, the individual decoupling filters can be replaced by a single electrolytic with high capacitance. Of course this will have an impact on the resulting sound quality. The change to RC coupling will also reduce the ehadroom in the driver since the driver tube will operate on about half the voltage. The other half will be consumed by the plate load resistor. Here is the conceptual schematic of the low cost version: Such a concept only needs 4 pieces of iron: 2 output transformer, power transformer and one choke. The choke could even be left out, but let's not make it to primitive it should still sound good. If a less oversized power transformer is used, the iron set would be around 250 for the output transformer (let's still use something very good like the Lundahls), 50 for the choke and 100 for the power transformer. That's 400 Euros for the iron set. Another 100 - 150 should be enough for the tubes, sockets, resistors and capacitors. And the beaty of this: You could start with the simple RC concept and later upgrade to transformer coupling. In the next parts of this series we will fill the concept with a bit more flesh. I will write about the driver tube selection and sizing of the resistor and capacitor values. After that I will present some power supply concepts for this amp and will also show how the design can be easily converted to use directly heated triodes like the 300B, 45, 2A3 or 801A in the output stage. But even there the journey will not end, the concept can be enhanced to an all DHT amplifier with directly heated triodes in the driver stage as well. Stay tuned! Best regards Thomas

Hi! The circuit and assembly of the UX 201A Sound Processor have been covered in previous posts, this and the next article in this series will be about the power supply. The power supply only has to deliver a single voltage of about 150VDC. But at a hefty current of 0.5A. So we need a power supply with some grunt. If rectification should be done with tubes, the average rectifier will not do. We need something tough which can handle this amount of current. Even among TV dampers, the choice is limited for such a high current. But there are some which can handle this job. For example the 6CG3. I have chosen a full wave bridge rectification scheme for the task. This is the circuit: Quite straight forward, no surprises here. Since the voltages in this power supply are rather low and the 6CG3 allows quite large voltage difference between heater an cathode all heaters could be wired in parallel and fed from a single winding, referenced to ground. But the 6CG3 draws a lot of heater current. 4 of them need almost 8A. Since the power transformer which I used has two separate 6.3V heater windings, I split the 6CG3s into two pairs which are heated from one winding. The two tubes in the middle deliver the raw B+ voltagesat their cathodes. These share one of the heater windings, which is connected to raw B+ at one end. The two other 6CG3s are fed from the other heater winding which is referenced to ground. The rectifier is followed by a choke input filter, using two LC sections. The chokes are Lundahl LL1638 gapped for 3Hy/500mA. The caps are 220uF/350V electrolytics. A 350V rating is used so that the capacitors will not get stressed with over voltage in case the power supply is turned on without the signal section attached. In that case the output voltage will rise significantly since the current draw would drop below the critical value to ensure proper choke input operation of the filter. The 20k bleeder resistor ensures that the high voltage gets drained after turn off if no load is attached. No worries about the use of electrolytics. Each channel is decoupled by a choke in the signal section. So far so good, but such beautiful tubes as the UX201A deserve some similarly awesome rectifier tubes. Here is an alternative PSU circuit using the 866A mercury vapour rectifier: Two of the 6CG3s are replaced by 866As. Since they require a different heater voltage a separate filament transformer is added which provides 2.5V. Each 866A needs 5A filament current. So it might be easier to use two separate heater windings which can provide 5A each. If a 10A filament transformer is available they can be heated in parallel as shown in the schematic. The raw B+ is derived from the center tap of the 866A filament winding. If two separate filament windings are used, their center taps need to be connected. The filter section remains the same. Due to the lower voltage drop of the 866As, the secondary voltage needs to be a bit lower. Two 6CG3s remain in the circuit in the ground path to complete the full wave bridge. They also provide a delayed high voltage to allow pre heating of the 866As without a separate switch. This has a cool effect when turning the supply on. At turn on there is no blue glow. As the 6CG3 heat up, a faint blue glow starts to appear between filament and plate of the 866As which slowly gets brighter as the TV dampers fully warm up. The assembly of the PSU will be shown next, stay tuned. Best regards Thomas

Hi! This article will show a variation of the transformer coupled single ended amplifier concept. It is based on the 46, which was presented as tube of the month in the previous post. The amp was built mostly with parts which I had available. The parts cost is moderate, even using a heavy duty power transformer, ASC oil caps and all Lundahl signal transformers and chokes, the parts add up to about 1100-1200 Euros, excluding chassis. It uses all the features of this amplifier concept which contribute to the overall sound quality: Multiple choke filtered power supply, high quality caps, interstage transformer coupling, ultrapath caps in each stage. I still have some power transformers in stock with center tapped secondary and 3 independent 6.3V windings. To keep cost down, I used this one. Only this single power transformer is needed for B+ heater and filament supplies. The 2.5V AC voltage is obtained through dropping resistors from two of the 6.3V windings. The third 6.3V winding is used to supply the heaters of the rectifiers and the 6N7 driver tubes. This requires the heater winding to be referenced to ground. Two 6AX4s are used in a full wave configuration. Since it can withstand a large voltage between heater and cathode, the heater winding can be shared with the driver tubes. The PSU is common for both channels, only the filament windings of the output tubes are separate: The B+ supply is the usual choke input PSU using two LC sections for smoothing. Heaters and filaments are fed with AC. Since the heater windings have no center taps, a hum buck potentiometer is wired across the filaments of each of the 46. Two resistors are wired from the wiper to the filament terminals to reduce the resistance of the 100 Ohm Pot which I had available. The rest of the amplifier circuit is similar as presented in previous parts of this series of posts. my favorite indirectly heated driver for small output tubes, the 6N7 is used again, both halves wired in parallel. The output tube is driven by a Lundahl LL1660/10mA, wired almost 1:1 (actually it is 1.125:1). I had a pair of Lundahl LL1682/50mA in stock. These fit quite well to the 46. With the primary impedance of 5.5kOhm and a secondary of 5 Ohm they will provide a good damping factor when used with a 46 on 8 Ohm speakers. The 46 is wired for Class A use, with the second grid tied to the plate at the socket. See the 46 datasheet for details of the tube. The B+ is around 300V. The 46 is biased at around 30V. With voltage drops across chokes and output transformer primaries this results in just under 250V across the 46. Below, some photos of the construction of the amplifier. The first picture shows all capacitors, sockets, connectors, switches and resistors mounted and some initial wiring. The power transformer is placed on the top side of the plate and will be hidden under a cover. The next photo shows the inside with all chokes, interstage and output transformers mounted and completely wired: The top view of the completed amplifier: The amp sounds very smooth and delicate. It shares a lot of qualities with the best 45 amps at a lower cost and size. The same circuit can be used with minor adaptions for 45 or 2A3 output tubes. Best regards Thomas

Hi! Just finished testing and tweaking of a line level crossover which I started working on in November. The crossover is meant to be used between the linestage and power amplifiers in an active speaker system. I made this for my good friend Stefano for his Gotorama system. It is meant to complement his preamplifier combo. There are several ways to implement such a crossover. It can be active, which is the most common method, or passive. This might create some confusion since mostly active crossovers are used in such systems. The speaker system setup is still active even when the crossover is done in passive fashion. Active speaker only means that each chassis has it's own amplifier. Only the best is good enough for Stefano. So the crossover had to be passive, thus minimizing the number of gain stages in the set up. Since he has a line stage with low output impedance the crossover could be done with low impedance LCR type networks which are similar circuit as in 600 Ohm LCR RIAA networks. Stefano consulted his friend Franz, who built the bass enclosures for his speaker and who did an excellent job setting it up. Topology and crossover frequencies have been extensively discussed and tested beforehand with a digital crossover. 6dB/octave slopes have been chosen since steeper slopes become very unpractical with LCR networks. The picture on the left shows the overall scheme. Not shown in the scheme are passive level controls on each of the outputs which allow level adjustments in 12 1dB steps for leveling each frequency range and possible adaption to the gain of different power amplifiers. The schematics on the right show the principle of LCR filter networks. The first schematic shows a low pass. This is the same type of filter as used for the 75uS time constant in LCR RIAAs. By simply swapping the coil and choke this can be turned into a high pass filter as shown in the second schematic. The values of the coil can be easily calculated: L = R / 2 * Pi * f the capacitor calculates C = 1 / 2 * Pi * R * f . R is the impedance of the networks. All the resistor values are the same, f is the desired corner frequency. For the lower and upper mid range band pass filters, two LCR networks need to be wired in series. When two filters are connected in series, the impedance of the second one terminates the first one so the first filter does not need the termination resistor. Of course in this case both need to be calculated for the same impedance. The LCR filters in the crossover have the same advantage as in a RIAA network. They have a constant input impedance which is independent of the frequency. So the filter behavior does not alter with variations of the driving impedance. This ensures constant performance over long time also when the output tube of the driving line stage changes it's plate resistance with age. Since Stefano had been very happy with the transformer volume control in his linestage, the same approach was chosen for the level adjustments at each output. Custom autoformer volume controls were made by Dave Slage of intactaudio for this project. The crossover was made in the same style as the existing preamp combination so the whole combo becomes a Triptychon for Stefanos audio shrine. The photo above shows the first assembly step. Mounting of the RCA inputs and outputs and the switches for the level adjustements. 12 step ELMA rotary switches are used. Then the autoformers got mounted close to the switches: Then the TVCs had to be wired up to the switches. Wiring a single TVC is already a quite tedious job. Wiring 8 of them is a challenge: Again only the best is good enough. So teflon insulated solid core silver wire is used. Correct wiring of all TVCs was tested before the next step. Since the filter networks had to be mounted in a second level above the TVCs it was essential to make sure there is no wiring mistake. Fixing it afterwards would be quite painful since they would be difficult to access below the filter coils: A total of 20 nickel cores. All wired up with about 15m of silver wire. After testing and trimming of the coils the Crossover measured as calculated and simulated and could be assembled into the wooden enclosure. Ready to be tested in the system. Since I'm also planning to build an active speaker set up, I will built another one for myself. Best regards Thomas

Hi! As you could already read in previous articles, I prefer TV damper tubes for rectification. My favorite among those is the 6AX4. The use of TV dampers in power supplies was advocated by JC Morrison in Sound Practices magazine back in the 1990ies. He mostly uses them as a slow turn on device in conjunction with silicon rectifier bridges. TV dampers have been developed for TV service. Their purpose was to dampen oszillations in the deflection system during the fly back period of the electron beam. This requires a very tough diode which can handle very high peak inverse voltages and peak currents. These properties made them perfectly suited for mains rectification as well. However this was not promoted by tube manufactures since they wanted to continue to sell their dedicated rectifier tubes, which where more expensive. There is still a prejudice against TV dampers as rectifiers because of this. Many years of experience with TV dampers in various power supplies proofed that they are extremely reliable and perfectly usable as rectifiers. Another advantage is their extremely low cost. The 6AX4 shares the same base diagram with other similar types like 6DE4, 6DM4, 6AU4 and many others. It is also available with different heater voltages and for series heater connection as types 12AX4, 17AX4 and 25AX4. A detailed datasheet can be found here. The 6AX4's current rating is on the low side compared to some of the other TV dampers. So why did I chose it as my favorite? The current rating is still way beyond the needs of most amps I build. More than enough to power both channels of a SE power amp. When followed by a choke input filter, a 6AX4 rectifier bridge can deliver more than 300mA continous. The advantage of the 6AX4 is it's lower heater current compared to others. It needs only 1.2A. When 4 of them are used in a bridge, the higher heater current of other types can become a difficulty. Therefor I stick with the 6AX4 for most applications. If higher current is required, I use some of the beefier TV dampers. Another big plus of TV dampers is their good insulation between heater and cathode. Please note that this voltage is quite different depending on the polarity. The cathode can be at substantial higher voltage than the heater, while they don't like the heaters to be too postive against the cathode. For the 6AX4 the cathode can be up to 900V DC above the heater potential. This means that all heaters of the 6AX4s in a Greatz rectifier bridge can be wired in parallel and fed from a secondary winding which is referenced to ground. This is safe for power supplies with choke input filters which deliver up to approximately 650VDC. Higher voltage power supplies like those for 211 amps will require at least 3 different heater windings for a rectifier bridge to ensure cathode to heater voltage is not exceeded. The schematic below illustrates how the heaters need to be supplied and biased in the case of higher voltage PSUs: The 6AX4 was available from almost any tube manufacturer. Although production of this tube has ceased a long time ago, it is still available in large numbers. Below is a selection of 6AX4 tube boxes of different brands: The 6AX4 was made in various forms, most with the common large Octal base, in later years it was also broduced with the flat 'coin base': The last photo shows a 6AX4 Graetz bridge in a power supply: Best regards Thomas

Hi! The previous part of this series showed a scaled down version of this concept based on the 6CB5A which minimized cost mainly by reduction of the iron content. But most people who were interested in an amp with the 6CB5A asked what can be done to get the best possible sound out of it: The most obvious way is of course to choose the best possible quality of interstage and output transformers. While the Lundahl transformers which were mentioned in previous posts will provide excellent sound quality, this can be improved by the use of suitable transformers from the Tango range. These come at a much higher cost though. The very early prototype of this amp has been built with the Tango XE20S which is an excellent performer. One level up is the type FC30-3.5S. Tango makes one of the best interstage transformers available, the famous NC20, which is especially well suited for the highish plate resistance of the 6N7. A change to these transformers does not require any other change to the circuit. The photo above shows the most elaborate implementation of the 6CB5A amplifier so far with Tango NC20 and FC30-3.5S transformers. Another possibility to scale up is the power supply. Rather than splitting the amp up into mono blocks, a split between amp and power supply is the better option in my opinion. Not only does it provide good isolation of signal section and PSU but also more room to use a heavy duty power transformer. The external PSU also provides enough space for 4 tube sockets which enable the use of an all vacuum tube rectifier bridge instead of the more traditional full wave rectifier scheme with a center tapped secondary. The schematic above shows the power supply as I use it in amps with external PSUs. It has a LCLC filter section as the previously shown supply. The transformer secondary only needs to be about half the voltage. No center tap required. The advantage of this rectifier scheme becomes obvious when you compare the voltage waveforms after the rectifier, or the secondary voltage on a scope. In the common full wave scheme the two diodes 'fight' with each other during the switch over between phases. This causes a distorted waveform. The full wave 'Graetz' bridge as shown above has a much smoother switching behaviour which means less potential interference. The rectifier bridge is composed of 4 6AX4 TV damper diodes. The 6AX4 deserves a separate post on it's own, so I'm not going much into detail about it here. All heaters of the 6AX4 can be wired in parallel and fed from a dedicated heater winding which should be referenced to ground, simply connecting one end of the winding to ground is sufficient. All other aspects of the PSU are the same as in the previously introduced version. Here some more photos of this implementation. As you can see, there is one deviation from the rectifier scheme above. Half of the bridge has been done with 866A mercury vapour rectifiers. This was done mainly for cosmetic reasons. Check out the blue glow of the 866As in operation and you understand why. Using mercury vapour rectifiers adds some complexity to the circuit since their filaments need to be preheated before the high voltage can be applied. So a separate filament transformer and a delay circuit was necessary to implement this preheating mechanism. A manual override switch for this mechanism is added on the back to allow longer preheating of 866As when new tubes are used the first time. All connections are brought out on the back side of the chassis. The main power transformer is mounted under one of the two black covers on the PSU. The second cover hides the oil caps. On the amplifier chassis, the caps are visibly mounted. Only six of the eight caps are placed on the top, two more are mounted inside of the chassis. Two vintage style panel meters have been added for the plate voltage and current. The current indicator shows the total current which is consumed. They are mainly there for stylish reasons. No manual bias adjustment in this amp. However they give some indication if the tubes age and the drawn current drops. As mentioned above, the 866As need some precaution. The main power switch connects power to the filament transformer and a small control transformer which powers a relais control circuitry. The secondary of that is rectified, smoothed and provides the voltage for a simple LM317 regulator. The resistors and capacitor at the adjust pin of that regulator are sized such that the output voltage of the LM317 rises slowly. After about a minute it reaches the threshold voltage of a relais which is connected to it. This relais then powers up the main power transformer and applies high vollage to the plates of the 866As. Since the Two 6AX4s which form the second half of the bridge are heated from the same mains power transformer, current doesn't flow immediately but only after the TV dampers cathodes are warmed up. This avoids voltage overshoots when the relais closes. As a nice side effect the blue glow in the 866As comes up slowly to full brightness as the 6AX4s warm up. The power supply schematic with the delay scheme is shown below: Not shown in the circuit is the manual override switch which allows to condition the 866As when they are new. New mercury vapour rectifiers should be preheated for 30 minutes before high voltage is applied. This ensures that all the mercury gets vaporized and no drops remain in the system which could potentially created shorts and damage the tube. Such a preconditioning should be done whenever a tube is first used in the power supply. Transport or horizontal storage will distribute the mercury all over the inside of the bulb. This photo shows a close up of the 866As. This tube will get it's own post in the Tube of the Month series in the future. And here another shot of the PSU in the dark so you can see the nice glow of the 866As: The amp as shown here is about as far as it makes sense to go with the 6CB5A. Of course the whole amplifier concept can be improved even further. But that would require to go to directly heated triodes. This will be covered in upcoming articles which will show how the output stage can be converted to directly heated triodes. 45, 2A3, 10Y, 801A and 300B tubes have been used with the same concept, keeping the same driver stage and power supply configuration. For the 300B it is recommened to use the 6J5 instead of the 6N7. The 300Bs grid needs a driver with a bit more power. For all the others, the 6N7 is perfect. I can provide all the parts needed for such an amplifier concept. Since I only use the Graetz rectifier scheme, I can only supply power transformers for these, none with center tap. Such power transformers can be easier specified for unicersal usage. My power transformers each have two different secondaries which can be combined in many different ways to obtain secondary voltages from as low as 100V to 600V in very small increments. So the power transformers can be used for many different projects and easily allow the change of the B+ voltage for different output tubes. The power transformers have separate heater windings for the rectifier bridge and signal tubes. The transformers are available in 3 sizes, with 100, 200 and 400mA current ratings of the secondary. Besides the use with Graetz bridges, they are also suitable for voltage doubler schemes to obtain volatges for procects with 211, 845 or 6HS5 tubes as well. 211 or 845s however will require a different concept from this one. This will be covered in another series of articles. Best regards Thomas

Hi! After the introduction of the 6BY5 tube in the previous post, I will write about a power supply that uses it in this article. As mentioned in the 6BY5 tube of the month post, two of them will be very well usable in a bridge rectifier configuration for preamplifier supplies. I'm using them this way in the power supplies for the phono gain stages of the modular preamplifier. In this modular preamplifier, the phonostage is split up into two separate gain stages (input and output stage), separate LCR RIAA module and MC transformer. Each gain stage has it's own separate power supply for maximum isolation and flexibility. The gain stages will use the E55L which was presented in a Tube of the Month article in last November. They will be run at around 150V B+ and about 35mA. Taking some voltage drop into account in the output teransformer primary and local LC decoupling which will be in the gain section, a voltage of approximately 175V is needed. The schematic shows the high voltage part. The two 6BY5s are wired as a bridge, both heaters fed from the same winding and referenced to ground. About 300V AC are needed from the transformer to get the 175V out with the voltage drop in the chokes and rectifier. Final voltage will be determined once the PSU is tested under load. The transformer allows adjustment of the voltage. Heaters will be supplied by DC, rectified with Schottky diodes and powered by a separate transformer. A choke input filter is used for low switching noise. About 12Vs AC are needed to get 6,3V out with the losses in the rectifiers and filter. Again the final voltage will have to be trimmed when tested under load. The 100mA B+ transformer and low voltage transformer for DC filaments are used from my own range of custom wound power transformers. Details about these transformers can be found here. All elements of the modular preamp will use the same chassis style as shown in the posts about passive line stage and MC transformer. The circuit is divided into sub modules which are premounted on metal plates: All these plates are assembled in sandwich fashion: The complete assembly slides into the wooden enclosure: There are holes on the top and bottom of the enclosure to allow airflow around the tubes. A rotary on/off switch was chosen so that the same knobs can be used as on the passive line stage Usually the glow of the tubes are good enough for me to work as on/off indicator. Since they are mounted inside, a LED was added to the front: The PSUs for phono input and phono output stages are identical. Assembly of the gain stages themselves will be shown in the next post. Stay tuned! Best regards Thomas

Hi! Many people know me for my LCR phonostages and directly heated line preamplifiers. After I published the single ended amplifier concept based on the 6CB5A, the wish for a simpler preamplifier came up, which would match to this power amp both sonically as wells as in terms of materials cost. To achieve this goal a fairly common phono architecture was chosen, a RC coupled circuit with passive split RIAA, using commonly available and reasonably priced tubes. Since the 6CB5A amp already introduced transformer coupling, this concept was carried over to the preamp as well. At least into the line output stage. This would provide a low output impedance and the option to use a transformer volume control. The tube choice was somewhat inspired by JC Morrison's Siren Song preamplifier which was presented in an article in the Sound Practices magazine in the 90ies. Also the passice split RIAA approach is shared with the Siren Song. But this is were the similarities end. While the Siren Song hat a differential front end, this preamp should be all single ended. In the first stage the 6SL7 provides a lot of gain to bring up the phono signal to a reasonable level. The first stage drives the 75uS part of the RIAA network. After that a stage with paralleled halves of a 6N7 further amplifies the signal and drives the 318/3180uS network. These two stages provide about 40dB gain (20dB loss in the RIAA EQ already deducted). A final stage with a 6SN7 driving a 4.5:1 Lundahl LL1660 line output transformer amplifies the signal by another 12dB and provides a low 350 Ohm output impedance. This is sufficient gain for MM cartridges. For MC a suitable step up transformer needs to be used. This is best directly integrated into the preamp. A suitable and very nice sounding MC step up is for example the Lundahl LL1681. Depending on the step up ratio, which can be chosen between 1:13 and 1:26. The overall gain of the preamp can reach well over 80dB, enough even for MC cartridges with very low output voltage. This is the first version of the schematic which was built in a prototype style and tested: This was the first version of the PSU: In order to minimize capacitors in the signal path, all cathode resistors are unbypassed. The last two stages are DC coupled and the output stage uses an ultrapath cap. If you only have an analog front end, this circuit is optimized for this purpose. But like this it is a bit difficult to get an entry point for line level sources, since the output stage is DC coupled. We will see how the circuit can be changed to enable switchable line inputs later. First let's discuss some further aspects of the design. How are the RIAA components calculated? The 75uS network provides a corner at 2122Hz. It gives a constant roll off of 3dB per octave from that frequency on. The filter which achieves this is a simple RC network, the resistance multiplied by the capacitance results in 75uS. But if you multiply the 200k series resistor with the 150pF, you get a different value. This is simply because the output impedance of the driving stage (this is the plate resistance of the tube paralleled by the plate load resistor) is in series with this resistance and needs to be added. Also the grid to ground resistor of the following stage plays a role. This is in parallel to the series resistance. Another component which is not directly visible but plays a role is the miller capacitance of the following stage. It is in parallel to the capacitor in the network which needs to be reduced in size accordingly. Unfortunately the datasheets of the 6N7 don't give electrode capacitance, probably because it was not intended for RF use which requires this information. So the actual cap value for the RC network was determined by measurements in circuit. As you can see, both the miller capacitance of the second stage as well as the plate resistance of the driving stage are a significant part of the RIAA network and will influence it's accuracy. This could be seen as a weak point, but this compromise was made intentionally. One way to reduce this dependency would be to use a lower impedance RIAA network, then the miller capacitance of the second stage would be negligible. But a lower impedance network in turn would require a driving tube which has low plate resistance. Such tubes either have low gain or high transconductance. Low gain tubes are not really suitable since we want a lot of amplification in the first stage. High transconductance tubes typically tend to oscillate without suitable counter measures, therefore they where ruled out as well. There is a series resistor to the cap to ground in the 75uS network. The purpose of this is to add the von Neuman time constant. This 4th timeconstant is not described in the RIAA standard. But it is used in the recording process. The reason is that the recording amplifier cannot increase the amplitude with rising frequencies endlessly, but stays flat beyond a certain frequency. The actual corner frequency was never really defined. It can be assumed to be around 50kHz. The series resistor stops the high frequency attenuation of the 150pF cap at a certain frequency. However this resistor cannot affect the roll off caused by the miller capacitance, which is a substantial part of the filter network. So it is debatable if it makes sense to have this resistor. I left it in, but did not do any further experimentation there. The second part of the filter network is less influenced from it's surrounding circuits. It can be made lower impedance due to the lower plate resistance of the 6N7. The two time constants can be simply achieved by a 100k resistor in series and a 32nF cap in series with a 10k resistor to ground. 100kOhm times 32nF equals 3200uS and 10k multiplied by 32nF equals 320uS. The cap can be formed by some standard values like 22nf parallel to 10nF. With some selection of the caps they can be brought closer to the ideal value of 31,8nF. Again the output impedance of the driving stage and grid to ground resistor of the output stage needs to be considered in the calculation of the series resistor. Best practice is to fine tune the RIAA network in circuit. First build up the circuit without the RIAA network, instead just place a 1:10 voltage divider after the second stage to avoid overload. Then scheck that the circuit works linear without the RIAA network. If it is ok, introduce the networks, possibly one by one and verify and fine tune their correct filter operation. The PSU is a classic full wave rectifier with a center tapped secondary, followed by a choke input filter. The heaters are all wired in parallel and also supplied by a choke input filtered DC supply. To minimize the impact of any residual ripple on the heater supply, it is biased slightly positive by a voltage divider across the B+. This voltage divider acts at the same time as a bleeder resistor. As mentioned above, in order to change the circuit into a full function preamp with line level inputs, the last two stages need to be changed to capacitor coupling. The second schematic shows how this is done: The schematic also illustrates the grounding approach. Each stage has it's local star ground. These ground points get connected with a heavy gauge solid copper wire. The connection between ground and the output is shown as a dotted line. This is an optional connection and can be done according to the system needs. In order to avoid ground loops it can be left open. Each channel is decoupled via it's own choke so the PSU can be shared for both channels. As volume control, I use Dave Slagle's AVCs. To minimze wiring, there is a PCB with relais. But of course it can also be done by routing each signal wire to the rotary switch. The control voltage for the relais is set for 6.3V so it can be conveniently supplied by the heater voltage. As a start, the preamp could also be used with a standard resistive potentiometer and upgraded to a transformer volume control later. If cost is an issue the preamp could also be built with an RC coupled output stage. Of course any number of line inputs can be implemented, shown are just 3. The PSU of the final version also uses the Graets bridge rectifier scheme which I introduced already in other articles: Since all the tubes in the preamplifier share the octal base, the name Octal preamplifier was created. Even the rectifiers are octal base types. In total the preamp has 8 tubes (1 6SL7, 1 6SN7, 2 6N7 and 4 6AX4). The preamplifier has been built several times in this form and works nicely. But as the suffix Mk1 in the title indicates, there is a new version on the way with some changes: The input and output stage both share one half of a double triode between the two channels. While this is not a real problem (channel separation is an overrated parameter) it can be easily changed. The simplest way would be to just use two each of the 6SL7 and 6SN7 and wire both triode systems in each bottle in parallel. The 75uS RIAA network would require some adaption if this is done and the line output transformer would need to be exchanged for a 18mA type. But since these two are very commonly used in the audio world, I'd also like to use different, less common tubes instead of them. For the line putput stage there is another reason for a change. The 6SN7 even working into a step down transformer provides a lot of gain of about 12dB. With most line sources having 2V RMS output, this is a bit on the high side. We want to have a sensible range on the volume control. So it will be substituted with another tube. The Mk2 version of this preamplifier will be presented in another article. Stay tuned! Best regards Thomas P.S.: I tried to explain all aspects of the circuit. If anything is left unclear, don't hesitate to ask.

Hi! Many people know me for my LCR phonostages and directly heated line preamplifiers. After I published the single ended amplifier concept based on the 6CB5A, the wish for a simpler preamplifier came up, which would match to this power amp both sonically as wells as in terms of materials cost. To achieve this goal a fairly common phono architecture was chosen, a RC coupled circuit with passive split RIAA, using commonly available and reasonably priced tubes. Since the 6CB5A amp already introduced transformer coupling, this concept was carried over to the preamp as well. At least into the line output stage. This would provide a low output impedance and the option to use a transformer volume control. The tube choice was somewhat inspired by JC Morrison's Siren Song preamplifier which was presented in an article in the Sound Practices magazine in the 90ies. Also the passice split RIAA approach is shared with the Siren Song. But this is were the similarities end. While the Siren Song hat a differential front end, this preamp should be all single ended. In the first stage the 6SL7 provides a lot of gain to bring up the phono signal to a reasonable level. The first stage drives the 75uS part of the RIAA network. After that a stage with paralleled halves of a 6N7 further amplifies the signal and drives the 318/3180uS network. These two stages provide about 40dB gain (20dB loss in the RIAA EQ already deducted). A final stage with a 6SN7 driving a 4.5:1 Lundahl LL1660 line output transformer amplifies the signal by another 12dB and provides a low 350 Ohm output impedance. This is sufficient gain for MM cartridges. For MC a suitable step up transformer needs to be used. This is best directly integrated into the preamp. A suitable and very nice sounding MC step up is for example the Lundahl LL1681. Depending on the step up ratio, which can be chosen between 1:13 and 1:26. The overall gain of the preamp can reach well over 80dB, enough even for MC cartridges with very low output voltage. This is the first version of the schematic which was built in a prototype style and tested: This was the first version of the PSU: In order to minimize capacitors in the signal path, all cathode resistors are unbypassed. The last two stages are DC coupled and the output stage uses an ultrapath cap. If you only have an analog front end, this circuit is optimized for this purpose. But like this it is a bit difficult to get an entry point for line level sources, since the output stage is DC coupled. We will see how the circuit can be changed to enable switchable line inputs later. First let's discuss some further aspects of the design. How are the RIAA components calculated? The 75uS network provides a corner at 2122Hz. It gives a constant roll off of 3dB per octave from that frequency on. The filter which achieves this is a simple RC network, the resistance multiplied by the capacitance results in 75uS. But if you multiply the 200k series resistor with the 150pF, you get a different value. This is simply because the output impedance of the driving stage (this is the plate resistance of the tube paralleled by the plate load resistor) is in series with this resistance and needs to be added. Also the grid to ground resistor of the following stage plays a role. This is in parallel to the series resistance. Another component which is not directly visible but plays a role is the miller capacitance of the following stage. It is in parallel to the capacitor in the network which needs to be reduced in size accordingly. Unfortunately the datasheets of the 6N7 don't give electrode capacitance, probably because it was not intended for RF use which requires this information. So the actual cap value for the RC network was determined by measurements in circuit. As you can see, both the miller capacitance of the second stage as well as the plate resistance of the driving stage are a significant part of the RIAA network and will influence it's accuracy. This could be seen as a weak point, but this compromise was made intentionally. One way to reduce this dependency would be to use a lower impedance RIAA network, then the miller capacitance of the second stage would be negligible. But a lower impedance network in turn would require a driving tube which has low plate resistance. Such tubes either have low gain or high transconductance. Low gain tubes are not really suitable since we want a lot of amplification in the first stage. High transconductance tubes typically tend to oscillate without suitable counter measures, therefore they where ruled out as well. There is a series resistor to the cap to ground in the 75uS network. The purpose of this is to add the von Neuman time constant. This 4th timeconstant is not described in the RIAA standard. But it is used in the recording process. The reason is that the recording amplifier cannot increase the amplitude with rising frequencies endlessly, but stays flat beyond a certain frequency. The actual corner frequency was never really defined. It can be assumed to be around 50kHz. The series resistor stops the high frequency attenuation of the 150pF cap at a certain frequency. However this resistor cannot affect the roll off caused by the miller capacitance, which is a substantial part of the filter network. So it is debatable if it makes sense to have this resistor. I left it in, but did not do any further experimentation there. The second part of the filter network is less influenced from it's surrounding circuits. It can be made lower impedance due to the lower plate resistance of the 6N7. The two time constants can be simply achieved by a 100k resistor in series and a 32nF cap in series with a 10k resistor to ground. 100kOhm times 32nF equals 3200uS and 10k multiplied by 32nF equals 320uS. The cap can be formed by some standard values like 22nf parallel to 10nF. With some selection of the caps they can be brought closer to the ideal value of 31,8nF. Again the output impedance of the driving stage and grid to ground resistor of the output stage needs to be considered in the calculation of the series resistor. Best practice is to fine tune the RIAA network in circuit. First build up the circuit without the RIAA network, instead just place a 1:10 voltage divider after the second stage to avoid overload. Then scheck that the circuit works linear without the RIAA network. If it is ok, introduce the networks, possibly one by one and verify and fine tune their correct filter operation. The PSU is a classic full wave rectifier with a center tapped secondary, followed by a choke input filter. The heaters are all wired in parallel and also supplied by a choke input filtered DC supply. To minimize the impact of any residual ripple on the heater supply, it is biased slightly positive by a voltage divider across the B+. This voltage divider acts at the same time as a bleeder resistor. As mentioned above, in order to change the circuit into a full function preamp with line level inputs, the last two stages need to be changed to capacitor coupling. The second schematic shows how this is done: The schematic also illustrates the grounding approach. Each stage has it's local star ground. These ground points get connected with a heavy gauge solid copper wire. The connection between ground and the output is shown as a dotted line. This is an optional connection and can be done according to the system needs. In order to avoid ground loops it can be left open. Each channel is decoupled via it's own choke so the PSU can be shared for both channels. As volume control, I use Dave Slagle's AVCs. To minimze wiring, there is a PCB with relais. But of course it can also be done by routing each signal wire to the rotary switch. The control voltage for the relais is set for 6.3V so it can be conveniently supplied by the heater voltage. As a start, the preamp could also be used with a standard resistive potentiometer and upgraded to a transformer volume control later. If cost is an issue the preamp could also be built with an RC coupled output stage. Of course any number of line inputs can be implemented, shown are just 3. The PSU of the final version also uses the Graets bridge rectifier scheme which I introduced already in other articles: Since all the tubes in the preamplifier share the octal base, the name Octal preamplifier was created. Even the rectifiers are octal base types. In total the preamp has 8 tubes (1 6SL7, 1 6SN7, 2 6N7 and 4 6AX4). The preamplifier has been built several times in this form and works nicely. But as the suffix Mk1 in the title indicates, there is a new version on the way with some changes: The input and output stage both share one half of a double triode between the two channels. While this is not a real problem (channel separation is an overrated parameter) it can be easily changed. The simplest way would be to just use two each of the 6SL7 and 6SN7 and wire both triode systems in each bottle in parallel. The 75uS RIAA network would require some adaption if this is done and the line output transformer would need to be exchanged for a 18mA type. But since these two are very commonly used in the audio world, I'd also like to use different, less common tubes instead of them. For the line putput stage there is another reason for a change. The 6SN7 even working into a step down transformer provides a lot of gain of about 12dB. With most line sources having 2V RMS output, this is a bit on the high side. We want to have a sensible range on the volume control. So it will be substituted with another tube. The Mk2 version of this preamplifier will be presented in another article. Stay tuned! Best regards Thomas P.S.: I tried to explain all aspects of the circuit. If anything is left unclear, don't hesitate to ask.

Hi! The concept of the amplifier has been defined in the first part. Now I will write a little about the search for a suitable driver tube. Also the concept will be detailed further with values for supply voltages and passive components. To have all requirements for the driver tube, we need to know the voltage swing and gain it has to deliver to drive the outout stage to full power with reasonable sensitivity. So let's flesh out the details of the output stage first. Since the 6CB5A is fairly cheap and can be expected to be very robust as is common with TV tubes, we have no fear to run it at maximum plate dissipation. Laying load lines on the plate curves and experiments with a 6CB5A in a prototype mock up came up with about 350V from plate to cathode and 70-75mA plate current. This requires about -70 to -75V on the grid. A 1kOhm cathode resistor will provide the correct operating point. This means the amp needs to be fed with a B+ voltage of about 425V. A bias voltage of 70 V means the output tube requires 140V peak to peak for full power. This translates to about 50V RMS (amplitude of the signal divided by the square root of 2). In my post about gain, headroom and power I already wrote about the preference of having not too much gain, so I would shoot for a input sensitivity between 1V and 2V RMS. This means we need a gain of 25 to 50 from the driver stage. This is a bit above the gain which the common 6SN7 would deliver, so let's look for something else. Choosing the one of the most widely used driver tubes would be boring anyways. We also want something which is not too expensive. The driver tube needs to be able to drive an interstage transformer so it cannot have a plate resistance which is too high. Brwosing through the databooks came up with two interesting types: the 6AM4 and the 7F8, the latter is a double triode with a common cathode, so both sections would need to be wired in parallel. The 6AM4 would even provide a higher gain than targetted, around 80. The 7F8 would be 38. I had some suitable output transformers lying around, A pair of Tango XE-20S. As interstage transformer the Lundahl LL1660/10mA could be used. A prototype was built with components out of the parts bin. The prototype had two sockets wired in parallel for a quick comparison of both the 6AM4 and 7F8 as drivers. As caps I used 25uF MKP types. The Tango XE20S is a universal type which allows to select primary impedances of 2.5, 3.5 and 5kOhms. For the power supply I reused something which I had assembled for another project. It allowed selection of different B+ voltages. It got set up for the required 425V. It also delivered the 6,3VAC for the heaters of 6CB5As and driver tubes. All heaters got wired up in parallel and referenced to ground via two 100 Ohm resistors. A group of people got invited for the initial listening. Some other amps were also available for direct comparison. Among them a 300B amp, a PP 46 and also a very elaborate 801A amp. The amp was an instant hit. It sounded much much better than I would have expected or hoped for. It instantly beated the SE300B and PP46 amps. It had no chance against the 801A amp which was built with 4 times the budget for parts. This was very encouraging and sparked quite some interest among local DIYers. The owner of the 300B amps immediately decided to sell them and switch to 6CB5A. Before I started to rebuild the amp in a nicer chassis, I spent some more thought on the driver tube. Although the 6AM4 and 7F8 both performed quite well they did not have enough headroom. It was just ok for transformer coupling but would not have been enough for RC coupled versions. So back to browsing the databooks. The 6N7 caught my interest. It would provide similar gain as the 7F8 but more headroom, enough to also use it RC coupled. It is quite linear and has an octal base as the 6CB5A. It is a double triode with common cathode like the 7F8. The prototype got modified for the 6N7 as driver. It has a max plate voltage of 300V so B+ needs to be dropped via a resistor form the output stage. This also serves as decoupling together with a capacitor. A 1kOhm cathode resistor gives an op point of about -6 to -7V on the grid, 6-7mA. That's a good 9-10dB headroom in the driver stage. A listening test confirmed that the 6N7 was a much better choice as driver. The amp got more refined, more neutral than before, retaining all the sonic qualities which it shares with other excellent single ended concepts. Now it was time to build a 'serious' version of this amp. Capacitors should be upgraded to oil types. For this purpose I selected ASC X386 series caps. 30uF/440VAC types. Since there was enough interest from other DIYers, I specified a heavy duty power transformer and got a batch of them wound from a local manufacturer. The power transformer was spec'ed such to allow some flexibility for adjustment of the voltages and flexibility to use other tubes as well. Here is the schematic of the signal section of the final version of the amp with all component values and approximate voltages. No need for 1% parts. Also the exact voltages are not critical. The design has enough headroom to allow for some leeway there. Let's go through each component and clarify it's purpose and value. Starting at the amps input, we see a 100k resistor to ground. This is providing a reference for the grid of the driver tube and a path to ground. The maximum allowable value for the 6N7 is 500k. 100k is a good value which will present an easy load to almost any preamp. This value can be lowered or increased, 47k or 200k would be fine too. No speacial wattage needed here, 0.5W is fine. This amp is DC coupled at the input. Some amps cap couple the input to provide protection if the preamp has some offset. This amp is fairly insensitive to DC offsets, even a offset of 100mV would not do any harm. If a preamp has more offset it should be replaced anyways. So let's avoid a cap in the signal path here. If you want to have DC protection however, don't hesitate to put one in. Many amps also make excessive use of grid stopper resistors. I left them out. These are not needed for lowish transconductance tubes like the ones in this concept. With careful layout there will be no danger of parasitic oscillation. Again, if you feel better with grid stoppers, put some in. But don't exaggerate. a few 100 Ohms is enough. The 6N7 has both cathodes wired in parallel, the pin numbers are indicated in the schematic. A 1kOhm cathode resistor takes care of the setting of the operating point. 0.5W or higher can be used for the cathode resistor. What is not shown in the schematic is the connection of Pin 1. This Pin is connected to the envelope if a metal verison of the 6N7 is used. Pin 1 should be connected to ground. Another nice touch about the 6N7 driver: If you compare it's pinout to the 6J5, you will realize that they are interchangeble. The 6J5 will bias up correctly in the same circuit if dropped in. It will just lower the gain. So if there is too much gain in the system, the 6J5 is an easy way to reduce it. The interstage transformer is a Lundahl LL1660/10mA it is wired 1:1.125 (connection alternative S in the datasheet). The 6CB5A does not need a grid to ground resistor since the secondary of the interstage transformer provides a ground path. This path is very low in DC resistance. In case the output tube is overdriven, which causes grid current it is routed to ground through this low DCR path. This allows the amp to recover quickly from overload conditions. You can experiment with a grid resistor which will terminate the transformer. This usually improves the square wave response. Sonically, I prefer to run transformers unloaded whenever possible. The 6CB5A is biased with a 1kOhm cathode resistor. Here a high wattage type is needed, 20W or more. This is because of the high voltage across the resistor. It gets quite hot! The 6CB5A is wired in triode mode, the screen grid is connected to the plate through a 100 Ohm screen stopper resistor. Place this resistor as close to the pins as possible. The output transformer can be any type between 3 and 5 k primary impedance which can be run with 70mA DC. Suitable types: Tango XE20S, Tango FC30-3.5S, Lundahl LL1663, LL1664, LL1682 and others. There are also types from winders like Tamura, Hashimoto, James and many others which would fit. As mentioned above I selected ASC X386 MP in oil caps. All values are 30uF/440VAC. These caps sound excellent in this amp, better than the MKP types used in the first prototype. Any other cap of similar value and sufficient voltage rating can be used. The exact cap value is not critical. Also higher capacitances can be used. The 6,8k/20W resistor provides dropped voltage to the driver and decouples the driver supply with the cap following it. The 22k resistor to ground from the driver supply acts as voltage divider together with the 6,8k and as bleeder resistor. It will ensure that the power supply is always discharged when the amp is switched off. In part 3 I will describe the power supply and also show some photos of the assembly process of the amp. Stay tuned! Best regards Thomas

Hi! The circuit of the amp his been detailed in the first two parts down to each component. What has not been covered yet is the power supply. So let's concentrate on this now. The requirements have already been set. The B+ is about 425VDC. It has to supply both channels. Each output draws 70-75mA. The drivers take 6-7mA each. The bleeder resistors swallow another 15mA. That sums up to almost 200mA. The heaters are all connected in parallel. The 6CB5A draws 2.5A per tube, the 6N7 0.8A. That is 6.3V/6.6A for the heater supply. Basically any power supply which can deliver these voltages at the given currents will do. But of course we will also propose a good solution to go with the circuit as presented in the first two parts. Some deviation from the targetted 425V is not critical. This can move up a bit, as long as the max plate dissipation is not exceeded. The plates of the 6CB5A will start to glow red if they are beyond their maximum. In that case reduce the B+ voltage until the glow disapears. If you cannot reduce the voltage in the supply, you can dial down the current by increasing the cathode resistor. The heater voltage should be as close as possible. I'd rather have the voltage a bit below than above the nominal 6.3V. Anything between 6 and 6.3V is ok. As has been mentioned in the first part, we don't want the average cheap PSU, but something nice, yet affordable. So let's go for a classic choke input filtered supply with tube rectification. The choke input supply has some advantages. Voltage regulation is better compared to cap input. Current draw is spread over the whole conduction angle of the diodes rather than in pulses as with cap input. This means less current spikes with the potential to creep into our amp circuit. Also the rectifier and transformer have to deliver smaller current amplitudes and get less stress. But nothing comes for free, there are also disadvantages. The resulting DC voltage is much smaller at a given secondary voltage, about 0.9 times the secondary voltage versus the theoretical maximum of 1.4 times secondary voltage for cap input. The first choke needs to be rated for choke input duty, otherwise it can exhibit mechanical buzz. Since we have chosen unusual and low cost tubes for the amp circuit, we will do the same for the rectifier. So we stay clear of the typical rectifiers seen in audio amps like GZ34, 5AR4, 5U4, etc. Looking into the TV tube arsenal we find the so called TV damper tubes. These have been developed to damp oscillations in the electron beam defelection system of TVs during the fly back of the beam. For this application they need to be capable of high current pulses and very high peak inverse voltages. There is an abundance of TV damper diodes with various bases, like 6AX4, 6AU4, 6CJ3, 6CG3. Unfortunately they are all single diodes, so two are needed for a full wave rectifer. But there is one exception. The 6BY5 contains 2 diodes in one tube. It has an octal base, so all tubes in the amp will use the same socket, which is a nice touch. Another good point is it's low cost. It lists for a few bucks in the catalogs of all major tube dealers. It's specs indicate more than enough current capability and peak inverse voltage rating. So let's use it. The schematic above shows the complete power supply. Let's go through it component for component, starting with the power transformer. The secondary voltage can be estimated as follows. We want 425V DC out. There will be some voltage drop in the rectifier and in the chokes. Let's assume 25V. So the actual DC voltage we need out of the rectifier is 450. To get the secondary voltage we need to divide this value by 0.9 which gives 500V per leg of the secondary. that is 1000V across the entire secondary. Depending on the DC resistance of the windings in the chokes and power transformer the actual voltage might differ. But as mentioned it is not necessary to reach the exact 425V Anything between 400 and 450 will be pretty much ok. There is a nice freeware tool available on the web which does all those PSU calculations for you: The power supply designer PSUD2. Download it and play around with it. Very useful and educational tool! As determined above, the supply needs to deliver 200mA. The choke input supply draws current continuoulsy during the conduction angle of the rectifier tube, but only from one half of the secondary winding at a time. The textbooks say that with choke input the current capability of the secondary winding should be about 10% higher than the DC current drawn. That would translate into 110mA across the whole secondary (220mA divided by 2 since each half only needs to deliver current 50% of the time). But it doesn't hurt to over specify the power transformer. This yields better voltage regulation and less heating of the transformer. So let's pick at least 150mA. For the first batch of power transformers I got wound for this project I spec'ed even 200mA. There are also the heaters of the tubes which need to be supplied with 6,3VAC. For AC heating there is no benefit in having these supplied from separate transformers, of course it would not hurt either. I got the heater voltages wound on the B+ transfomer as well. We need separate heater windings for the rectifier and for the amplifier tubes. The 6BY5 does not withstand a very high voltage difference between heater and cathode as other TV dampers, so best to have the heater on the cathode potential. This is achieved by simply connecting one end of the heater directly to the cathodes as indicated in the schematic. Of course that means we need separate windings since we want to reference the other heaters to ground. The requirements for the heater windings are 1,6A for the 6BY5 and a total of 6.6A for the signal tube heater winding. Both heater windings need to have sufficient isolation between them. Another important feature of the power transformer is the screen between primary and secondaries. Without a screen winding, there is capacitive coupling between the windings. This would allow high frequency crap from the mains to pass straight through. With the screen winding in between both primary and secondary now have a coupling capacitance to ground rather than between them. As indicated in the schematic I got my transformers wound with two independent screens. One is connected to protective earth and chassis and the second to signal ground. Not indicated in the schematic is a feature which I get on all my power transformers. Taps on the primary for some fine adjustment of roughly + or - 5%. Instead of 0-230V the primary actually is 0-220-230-240V. Also not shown are mains on/off switch and the mains fuse. The average DIYer should be able to sort out these details himself. The schematic shows two LC sections, each with a 10Hy choke and 40uF cap. As mentioned in the first parts, amp circuits with ultrapath connection and no cathode bypass cap need very well filtered B+. You might get away with just a single LC section. Then it is advisable to increase the capacitance and use cathode bypass caps at least on the driver tubes if there is still some hum. With a supply as drawn, there will be no hum even with very sensitive speakers. The first inductor needs to be wound for choke input duty. Otherwise it can exhibit mechanical buzz due to the large AC voltage across it. You might get away with a choke for cap input service if it is sufficiently derated. A small input cap of 100nF can help if you have some mechanical buzz. This cap should be rated for at least 1000V since it can see high voltage spikes. This optional cap is shown in grey in the schematic. I use Lundahl LL1673 chokes. They are made for use in choke input supplies and work excellently in this PSU. The heater supply for the signal tubes is referenced to ground via two 100 Ohm resistors. 2W types are sufficient for this. The third resistor is a bleeder. It ensures that the minimum current is drawn from the supply to maintain proper choke input operation. As a rough guideline 1kOhm load is needed per Henry inductance of the first choke. The first choke is 10Hy, so a load of roughly 10kOhm is needed. Since there are already bleeder circuits in the amp (22k and 6,8k in series per channel) we only need a 33k which in parallel with the other resistors gives about about 10k. Since this resistor dissipates quite some power, a 20W type is needed. This ensures that even with tubes unplugged or failing tubes, the current will not fall below the critical value. This resistor can be left out if the capacitors have sufficient voltage rating. If sub critical current is drawn the filter will stop working as a choke input but behave like a cap input and the voltage will rise by up to 50%. To be safe in a fault condition or when the power supply is tested without amp circuit attached we don't want the caps to blow. So either overrate them or install the bleeder circuit, or both to be on the save side. I selected ASC X386 440VAC caps. The DC rating of these caps is 630VDC minimum. With this post all details have been laid out about this amplifier. In the next parts I will first show the cost down version in detail including it's power supply. After that I plan an article about an improved version of the power supply using a full wave Graetz-bridge with tubes. And after that we will see how this concept can be adapted for directly heated tubes as well. Let's close this post with some photos. The first row shows the interior of 6CB5A amps based on the same circuit, built by different people Here some 6CB5A amps with different chassis styles and output transformers: The top row shows implementations with Tango, James and Hashimoto transformers. The second row shows a pair of monoblocks using Lundahl tranformers and the blue one is a Push Pull version, again with Tango. The last picture shows an amplifier based on the very same circuit but with the power supply separated into an external chassis. There is a minor difference in the power supply though, instead of a 6BY5, two 6AX4s are used as rectifier. Both interstage and output transformers are Tango. NC20F interstage and FC30-3.5S output transformer: Best regards Thomas

Hi! I often hear people complaining about tube prices and how much better it has been in the old days when prices had been more affordable. I wholeheartedly disagree with this point of view. In fact I think we are in the best possible times when it comes to tube audio. While it is true that the prices of some overly hyped tube types skyrocketed, there are still a lot of dirt cheap tubes which are available in abundance as NOS. Thanks to the internet it is easy to find suppliers for such tubes and data sheets of even the most obscure types are just a few clicks away. I remember when I started to get into tube audio that it took me almost a year just to get the most essential tube data books. Also the availability of other parts is gerat nowadays. Audio transformers are available in a larger variety than ever. And there are new tubes still being made with the Elrog 300B being the latest introduction. If you think some tube types are expensive, compare the prices of them from 40, 50 or even more years ago with inflation in mind. Suddenly even many of the highly sought after tubes appear not that expensive compared to the old days. But as mentioned above, no need to hunt for those mainstream tubes if you are on a budget. I already introduced the 6CB5A as a cheap alternative to the 300B and many amplifiers have been build with this tube. In the Tube of the month post about the 6GE5 I suggested that it could be used triode connected in 2A3 designs. In the November Tube of the Month post I presented the triode section of the 6AG9 as a possible driver for a triode strapped 6GE5 in single ended mode. All that was missing is a suitable circuit. In order to evaluate the sound potential of these tubes I decided to not use the cheapest possible design but something a bit more advanced, with oil caps and interstage transformer coupling. Once the tubes have been proven to work as expected, simpler circuits can be explored as well as more elaborate ones. In order to minimize variables, I decided to adapt a proven circuit which is known to sound good. So it will be just the tubes which changed. The circuit which I usually use with the 6CB5A could be adapted with minor changes. The same circuit description as in the article about the single ended amplifier concept applies, so I am not going to repeat it. The voltages are adapted to the 6GE5 and 6AG9. The output tube runs at about 50-55mA with 300V from plate to cathode, so about 15W plate dissipation. The driver operates at about 200V and 6mA. The power supply is basically the same as described in an earlier Making of a 6CB5A amp post. Read it for a description of the supply. Just for the fun of it I decided to build two cute little mono blocks. So currents, choke and transformer ratings have been adapted accordingly. To stay consistent with the 12-pin compactron theme 6BE3 TV dampers are used as rectifiers. The construction of the amps will be shown in the next part. Stay tuned! I am making parts kits available for this design, both for mono and stereo versions. Best regards Thomas

Hi! The previous part of this series showed a scaled down version of this concept based on the 6CB5A which minimized cost mainly by reduction of the iron content. But most people who were interested in an amp with the 6CB5A asked what can be done to get the best possible sound out of it: The most obvious way is of course to choose the best possible quality of interstage and output transformers. While the Lundahl transformers which were mentioned in previous posts will provide excellent sound quality, this can be improved by the use of suitable transformers from the Tango range. These come at a much higher cost though. The very early prototype of this amp has been built with the Tango XE20S which is an excellent performer. One level up is the type FC30-3.5S. Tango makes one of the best interstage transformers available, the famous NC20, which is especially well suited for the highish plate resistance of the 6N7. A change to these transformers does not require any other change to the circuit. The photo above shows the most elaborate implementation of the 6CB5A amplifier so far with Tango NC20 and FC30-3.5S transformers. Another possibility to scale up is the power supply. Rather than splitting the amp up into mono blocks, a split between amp and power supply is the better option in my opinion. Not only does it provide good isolation of signal section and PSU but also more room to use a heavy duty power transformer. The external PSU also provides enough space for 4 tube sockets which enable the use of an all vacuum tube rectifier bridge instead of the more traditional full wave rectifier scheme with a center tapped secondary. The schematic above shows the power supply as I use it in amps with external PSUs. It has a LCLC filter section as the previously shown supply. The transformer secondary only needs to be about half the voltage. No center tap required. The advantage of this rectifier scheme becomes obvious when you compare the voltage waveforms after the rectifier, or the secondary voltage on a scope. In the common full wave scheme the two diodes 'fight' with each other during the switch over between phases. This causes a distorted waveform. The full wave 'Graetz' bridge as shown above has a much smoother switching behaviour which means less potential interference. The rectifier bridge is composed of 4 6AX4 TV damper diodes. The 6AX4 deserves a separate post on it's own, so I'm not going much into detail about it here. All heaters of the 6AX4 can be wired in parallel and fed from a dedicated heater winding which should be referenced to ground, simply connecting one end of the winding to ground is sufficient. All other aspects of the PSU are the same as in the previously introduced version. Here some more photos of this implementation. As you can see, there is one deviation from the rectifier scheme above. Half of the bridge has been done with 866A mercury vapour rectifiers. This was done mainly for cosmetic reasons. Check out the blue glow of the 866As in operation and you understand why. Using mercury vapour rectifiers adds some complexity to the circuit since their filaments need to be preheated before the high voltage can be applied. So a separate filament transformer and a delay circuit was necessary to implement this preheating mechanism. A manual override switch for this mechanism is added on the back to allow longer preheating of 866As when new tubes are used the first time. All connections are brought out on the back side of the chassis. The main power transformer is mounted under one of the two black covers on the PSU. The second cover hides the oil caps. On the amplifier chassis, the caps are visibly mounted. Only six of the eight caps are placed on the top, two more are mounted inside of the chassis. Two vintage style panel meters have been added for the plate voltage and current. The current indicator shows the total current which is consumed. They are mainly there for stylish reasons. No manual bias adjustment in this amp. However they give some indication if the tubes age and the drawn current drops. As mentioned above, the 866As need some precaution. The main power switch connects power to the filament transformer and a small control transformer which powers a relais control circuitry. The secondary of that is rectified, smoothed and provides the voltage for a simple LM317 regulator. The resistors and capacitor at the adjust pin of that regulator are sized such that the output voltage of the LM317 rises slowly. After about a minute it reaches the threshold voltage of a relais which is connected to it. This relais then powers up the main power transformer and applies high vollage to the plates of the 866As. Since the Two 6AX4s which form the second half of the bridge are heated from the same mains power transformer, current doesn't flow immediately but only after the TV dampers cathodes are warmed up. This avoids voltage overshoots when the relais closes. As a nice side effect the blue glow in the 866As comes up slowly to full brightness as the 6AX4s warm up. The power supply schematic with the delay scheme is shown below: Not shown in the circuit is the manual override switch which allows to condition the 866As when they are new. New mercury vapour rectifiers should be preheated for 30 minutes before high voltage is applied. This ensures that all the mercury gets vaporized and no drops remain in the system which could potentially created shorts and damage the tube. Such a preconditioning should be done whenever a tube is first used in the power supply. Transport or horizontal storage will distribute the mercury all over the inside of the bulb. This photo shows a close up of the 866As. This tube will get it's own post in the Tube of the Month series in the future. And here another shot of the PSU in the dark so you can see the nice glow of the 866As: The amp as shown here is about as far as it makes sense to go with the 6CB5A. Of course the whole amplifier concept can be improved even further. But that would require to go to directly heated triodes. This will be covered in upcoming articles which will show how the output stage can be converted to directly heated triodes. 45, 2A3, 10Y, 801A and 300B tubes have been used with the same concept, keeping the same driver stage and power supply configuration. For the 300B it is recommened to use the 6J5 instead of the 6N7. The 300Bs grid needs a driver with a bit more power. For all the others, the 6N7 is perfect. I can provide all the parts needed for such an amplifier concept. Since I only use the Graetz rectifier scheme, I can only supply power transformers for these, none with center tap. Such power transformers can be easier specified for unicersal usage. My power transformers each have two different secondaries which can be combined in many different ways to obtain secondary voltages from as low as 100V to 600V in very small increments. So the power transformers can be used for many different projects and easily allow the change of the B+ voltage for different output tubes. The power transformers have separate heater windings for the rectifier bridge and signal tubes. The transformers are available in 3 sizes, with 100, 200 and 400mA current ratings of the secondary. Besides the use with Graetz bridges, they are also suitable for voltage doubler schemes to obtain volatges for procects with 211, 845 or 6HS5 tubes as well. 211 or 845s however will require a different concept from this one. This will be covered in another series of articles. Best regards Thomas

Hi! This article will show a variation of the transformer coupled single ended amplifier concept. It is based on the 46, which was presented as tube of the month in the previous post. The amp was built mostly with parts which I had available. The parts cost is moderate, even using a heavy duty power transformer, ASC oil caps and all Lundahl signal transformers and chokes, the parts add up to about 1100-1200 Euros, excluding chassis. It uses all the features of this amplifier concept which contribute to the overall sound quality: Multiple choke filtered power supply, high quality caps, interstage transformer coupling, ultrapath caps in each stage. I still have some power transformers in stock with center tapped secondary and 3 independent 6.3V windings. To keep cost down, I used this one. Only this single power transformer is needed for B+ heater and filament supplies. The 2.5V AC voltage is obtained through dropping resistors from two of the 6.3V windings. The third 6.3V winding is used to supply the heaters of the rectifiers and the 6N7 driver tubes. This requires the heater winding to be referenced to ground. Two 6AX4s are used in a full wave configuration. Since it can withstand a large voltage between heater and cathode, the heater winding can be shared with the driver tubes. The PSU is common for both channels, only the filament windings of the output tubes are separate: The B+ supply is the usual choke input PSU using two LC sections for smoothing. Heaters and filaments are fed with AC. Since the heater windings have no center taps, a hum buck potentiometer is wired across the filaments of each of the 46. Two resistors are wired from the wiper to the filament terminals to reduce the resistance of the 100 Ohm Pot which I had available. The rest of the amplifier circuit is similar as presented in previous parts of this series of posts. my favorite indirectly heated driver for small output tubes, the 6N7 is used again, both halves wired in parallel. The output tube is driven by a Lundahl LL1660/10mA, wired almost 1:1 (actually it is 1.125:1). I had a pair of Lundahl LL1682/50mA in stock. These fit quite well to the 46. With the primary impedance of 5.5kOhm and a secondary of 5 Ohm they will provide a good damping factor when used with a 46 on 8 Ohm speakers. The 46 is wired for Class A use, with the second grid tied to the plate at the socket. See the 46 datasheet for details of the tube. The B+ is around 300V. The 46 is biased at around 30V. With voltage drops across chokes and output transformer primaries this results in just under 250V across the 46. Below, some photos of the construction of the amplifier. The first picture shows all capacitors, sockets, connectors, switches and resistors mounted and some initial wiring. The power transformer is placed on the top side of the plate and will be hidden under a cover. The next photo shows the inside with all chokes, interstage and output transformers mounted and completely wired: The top view of the completed amplifier: The amp sounds very smooth and delicate. It shares a lot of qualities with the best 45 amps at a lower cost and size. The same circuit can be used with minor adaptions for 45 or 2A3 output tubes. Best regards Thomas

Hi! Earlier this year, I already introduced the modular preamplifier concept. Here a post about the first modular phonostage in this style. The phono got broken up into input and output gain stages and separate MC step up and EQ units to be able to experiment with different gain modules. One of the ideas behind this was to experiment with directly heated triodes in the phono section. As a first step the phono output stage can be changed to DHT. And later maybe the input stage as well, but the usually high mircrophonics of DHTs might prevent this. The first circuit draft of a directly heated gain stage is shown above. The plan is to use the 841 as the gain element, because it is one of the few DHTs with considerably high amplifaciotn factor. Since it has a rather high plate resistance, it will be choke loaded and DC coupled to a 801 which will drive a step down line output transformer for low output impedance. To minimize capacitors in the signal path, the 841 will use filament bias. The output stage uses an ultra path cap from B+ to cathode (filament) of the 801. The other caps are there for decoupling and are not directly in the signal path. The 841 will run at about 5mA. Since the 801 operates at a higher current, a bypass resistor of about 47k is needed for the current difference. Final values need to be checked and if necessary adjusted once the circuit is built. The power supplies will follow my usual approach with passive choke filtering and tube rectification. Since this is meant for phono use, the usual LCL approach for the filaments will not be sufficient, so another LC stage needs to be added which ends up in 3 chokes per tube, 12 total! 8 of these chokes will be placed in the PSU unit and the final chokes in the preamp section. High voltage will be rectified with a TV Damper bridge, followed by LCLCLC filtering and the final LC decoupling separate per channel in the preamp section. This is a lot of iron, so 3 chassis are needed. Separate PSUs for filament supplies and high voltage. Even with the three chassis, there are too many components to use the enclosed wooden frames, so some parts need to be placed on top, similar to the 10Y preamplifier The initial construction steps for the filament supply chassis. Power transformers placed on the top: The filament transformer which is used is described here. Due to the high voltage drop through the chokes, 1 per tube is not enough. 2 more transformers are added. The separate secondaries of those are shared between two supplies. The initial wiring of the filament transformers: The filament chokes are mounted on sub plates which will be placed inside the wood chassis: The rectifier bridges using Schottky diodes are directly soldered to the input choke along with the filter caps: The high voltage supply will be constructed in a similar style, transformers, input choke and rectifiers placed on top of the chassis: Since the circuit uses 800V B+, two transformers are needed with the secondaries in series. To keep the heater to cathode voltages for the rectifiers within specs, 3 separate heater windings are required. Since each of the B+ transformers has only one heater winding, a separate heater transformer is added for the third one. The initial wiring: And finally the first construction steps of the signal chassis. Sockets mounted on vibration damped sub assemblies. The capacitors are Sprague paper in oil types with 2kV voltage rating. Here painted in white: The initial signal wiring: The filament bias resistors are at the front. The connection between them serves as star ground point. All signal wiring is done with solid core silver wire. This is probably one of my crazier projects. You might wonder why so much effort and why not try smaller DHTs which need less filament current. And why no regulated supplies instead of those heaps of iron. Such an approach might follow later. First I want to create the ultimate DHT phonostage. And that has to use tubes with thoriated tungsten filaments in my book. While there are good solutions for filament regulators, I still prefer the passive approach. Such regulators work best if preceeded with a good supply with choke input filter anyways, so staying completely passive is not that much more in terms of parts. Stay tuned for updates as the construction of this DHT gain stage progresses. Best regards Thomas

Hi! Many people know me for my LCR phonostages and directly heated line preamplifiers. After I published the single ended amplifier concept based on the 6CB5A, the wish for a simpler preamplifier came up, which would match to this power amp both sonically as wells as in terms of materials cost. To achieve this goal a fairly common phono architecture was chosen, a RC coupled circuit with passive split RIAA, using commonly available and reasonably priced tubes. Since the 6CB5A amp already introduced transformer coupling, this concept was carried over to the preamp as well. At least into the line output stage. This would provide a low output impedance and the option to use a transformer volume control. The tube choice was somewhat inspired by JC Morrison's Siren Song preamplifier which was presented in an article in the Sound Practices magazine in the 90ies. Also the passice split RIAA approach is shared with the Siren Song. But this is were the similarities end. While the Siren Song hat a differential front end, this preamp should be all single ended. In the first stage the 6SL7 provides a lot of gain to bring up the phono signal to a reasonable level. The first stage drives the 75uS part of the RIAA network. After that a stage with paralleled halves of a 6N7 further amplifies the signal and drives the 318/3180uS network. These two stages provide about 40dB gain (20dB loss in the RIAA EQ already deducted). A final stage with a 6SN7 driving a 4.5:1 Lundahl LL1660 line output transformer amplifies the signal by another 12dB and provides a low 350 Ohm output impedance. This is sufficient gain for MM cartridges. For MC a suitable step up transformer needs to be used. This is best directly integrated into the preamp. A suitable and very nice sounding MC step up is for example the Lundahl LL1681. Depending on the step up ratio, which can be chosen between 1:13 and 1:26. The overall gain of the preamp can reach well over 80dB, enough even for MC cartridges with very low output voltage. This is the first version of the schematic which was built in a prototype style and tested: This was the first version of the PSU: In order to minimize capacitors in the signal path, all cathode resistors are unbypassed. The last two stages are DC coupled and the output stage uses an ultrapath cap. If you only have an analog front end, this circuit is optimized for this purpose. But like this it is a bit difficult to get an entry point for line level sources, since the output stage is DC coupled. We will see how the circuit can be changed to enable switchable line inputs later. First let's discuss some further aspects of the design. How are the RIAA components calculated? The 75uS network provides a corner at 2122Hz. It gives a constant roll off of 3dB per octave from that frequency on. The filter which achieves this is a simple RC network, the resistance multiplied by the capacitance results in 75uS. But if you multiply the 200k series resistor with the 150pF, you get a different value. This is simply because the output impedance of the driving stage (this is the plate resistance of the tube paralleled by the plate load resistor) is in series with this resistance and needs to be added. Also the grid to ground resistor of the following stage plays a role. This is in parallel to the series resistance. Another component which is not directly visible but plays a role is the miller capacitance of the following stage. It is in parallel to the capacitor in the network which needs to be reduced in size accordingly. Unfortunately the datasheets of the 6N7 don't give electrode capacitance, probably because it was not intended for RF use which requires this information. So the actual cap value for the RC network was determined by measurements in circuit. As you can see, both the miller capacitance of the second stage as well as the plate resistance of the driving stage are a significant part of the RIAA network and will influence it's accuracy. This could be seen as a weak point, but this compromise was made intentionally. One way to reduce this dependency would be to use a lower impedance RIAA network, then the miller capacitance of the second stage would be negligible. But a lower impedance network in turn would require a driving tube which has low plate resistance. Such tubes either have low gain or high transconductance. Low gain tubes are not really suitable since we want a lot of amplification in the first stage. High transconductance tubes typically tend to oscillate without suitable counter measures, therefore they where ruled out as well. There is a series resistor to the cap to ground in the 75uS network. The purpose of this is to add the von Neuman time constant. This 4th timeconstant is not described in the RIAA standard. But it is used in the recording process. The reason is that the recording amplifier cannot increase the amplitude with rising frequencies endlessly, but stays flat beyond a certain frequency. The actual corner frequency was never really defined. It can be assumed to be around 50kHz. The series resistor stops the high frequency attenuation of the 150pF cap at a certain frequency. However this resistor cannot affect the roll off caused by the miller capacitance, which is a substantial part of the filter network. So it is debatable if it makes sense to have this resistor. I left it in, but did not do any further experimentation there. The second part of the filter network is less influenced from it's surrounding circuits. It can be made lower impedance due to the lower plate resistance of the 6N7. The two time constants can be simply achieved by a 100k resistor in series and a 32nF cap in series with a 10k resistor to ground. 100kOhm times 32nF equals 3200uS and 10k multiplied by 32nF equals 320uS. The cap can be formed by some standard values like 22nf parallel to 10nF. With some selection of the caps they can be brought closer to the ideal value of 31,8nF. Again the output impedance of the driving stage and grid to ground resistor of the output stage needs to be considered in the calculation of the series resistor. Best practice is to fine tune the RIAA network in circuit. First build up the circuit without the RIAA network, instead just place a 1:10 voltage divider after the second stage to avoid overload. Then scheck that the circuit works linear without the RIAA network. If it is ok, introduce the networks, possibly one by one and verify and fine tune their correct filter operation. The PSU is a classic full wave rectifier with a center tapped secondary, followed by a choke input filter. The heaters are all wired in parallel and also supplied by a choke input filtered DC supply. To minimize the impact of any residual ripple on the heater supply, it is biased slightly positive by a voltage divider across the B+. This voltage divider acts at the same time as a bleeder resistor. As mentioned above, in order to change the circuit into a full function preamp with line level inputs, the last two stages need to be changed to capacitor coupling. The second schematic shows how this is done: The schematic also illustrates the grounding approach. Each stage has it's local star ground. These ground points get connected with a heavy gauge solid copper wire. The connection between ground and the output is shown as a dotted line. This is an optional connection and can be done according to the system needs. In order to avoid ground loops it can be left open. Each channel is decoupled via it's own choke so the PSU can be shared for both channels. As volume control, I use Dave Slagle's AVCs. To minimze wiring, there is a PCB with relais. But of course it can also be done by routing each signal wire to the rotary switch. The control voltage for the relais is set for 6.3V so it can be conveniently supplied by the heater voltage. As a start, the preamp could also be used with a standard resistive potentiometer and upgraded to a transformer volume control later. If cost is an issue the preamp could also be built with an RC coupled output stage. Of course any number of line inputs can be implemented, shown are just 3. The PSU of the final version also uses the Graets bridge rectifier scheme which I introduced already in other articles: Since all the tubes in the preamplifier share the octal base, the name Octal preamplifier was created. Even the rectifiers are octal base types. In total the preamp has 8 tubes (1 6SL7, 1 6SN7, 2 6N7 and 4 6AX4). The preamplifier has been built several times in this form and works nicely. But as the suffix Mk1 in the title indicates, there is a new version on the way with some changes: The input and output stage both share one half of a double triode between the two channels. While this is not a real problem (channel separation is an overrated parameter) it can be easily changed. The simplest way would be to just use two each of the 6SL7 and 6SN7 and wire both triode systems in each bottle in parallel. The 75uS RIAA network would require some adaption if this is done and the line output transformer would need to be exchanged for a 18mA type. But since these two are very commonly used in the audio world, I'd also like to use different, less common tubes instead of them. For the line putput stage there is another reason for a change. The 6SN7 even working into a step down transformer provides a lot of gain of about 12dB. With most line sources having 2V RMS output, this is a bit on the high side. We want to have a sensible range on the volume control. So it will be substituted with another tube. The Mk2 version of this preamplifier will be presented in another article. Stay tuned! Best regards Thomas P.S.: I tried to explain all aspects of the circuit. If anything is left unclear, don't hesitate to ask.

Hi! Many tube amplifers have several output taps for various speaker impedances. Typically 4 and 8 Ohms sometimes also 16 Ohms. There is a lot of confusion about which is the right tap to use. With this article I try to shed a bit of light onto this subject. There is no technical standard which defines the parameters that would qualify an output as 4 or 8 Ohm. The difference between the two is the output impedance and matching to the output stage. Different amplifier topologies and philosophies will yield very different output impedances though. On the 8 Ohm output for example the output impedance can be anywhere between 1 and 4 Ohm for sensible designs. Even outside these limits. For the 4 Ohm tap it will be between 0.5 and 2 Ohms. As you can see there is some overlap in these ranges. So what one amplifier designer would label as a 4 Ohm output, another one would find suitable for 8 Ohm speakers only. So this can be understood more as a recommendation, not a hard rule which terminal is to be used with which speaker. Then look at the impedance of a typical loudspeaker. The nominal speaker impedance is only a rough approximation. The impedance typically varies widely over the frequency band. A 8 Ohm speaker for example might have an impedance close to 8 Ohm somewhere in the midband. In the bass region you will typically see resonance peaks at which the impedance can rise to 20-30 Ohms or even more. In the midband and treble there can be dips significantly below 8 Ohms. Dips to 5-6 Ohms are not uncommon. Only few speaker designers linearize the impedance. Again there are no standards which define how the nominal speaker impedance is derived from the impedance curve. So again this number is widely subject to interpretation and different speaker designers will declare different nominal impedances. Due to these two uncertainties in the impedance numbers, there is no need to slavishly connect 8 Ohm speakers to the 8 Ohm tap only. Experimenting makes sense. Connect your speakers to the tap which sounds best to you. Most often hooking a 8 Ohm speaker to the 4 Ohm tap can yield some improvement. Due to the lower output impedance of this tap, the impedance variation of the speaker will have less impact on the frequency response as with the higher output impedance, the latter can actually cause some coloration. The lower output impedance will also mean more 'control' of the amp over the speaker (better damping factor). However some speakers actually sound better with a smaller damping factor. The disadvantage of such mismatching will be that the maximum possible power output of the amp gets a bit reduced. But this is negligible in most cases. If you have a speaker with a bi-wiring terminal, or better yet, if you build your own speakers and have full control over the crossover, there are more ways to experiment with output taps. For transformers, the relationship between winding and impedance ratio follows a square law. Besides impedances the transformer also transforms voltage and current. The ratios of voltage and current correspond linearily to the winding ratio. What does this mean? At a tapped secondary, the voltage at the midpoint (center tap) will be half the voltage which is seen across the entire secondary. The reflected impedance however will be only one fourth. This means that in case you have 4, 8 and 16 Ohm output taps, the 4 Ohm tap is actually the center tap of the secondary winding of the output transformer. There is the same output impedance, output voltage and current across 0-4 and 4-16. Now if you have a speaker with separate terminals for low frequency and high frequency, you can utilize the full winding, even if the speaker is not rated 16 Ohms. For this to work properly, low and high frequency sections need to be completely isolated. This can be checked with a Ohm meter. There should be an open between both ground terminals. Now you can connect the low frequency section between 0 and 4 Ohm taps and the high frequency section between 4 and 16 Ohm. Polarity needs to be observed. See the illustration below, how this looks like. No more unused parts of the output winding, dangling in the air! You can even use the taps to attenuate the high frequency part if necessary. The output voltage at the 8 Ohm tap is about 3dB above the output voltage on the 4 Ohm tap. This is neglecting load and output impedance, depending on the actual output impedance and speaker impedance the difference will be more like 2dB or less. Calculation or measurement is necessary to get the exact difference. In most speakers the woofer has a lower efficiency compared to the rest. So tweeter and/or midrange need to be attenuated which is usually done by a resistor divider network. With a multi tapped output transformer this adaption of the levels can probably be done without resistors, if in a given system a certain combination gives the right attenuation. Below some possibilities how LF and HF part can be hooked up to achieve different levels of attenuation: The two alternatives in the first row provide 6dB and 3dB raw voltage difference (HF section attenuated). With real life impedances probably more like 4 and 2 dB. In the two examples in the bottom part, HF is attenuated by about 4.5 and 7,5dB (unloaded). So there are quite a few possibilities to adjust the level of the HF part through this technique. This leaves quite a lot of room for experimentation and exploitation of all the transformer taps which might otherwise be unused. The sonic result of this technique heavily depends on the amplifier, transformer and speaker used. Keep in mind that this is playing with mismatching. In some configurations it might not work well. Also the full output power of the amplifier will not be achieved with this. If the system has enough headroom it is worth playing with this though. In addition to purposely mismatching parts of the speaker to the amp to achieve the required attenuation, this technique can also be used to achieve correct matching, when woofer and tweeter have different impedances. For example if a 8 Ohm woofer and 4 Ohm tweeter are combined in a speaker, both can be hooked up to appropriate parts of the secondary winding, through their respective crossovers. If 4 Ohm woofers are used with 8 Ohm tweeters, the tweeter impedance can be adapted to the 4 Ohm with a parallel resistor. Since tweeters are typically more efficient, this does not hurt. The other way around it is more difficult, you would not want to bring a 8 Ohm woofer impedance down with parallel networks to match a 4 ohm tweeter. Creative usage of output taps could make this more easy. Best regards Thomas

Hi! As mentioned in the first part of this series, this amplifier concept can be scaled down for a low cost version. Although this will impact the sound quality the scaled down version will still perform very nicely. There are three areas which we can look at to reduce cost: capacitors, interstage transformer and power supply. The basic concept of such an amp has already been shown in the first part. Instead of an interstage transformer, the more common RC coupling can be used. The interstage transformer will be replaced by a plate load resistor and a coupling capacitor. A 50kOhm resistor will work nicely. This resistor will dissipate some power so it should have at least a 10W rating. Two 100k resistors can be used in parallel. If you want to use a single resistor, a 47kOhm will work just as well. The grid of the output tube will need a resistor to ground. The datasheet of the 6CB5A gives 500k max for this resistor. A coupling cap of 220nF or higher will provide a very low roll of frequency with this 500k. Obviously the quality of the coupling cap will have a large influence on the sound. A good MKP type is what should be used. Expensive 'boutique' caps should be avoided however. This is a cost down amp, so an expensive cap would not fit into the concept, we could rather stick with the interstage transformer. The voltage rating of the coupling cap should be chosen such that it can withstand the maximum voltage which can appear at start up or during fault conditions. It should be 450V or higher. Here is the detailed schematic of the 'low cost' amp with all parts values and approximate voltages: Both stages and both channels use a single common filter cap, which is shown in the PSU section further down. Since we are not applying the ultrapath concept in this version, the cathode resistor of the 6CB5A needs a bypass cap as shown. Also the driver stage uses a cathode bypass cap. Otherwise the gain would be lower. Gain is already reduced by about 3dB compared to the transformer coupled version, so let's not reduce it further by omitting the bypass cap. We also want to keep the output impedance of the driver as low as possible. All other parts remain the same as for the transformer coupled version. The power supply also offers some possibilities to reduce cost. A capacitor input filter will simplify the PSU since it requires less secondary voltage from the transformer. But we want to keep at least one choke in the supply. Going to a solid state diode bridge for the rectification will reduce complexity of the power transformer further, since it only needs one heater winding and the HV voltage does not need to be center tapped: Just two windings on the power transformer, a Single 350V winding which should have at least a 300mA rating. As mentioned above, the capacitors can be electrolytics. They need to be of sufficient voltage rating of 500V minimum. We need the voltage rating to be above the B+ level since the voltage will rise somewhat in case the PSU is not loaded. not as much as a choke input filter though. Since 500V electrolytics are difficult to find, a series combination of 2 250V or 350V capacitors will do. If you use series combinations for the caps, each needs a resistor in parallel to equalize the voltages as shown in the schematic. These should be 2W types or higher. They serve at the same time as bleeder resistors which ensure a save discharge of the PSU when turned off. If single capacitors are used rather than series combinations, a separate bleeder resistor should be installed. It is not important to have the exact capacitor values as shown in the schematic. Anything which is in the same range will do. You might want to experiment with film bypass caps across the electrolytics. I myself am not fond of bypassing. It can create more colorations than advantages. But that is another topic. You can experiment as you wish. But keep in mind this is a scaled down version, so probably a better path for upgrading will be to change to interstage transformer and oil caps. Of course any mixed form between this RC coupled and the transformer coupled version can be built. As shown the cost of this version is about 50%. For example you can start with the RC coupled driver but immediately apply the oil caps and ultrapath concept in the output stage. This way less changes will be necessary when the amp gets upgraded later on. Actually by far the most 6CB5A amps I know of have been built in the transformer coupled version. In fact I got many questions about how the concept can be improved rather than scaled down. In the coming installments of this series I will show some beefed up versions with Tango transformers and a yet improved power supply. Best regards Thomas

Hi! After the introduction of the 6BY5 tube in the previous post, I will write about a power supply that uses it in this article. As mentioned in the 6BY5 tube of the month post, two of them will be very well usable in a bridge rectifier configuration for preamplifier supplies. I'm using them this way in the power supplies for the phono gain stages of the modular preamplifier. In this modular preamplifier, the phonostage is split up into two separate gain stages (input and output stage), separate LCR RIAA module and MC transformer. Each gain stage has it's own separate power supply for maximum isolation and flexibility. The gain stages will use the E55L which was presented in a Tube of the Month article in last November. They will be run at around 150V B+ and about 35mA. Taking some voltage drop into account in the output teransformer primary and local LC decoupling which will be in the gain section, a voltage of approximately 175V is needed. The schematic shows the high voltage part. The two 6BY5s are wired as a bridge, both heaters fed from the same winding and referenced to ground. About 300V AC are needed from the transformer to get the 175V out with the voltage drop in the chokes and rectifier. Final voltage will be determined once the PSU is tested under load. The transformer allows adjustment of the voltage. Heaters will be supplied by DC, rectified with Schottky diodes and powered by a separate transformer. A choke input filter is used for low switching noise. About 12Vs AC are needed to get 6,3V out with the losses in the rectifiers and filter. Again the final voltage will have to be trimmed when tested under load. The 100mA B+ transformer and low voltage transformer for DC filaments are used from my own range of custom wound power transformers. Details about these transformers can be found here. All elements of the modular preamp will use the same chassis style as shown in the posts about passive line stage and MC transformer. The circuit is divided into sub modules which are premounted on metal plates: All these plates are assembled in sandwich fashion: The complete assembly slides into the wooden enclosure: There are holes on the top and bottom of the enclosure to allow airflow around the tubes. A rotary on/off switch was chosen so that the same knobs can be used as on the passive line stage Usually the glow of the tubes are good enough for me to work as on/off indicator. Since they are mounted inside, a LED was added to the front: The PSUs for phono input and phono output stages are identical. Assembly of the gain stages themselves will be shown in the next post. Stay tuned! Best regards Thomas

Hi! Many tube amplifers have several output taps for various speaker impedances. Typically 4 and 8 Ohms sometimes also 16 Ohms. There is a lot of confusion about which is the right tap to use. With this article I try to shed a bit of light onto this subject. There is no technical standard which defines the parameters that would qualify an output as 4 or 8 Ohm. The difference between the two is the output impedance and matching to the output stage. Different amplifier topologies and philosophies will yield very different output impedances though. On the 8 Ohm output for example the output impedance can be anywhere between 1 and 4 Ohm for sensible designs. Even outside these limits. For the 4 Ohm tap it will be between 0.5 and 2 Ohms. As you can see there is some overlap in these ranges. So what one amplifier designer would label as a 4 Ohm output, another one would find suitable for 8 Ohm speakers only. So this can be understood more as a recommendation, not a hard rule which terminal is to be used with which speaker. Then look at the impedance of a typical loudspeaker. The nominal speaker impedance is only a rough approximation. The impedance typically varies widely over the frequency band. A 8 Ohm speaker for example might have an impedance close to 8 Ohm somewhere in the midband. In the bass region you will typically see resonance peaks at which the impedance can rise to 20-30 Ohms or even more. In the midband and treble there can be dips significantly below 8 Ohms. Dips to 5-6 Ohms are not uncommon. Only few speaker designers linearize the impedance. Again there are no standards which define how the nominal speaker impedance is derived from the impedance curve. So again this number is widely subject to interpretation and different speaker designers will declare different nominal impedances. Due to these two uncertainties in the impedance numbers, there is no need to slavishly connect 8 Ohm speakers to the 8 Ohm tap only. Experimenting makes sense. Connect your speakers to the tap which sounds best to you. Most often hooking a 8 Ohm speaker to the 4 Ohm tap can yield some improvement. Due to the lower output impedance of this tap, the impedance variation of the speaker will have less impact on the frequency response as with the higher output impedance, the latter can actually cause some coloration. The lower output impedance will also mean more 'control' of the amp over the speaker (better damping factor). However some speakers actually sound better with a smaller damping factor. The disadvantage of such mismatching will be that the maximum possible power output of the amp gets a bit reduced. But this is negligible in most cases. If you have a speaker with a bi-wiring terminal, or better yet, if you build your own speakers and have full control over the crossover, there are more ways to experiment with output taps. For transformers, the relationship between winding and impedance ratio follows a square law. Besides impedances the transformer also transforms voltage and current. The ratios of voltage and current correspond linearily to the winding ratio. What does this mean? At a tapped secondary, the voltage at the midpoint (center tap) will be half the voltage which is seen across the entire secondary. The reflected impedance however will be only one fourth. This means that in case you have 4, 8 and 16 Ohm output taps, the 4 Ohm tap is actually the center tap of the secondary winding of the output transformer. There is the same output impedance, output voltage and current across 0-4 and 4-16. Now if you have a speaker with separate terminals for low frequency and high frequency, you can utilize the full winding, even if the speaker is not rated 16 Ohms. For this to work properly, low and high frequency sections need to be completely isolated. This can be checked with a Ohm meter. There should be an open between both ground terminals. Now you can connect the low frequency section between 0 and 4 Ohm taps and the high frequency section between 4 and 16 Ohm. Polarity needs to be observed. See the illustration below, how this looks like. No more unused parts of the output winding, dangling in the air! You can even use the taps to attenuate the high frequency part if necessary. The output voltage at the 8 Ohm tap is about 3dB above the output voltage on the 4 Ohm tap. This is neglecting load and output impedance, depending on the actual output impedance and speaker impedance the difference will be more like 2dB or less. Calculation or measurement is necessary to get the exact difference. In most speakers the woofer has a lower efficiency compared to the rest. So tweeter and/or midrange need to be attenuated which is usually done by a resistor divider network. With a multi tapped output transformer this adaption of the levels can probably be done without resistors, if in a given system a certain combination gives the right attenuation. Below some possibilities how LF and HF part can be hooked up to achieve different levels of attenuation: The two alternatives in the first row provide 6dB and 3dB raw voltage difference (HF section attenuated). With real life impedances probably more like 4 and 2 dB. In the two examples in the bottom part, HF is attenuated by about 4.5 and 7,5dB (unloaded). So there are quite a few possibilities to adjust the level of the HF part through this technique. This leaves quite a lot of room for experimentation and exploitation of all the transformer taps which might otherwise be unused. The sonic result of this technique heavily depends on the amplifier, transformer and speaker used. Keep in mind that this is playing with mismatching. In some configurations it might not work well. Also the full output power of the amplifier will not be achieved with this. If the system has enough headroom it is worth playing with this though. In addition to purposely mismatching parts of the speaker to the amp to achieve the required attenuation, this technique can also be used to achieve correct matching, when woofer and tweeter have different impedances. For example if a 8 Ohm woofer and 4 Ohm tweeter are combined in a speaker, both can be hooked up to appropriate parts of the secondary winding, through their respective crossovers. If 4 Ohm woofers are used with 8 Ohm tweeters, the tweeter impedance can be adapted to the 4 Ohm with a parallel resistor. Since tweeters are typically more efficient, this does not hurt. The other way around it is more difficult, you would not want to bring a 8 Ohm woofer impedance down with parallel networks to match a 4 ohm tweeter. Creative usage of output taps could make this more easy. Best regards Thomas

Hi! The schematic of the signal section of the Octal phono stage was already shown in part 1. What is still missing is the power supply. In this post I will show two possible variants. The first one is quite simple without any chokes and only a single power transformer. This PSU uses a hybrid full wave rectifier with a 6BY5 and two UF4007 silicon diodes. The smoothing of the B+ voltage is done through a couple RC filter segments using electrolytic caps. I used my smallest power transformer which only has a single 6.3V heater winding. So this had to be used for the rectifier and signal tubes. The rectifier is hooked up directly to the heater winding, while the heater voltages for the signal tubes are rectified by a bridge of Schottky diodes. A small dropping resistor brings the voltage to the desired 6.3V. Smoothing is done by a bank of 4 10.000uF electrolytic caps. Not the most elegant solution. Better would be to have separate heater windings. But it works like this. I wanted to use the smallest power transformer which I have so that the whole phone stage fits into a single wooden chassis as used with my 6AH4 line stage. All the resistors in this circuit should be at least 5W or higher rated. The 100K/10K voltage divider at the end of the B+ filter chain serves as bleeder resistor and provides a positive bias voltage to elevate the heater potential. The supply was used like this in the first prototype build and works nicely without hum, even with the power transformer in the same chassis. The first user of this phonostage however asked if he can have a chassis style which has the tubes visibly exposed on the top. Something in the style of the recently built D3a phono stage. With only the tubes on the top of the chassis, this would be a bit too empty. So it would need something else to fill the surface. This would be possible by bringing the power transformer on the top as well and by adding some chokes. So I designed another power supply which is a bit more elaborate: Separate transformers for the B+ and heater voltages of the signal tubes. Both supplies with choke input filter. The assembly of this version will be covered in part 4. Stay tuned. Best regards Thomas

Hi! As promised in the first article about the Octal Line Preamplifier I will present the power supply schematic today. Similar to the Octal Phono stage, there are two possibilities. One using a choke and a very simple one using only resistors and capacitors for the smoothing. Both power supply options are very similar to the power supplies of the Octal phono stage. They use the 6BY5 as rectifier tube. The 6GL7 can be AC heated. Since we are dealing with much higher signal levels as in a phono stage, no DC heating is necessary and still the preamp is hum free. This is the schematic of the version with choke input filter as is used in the first version which I built: Very straight forward, a hybrid rectifier bridge with four UF4007 diodes augmenting the 6BY5. I used two of them in series in each leg (4 total) to increase peak inverse voltage capability. With choke input filters some very high peak inverse voltages can occur at turn on. The rectifier feeds the input choke, a 40Hy unit, followed by a 47uF electrolytic cap. Two RC sections with 500 Ohms and 47uF each provide sufficient filtering for hum free operation. The 100k resistor serves as bleeder resistor. Together with the two other 100k bleeders, one in each channel of the signal section, critical current is always guaranteed even if the signal tubes are unplugged. This avoids a steep voltage rise which could otherwise occur in case the tubes are unplugged and the PSU is turned on. The rectifier tube and the signal tubes are heated from separate windings. The 6GL7 heaters are referenced to ground via the two 100 Ohm resistors. The 6AX5, which I presented last month, could be used as an alternative to the 6BY5. If you want to save the cost of the choke, use this simpler PSU: The same rectification scheme, but only RC sections are used for smoothing. This version needs a lower secondary voltage to get the desired B+. Also only two UF4007 are needed. In this schematic signal tubes and rectifier share the same heater winding. Therefor the heater winding is elevated to a positive voltage by connecting one side to a tap of a voltage divider between B+ and ground. This is not very elegant and separate heater windings would be preferable. But I used such a scheme already with the Octal phono stage and it works fine. This second option is not tested, so some minor adaptions might be necessary if you build it. Here a view of the backside of the preamp showing the rectifier and the connectors: Best regards Thomas

Hi! The schematic of the signal section of the Octal phono stage was already shown in part 1. What is still missing is the power supply. In this post I will show two possible variants. The first one is quite simple without any chokes and only a single power transformer. This PSU uses a hybrid full wave rectifier with a 6BY5 and two UF4007 silicon diodes. The smoothing of the B+ voltage is done through a couple RC filter segments using electrolytic caps. I used my smallest power transformer which only has a single 6.3V heater winding. So this had to be used for the rectifier and signal tubes. The rectifier is hooked up directly to the heater winding, while the heater voltages for the signal tubes are rectified by a bridge of Schottky diodes. A small dropping resistor brings the voltage to the desired 6.3V. Smoothing is done by a bank of 4 10.000uF electrolytic caps. Not the most elegant solution. Better would be to have separate heater windings. But it works like this. I wanted to use the smallest power transformer which I have so that the whole phone stage fits into a single wooden chassis as used with my 6AH4 line stage. All the resistors in this circuit should be at least 5W or higher rated. The 100K/10K voltage divider at the end of the B+ filter chain serves as bleeder resistor and provides a positive bias voltage to elevate the heater potential. The supply was used like this in the first prototype build and works nicely without hum, even with the power transformer in the same chassis. The first user of this phonostage however asked if he can have a chassis style which has the tubes visibly exposed on the top. Something in the style of the recently built D3a phono stage. With only the tubes on the top of the chassis, this would be a bit too empty. So it would need something else to fill the surface. This would be possible by bringing the power transformer on the top as well and by adding some chokes. So I designed another power supply which is a bit more elaborate: Separate transformers for the B+ and heater voltages of the signal tubes. Both supplies with choke input filter. The assembly of this version will be covered in part 4. Stay tuned. Best regards Thomas

Hi! I often hear people complaining about tube prices and how much better it has been in the old days when prices had been more affordable. I wholeheartedly disagree with this point of view. In fact I think we are in the best possible times when it comes to tube audio. While it is true that the prices of some overly hyped tube types skyrocketed, there are still a lot of dirt cheap tubes which are available in abundance as NOS. Thanks to the internet it is easy to find suppliers for such tubes and data sheets of even the most obscure types are just a few clicks away. I remember when I started to get into tube audio that it took me almost a year just to get the most essential tube data books. Also the availability of other parts is gerat nowadays. Audio transformers are available in a larger variety than ever. And there are new tubes still being made with the Elrog 300B being the latest introduction. If you think some tube types are expensive, compare the prices of them from 40, 50 or even more years ago with inflation in mind. Suddenly even many of the highly sought after tubes appear not that expensive compared to the old days. But as mentioned above, no need to hunt for those mainstream tubes if you are on a budget. I already introduced the 6CB5A as a cheap alternative to the 300B and many amplifiers have been build with this tube. In the Tube of the month post about the 6GE5 I suggested that it could be used triode connected in 2A3 designs. In the November Tube of the Month post I presented the triode section of the 6AG9 as a possible driver for a triode strapped 6GE5 in single ended mode. All that was missing is a suitable circuit. In order to evaluate the sound potential of these tubes I decided to not use the cheapest possible design but something a bit more advanced, with oil caps and interstage transformer coupling. Once the tubes have been proven to work as expected, simpler circuits can be explored as well as more elaborate ones. In order to minimize variables, I decided to adapt a proven circuit which is known to sound good. So it will be just the tubes which changed. The circuit which I usually use with the 6CB5A could be adapted with minor changes. The same circuit description as in the article about the single ended amplifier concept applies, so I am not going to repeat it. The voltages are adapted to the 6GE5 and 6AG9. The output tube runs at about 50-55mA with 300V from plate to cathode, so about 15W plate dissipation. The driver operates at about 200V and 6mA. The power supply is basically the same as described in an earlier Making of a 6CB5A amp post. Read it for a description of the supply. Just for the fun of it I decided to build two cute little mono blocks. So currents, choke and transformer ratings have been adapted accordingly. To stay consistent with the 12-pin compactron theme 6BE3 TV dampers are used as rectifiers. The construction of the amps will be shown in the next part. Stay tuned! I am making parts kits available for this design, both for mono and stereo versions. Best regards Thomas

This is Thomas Mayer's blog about vacuum tube audio, to share updates about new amplifiers and preamplifiers and ELROG vacuum tubes.

Hi! In the previous article the 6AH4 linestage was mentioned, which is meant to be used with the 6CB5A amp. As with the power amp, I will describe circuit and assembly process of the line stage in two separate articles. The linestage shares the same circuit with the two chassis version which was already presented. Only this time power supply and preamp will be in one housing which will be in the same style as the power amp it is supposed to drive. The circuit is basically the linestage section of my Octal preamplifier Mk2. The only difference is in the heater supply, which can be AC since this is a linelevel preamp only. The picture below shows the complete schematic with both channels and the power supply: The TVCs are done with 24*2 db/step autoformer modules from intactaudio. Volume control and input select switches are swiss made from Elma. All capacitors are ASC MP in oil types. No electrolytics. The only amplifying element is a 6AH4GT triode per channel. This is a low mu triode with low plate resistance. With the 3.5:1 step down this yields a very low output impedance, more than suitable to drive a TVC and long cables to the power amp. As in the 6CB5 amp, a hybrid rectifier bridge was chosen. UF4007 diodes complement the 6AX4 TV dampers. Two diodes are in series in each leg, since substantial peak inverse voltages can occur in a choke input filter supply. The power transformer is of the same type as the one used in the power amp. Only a smaller version since the preamp has much less current draw. The power transformer has grain oriented core laminations and dual electrostratic screens between primary and secondaries. In addition there is a flux band around the outside of the winding stack. The output is basically floating to allow differential connection to the power amp. One side is connected to ground through a 1M resistor to avoid static charge build up in the output which could cause a plop when plugged in while power amp is turned on. The heater supply of the 6AH4 is referenced to ground through 2 100 Ohm resistors. The preamp has the same ground lift option as the power amp to allow adjustment to any grounding situation in the system. The construction will be done on the same style metal plates as for the power amp: Stay tuned for the second part which will show the assembly process. Best regards Thomas

Hi! In 2011 I already introduced a low cost version of the 6CB5A amplifier concept in the article single ended amplifier concept part 4. I occasionally get complaints that most of the designs which I present and offer as kit are too expensive. Therefore I would like to give this low cost amplifier design more attention. I recently finished mono blocks based on this circuit. I will outline the schematic and building process of these amps in detail in this and the upcoming articles. Why mono blocks? You might wonder since most of my amps are stereo builds and I am a proponent of stereo amps rather than mono blocks. Two reasons: I had two small wooden chassis on hand which are just big enough for a mono block, but not a stereo amplifier. And secondly, mono blocks seem to be very fashionable, although there is no real advantage to a mono design over a well implemented stereo amplifier. So I follow the taste of the crowd on this one, although it doesn't make a lot of sense, especially for a low cost object. In order to keep cost as low as possible, certain compromises are necessary. An obvious one is the interstage transformer which has to go. A RC coupled driver stage will do the job. Also no expensive oil caps. Electrolytics will fit into the restricted space and are the lowest cost option. This probably is the biggest compromise. Also the PSU needs to be a bit simpler, no external PSU. The schematic shows the whole circuit including power supply. Unmarked resistors can be 0.5W types. Approximate voltages are given for the most critical nodes. Let's go through the schematic step by step, starting at the input. The resistor to ground after the input defines the input impedance of the amp which will be 100kOhms in this case. It also serves as grid to ground resistor for the 6N7 input tube, to ensure the grid is at ground potential. The 6N7 contains two triodes with a common cathode. The grids and plates of the two sections are tied together. The input tube is cathode biased with a 1k resistor to ground. This will set the bias approximately at -4.5V and will result in 4.5mA current through the tube. The cathode resistor is bypassed with a 100uF electrolytic. Gain of the amp is on the low side, so we need to get all the gain from the 6N7 we can, therefore we need that bypass cap. The tube has a 47k plate load resistor. At minimum a 10W type should be used. I use 20W Dale aluminium resistors there. The plate of the 6N7 will settle at about half the B+ voltage. The driver is coupled through a 100nF capacitor to the output tube grid. I used a 1000V MKP foil there. At minimum a 450V should be used. The output stage is also cathode biased with a 1k resistor. Since we do not use the ultrapath approach as in my usual designs, this needs to be bypassed with a 100uF cap. The cathode resistor dissipates quite some power, so a 20W type is recommended. Again an aluminum Dale resistor does it for me. The 6CB5A needs a grid to ground resistor of max 500k for save operation. Since I didn't have 500k at hand, I wired two 1M resistors in parallel. The screen grid is connected to the plate through a 100 Ohm resistor which should be placed close to the screen grid pin. The 6CB5A works well with plate loads of 3.5k upwards. I prefer amps with a decent damping factor to make them usable with a wider range of speakers so I recommend 5k. Lundahl has some quite reasonably priced output transformers which fit the bill. LL1663/70mA would work well. For this particular build I chose the LL1682/70mA for an even lower output impedance. It is a 5.5k:5 transformer. This way the amp can be used with either 4 or 8 Ohm speakers. Let's have a look at the power supply section. I gave up my favorite choke input supply concept since this usually needs at least two chokes to get the ripple down. I didn't want to give up chokes all together, so a single choke stayed in there. A Lundahl LL1673 20Hy/100mA choke or anything similar will fit the bill. No compromises in the power transformer for me. So I picked a medium sized type from my power transformer range. It has a secondary which can be configured from 100 to 600V in 50V steps at 200mA. There are two 6.3V/5A heater windings. One of the heater windings is used for the 6N7 and 6CB5A. This winding is referenced to ground through the two 100 Ohm/5W resistors. Since there is another heater winding, a Tube rectifier can be used. I chose the 6BY5 which will be augmented by two UF4007 silicon diodes. This creates a hybrid rectifier bridge which maintains most of the advantages of the tube rectifier. The secondary is configured for 350V. This yields about 425V DC after the CLC filter. Since there is considerable ripple across the first capacitor after the rectifier, two 22uF/450V electrolytics are wired in series. Two 100k/5W resistors in parallel to these ensure that the voltage divides equally between the two capacitors. At the same time they act as bleeder resistors. A big capacitance is needed after the choke since it supplies both the driver and output stage and it needs to reduce the ripple to an acceptable level. A minimum of 100uF/450V is needed. More will not hurt I would recommend 100-200uF. This circuit can be adapted for stereo with a single common supply. The supply sections needs to be doubled in current capacity and I would recommend a minimum of 200uF B+ smoothing capacitance after the choke. Although a bigger power transformer would be about 50% more expensive, only one of them is needed in a stereo amplifier and only one choke. On top of that only a single chassis. The smart cheap skate would go for a stereo amp. Stay tuned for the next parts of this series which will outline the construction in detail. Best regards Thomas

Hi! The circuit and assembly of the UX 201A Sound Processor have been covered in previous posts, this and the next article in this series will be about the power supply. The power supply only has to deliver a single voltage of about 150VDC. But at a hefty current of 0.5A. So we need a power supply with some grunt. If rectification should be done with tubes, the average rectifier will not do. We need something tough which can handle this amount of current. Even among TV dampers, the choice is limited for such a high current. But there are some which can handle this job. For example the 6CG3. I have chosen a full wave bridge rectification scheme for the task. This is the circuit: Quite straight forward, no surprises here. Since the voltages in this power supply are rather low and the 6CG3 allows quite large voltage difference between heater an cathode all heaters could be wired in parallel and fed from a single winding, referenced to ground. But the 6CG3 draws a lot of heater current. 4 of them need almost 8A. Since the power transformer which I used has two separate 6.3V heater windings, I split the 6CG3s into two pairs which are heated from one winding. The two tubes in the middle deliver the raw B+ voltagesat their cathodes. These share one of the heater windings, which is connected to raw B+ at one end. The two other 6CG3s are fed from the other heater winding which is referenced to ground. The rectifier is followed by a choke input filter, using two LC sections. The chokes are Lundahl LL1638 gapped for 3Hy/500mA. The caps are 220uF/350V electrolytics. A 350V rating is used so that the capacitors will not get stressed with over voltage in case the power supply is turned on without the signal section attached. In that case the output voltage will rise significantly since the current draw would drop below the critical value to ensure proper choke input operation of the filter. The 20k bleeder resistor ensures that the high voltage gets drained after turn off if no load is attached. No worries about the use of electrolytics. Each channel is decoupled by a choke in the signal section. So far so good, but such beautiful tubes as the UX201A deserve some similarly awesome rectifier tubes. Here is an alternative PSU circuit using the 866A mercury vapour rectifier: Two of the 6CG3s are replaced by 866As. Since they require a different heater voltage a separate filament transformer is added which provides 2.5V. Each 866A needs 5A filament current. So it might be easier to use two separate heater windings which can provide 5A each. If a 10A filament transformer is available they can be heated in parallel as shown in the schematic. The raw B+ is derived from the center tap of the 866A filament winding. If two separate filament windings are used, their center taps need to be connected. The filter section remains the same. Due to the lower voltage drop of the 866As, the secondary voltage needs to be a bit lower. Two 6CG3s remain in the circuit in the ground path to complete the full wave bridge. They also provide a delayed high voltage to allow pre heating of the 866As without a separate switch. This has a cool effect when turning the supply on. At turn on there is no blue glow. As the 6CG3 heat up, a faint blue glow starts to appear between filament and plate of the 866As which slowly gets brighter as the TV dampers fully warm up. The assembly of the PSU will be shown next, stay tuned. Best regards Thomas

Hi! All the previous posts in this series showed indirectly heated tubes in the output stage. Today we will see how the same concept can be applied to directly heated tubes as well: This is a generic schematic which shows how this can be adapted to DHTs like 45, 2A3, 300B, etc. The driver stage can stay the same as in the 6CB5A version. The main point which is different is the cathode of the output tube which is now directly heated, that means the filament itself becomes the cathode and thus is in the signal path. So it needs to be handled with some care. DHTs can be heated with AC or with DC. The schematic shows AC heating which is the simplest form and can easily be done such that the filament supply has no negative impact on the sound. This is much more difficult in the case of DC heating and will be covered in a later post. AC heated DHTs have one draw back: There will be some remaining hum. Often hum bucking pots are seen in these cases. However we want to avoid such an ugly pot right in the signal path. This requires a filament transformer which has center tapped windings. This center tap becomes the cathode connection of the tube and is hooked up to the cathode bias resistor. It is also the connection point for the ultrapath capacitor (C2). This method does not allow any hum adjustment and relies on symmetry of the filament winding and the filament itself. Hum level can vary a bit from tube to tube. Very well suited for this scheme are the 2.5V filament types like the 45 or 2A3. Residual hum will be negligible with these. With 300Bs, hum level could become too much with this scheme as they are heated with 5V. I personally prefer DC heating for 300Bs for this reason. With 45s and 2A3s I stick to the AC scheme as pictured. As mentioned already, the filament supply is very critical. Any noise present here will be injected into the signal. For this reason the filament transformer should be of high quality and have a screen between primary and secondary sides. As usual I even use transformers with two screens. The filaments should be supplied from a separate transformer. The filament windings should not be on the same core as the B+ windings. This avoids any switching noise from the B+ rectifier to be injected into the filament circuit. In a stereo amp, both windings for the two channels can be on the same transformer. Even if the power supply of the amp is separated into an external chassis, the filament transformer for the output tubes should stay in the amplifier section, close to the output tubes. The rest of the schematic is the same as shown in previous posts. The interstage transformer needs to fit to the driver tube. Again the 6N7 works nicely with a LL1660/10mA driving small DHTs like 45 or 2A3. For the 300B it is advisable to use the beefier 6J5 instead of the 6N7. The output transformer also needs to be matched to the tube used. For example a Tango XE20S, configured for 5k primary for the 45. Also the Lundahl LL1663 would work nicely. C1 and C2 are the ultrapath capacitors as has been described in previous posts. C3 is the B+ decoupling cap of the driver stage. C4 decouples the output stage separately for each channel. R1 is the grid to ground resistor of the input tube and also sets the input impedance of the amp. Values of 100kOhm or 200kOhm are suitable. R2 is the cathode resistor of the input tube which sets it's operating point. In case of the 6N7, 1kOhm is a good value. A 1W rating is sufficient here. R3 is the cathode bias resistor of the output tube. In case of the 45 1.5kOhm/5W would be suitable. It needs to be chosen correctly for other DHTs. In both cases trials with and without cathode bypass capacitors can be made. Depending on the plate resistance of the tubes and the size of the ultrapath cap, a bypass cap can be necessary to avoid early low frequency roll off. A good starting point would be 25-30uF for all the caps shown in the schematic. The choke decouples the two channels from each other and from the power supply which can be common to both. The current rating of the choke needs to be sufficient, depending on the current draw of the tubes. R5 decouples the driver B+ from the output tube. It needs to be set according to tube choice. R4 forms a voltage divider with R5 and also acts as a bleeder resistor. B+ and the value of these resistors depends on the voltage requirement of the tube chosen. For example a 45 would require around 300V B+. Of course the same RC coupled driver stage which was shown as a lower cost alternative in an earlier post, can be used with a DHT as well. The same type power supply as shown before can be used. The voltages need to be adapted accordingly. The photo above shows a 45 amp with external PSU based on this concept. The amp is equipped with globe UX245 tubes. The next photo shows another implementation. This one can accept both 45 or 2A3s as output tubes. The correct operating point for each is chosen by a switch which seloects the appropriate cathode resistance: In future installments of this series we will see how this concept looks like with DC heated filaments and how this can be modified for an all directly heated amp with tubes like 26 or 10Y in the driver stage. Best regards Thomas

Hi! In the previous article the 6AH4 linestage was mentioned, which is meant to be used with the 6CB5A amp. As with the power amp, I will describe circuit and assembly process of the line stage in two separate articles. The linestage shares the same circuit with the two chassis version which was already presented. Only this time power supply and preamp will be in one housing which will be in the same style as the power amp it is supposed to drive. The circuit is basically the linestage section of my Octal preamplifier Mk2. The only difference is in the heater supply, which can be AC since this is a linelevel preamp only. The picture below shows the complete schematic with both channels and the power supply: The TVCs are done with 24*2 db/step autoformer modules from intactaudio. Volume control and input select switches are swiss made from Elma. All capacitors are ASC MP in oil types. No electrolytics. The only amplifying element is a 6AH4GT triode per channel. This is a low mu triode with low plate resistance. With the 3.5:1 step down this yields a very low output impedance, more than suitable to drive a TVC and long cables to the power amp. As in the 6CB5 amp, a hybrid rectifier bridge was chosen. UF4007 diodes complement the 6AX4 TV dampers. Two diodes are in series in each leg, since substantial peak inverse voltages can occur in a choke input filter supply. The power transformer is of the same type as the one used in the power amp. Only a smaller version since the preamp has much less current draw. The power transformer has grain oriented core laminations and dual electrostratic screens between primary and secondaries. In addition there is a flux band around the outside of the winding stack. The output is basically floating to allow differential connection to the power amp. One side is connected to ground through a 1M resistor to avoid static charge build up in the output which could cause a plop when plugged in while power amp is turned on. The heater supply of the 6AH4 is referenced to ground through 2 100 Ohm resistors. The preamp has the same ground lift option as the power amp to allow adjustment to any grounding situation in the system. The construction will be done on the same style metal plates as for the power amp: Stay tuned for the second part which will show the assembly process. Best regards Thomas

Hi! As mentioned in the first part of this series, this amplifier concept can be scaled down for a low cost version. Although this will impact the sound quality the scaled down version will still perform very nicely. There are three areas which we can look at to reduce cost: capacitors, interstage transformer and power supply. The basic concept of such an amp has already been shown in the first part. Instead of an interstage transformer, the more common RC coupling can be used. The interstage transformer will be replaced by a plate load resistor and a coupling capacitor. A 50kOhm resistor will work nicely. This resistor will dissipate some power so it should have at least a 10W rating. Two 100k resistors can be used in parallel. If you want to use a single resistor, a 47kOhm will work just as well. The grid of the output tube will need a resistor to ground. The datasheet of the 6CB5A gives 500k max for this resistor. A coupling cap of 220nF or higher will provide a very low roll of frequency with this 500k. Obviously the quality of the coupling cap will have a large influence on the sound. A good MKP type is what should be used. Expensive 'boutique' caps should be avoided however. This is a cost down amp, so an expensive cap would not fit into the concept, we could rather stick with the interstage transformer. The voltage rating of the coupling cap should be chosen such that it can withstand the maximum voltage which can appear at start up or during fault conditions. It should be 450V or higher. Here is the detailed schematic of the 'low cost' amp with all parts values and approximate voltages: Both stages and both channels use a single common filter cap, which is shown in the PSU section further down. Since we are not applying the ultrapath concept in this version, the cathode resistor of the 6CB5A needs a bypass cap as shown. Also the driver stage uses a cathode bypass cap. Otherwise the gain would be lower. Gain is already reduced by about 3dB compared to the transformer coupled version, so let's not reduce it further by omitting the bypass cap. We also want to keep the output impedance of the driver as low as possible. All other parts remain the same as for the transformer coupled version. The power supply also offers some possibilities to reduce cost. A capacitor input filter will simplify the PSU since it requires less secondary voltage from the transformer. But we want to keep at least one choke in the supply. Going to a solid state diode bridge for the rectification will reduce complexity of the power transformer further, since it only needs one heater winding and the HV voltage does not need to be center tapped: Just two windings on the power transformer, a Single 350V winding which should have at least a 300mA rating. As mentioned above, the capacitors can be electrolytics. They need to be of sufficient voltage rating of 500V minimum. We need the voltage rating to be above the B+ level since the voltage will rise somewhat in case the PSU is not loaded. not as much as a choke input filter though. Since 500V electrolytics are difficult to find, a series combination of 2 250V or 350V capacitors will do. If you use series combinations for the caps, each needs a resistor in parallel to equalize the voltages as shown in the schematic. These should be 2W types or higher. They serve at the same time as bleeder resistors which ensure a save discharge of the PSU when turned off. If single capacitors are used rather than series combinations, a separate bleeder resistor should be installed. It is not important to have the exact capacitor values as shown in the schematic. Anything which is in the same range will do. You might want to experiment with film bypass caps across the electrolytics. I myself am not fond of bypassing. It can create more colorations than advantages. But that is another topic. You can experiment as you wish. But keep in mind this is a scaled down version, so probably a better path for upgrading will be to change to interstage transformer and oil caps. Of course any mixed form between this RC coupled and the transformer coupled version can be built. As shown the cost of this version is about 50%. For example you can start with the RC coupled driver but immediately apply the oil caps and ultrapath concept in the output stage. This way less changes will be necessary when the amp gets upgraded later on. Actually by far the most 6CB5A amps I know of have been built in the transformer coupled version. In fact I got many questions about how the concept can be improved rather than scaled down. In the coming installments of this series I will show some beefed up versions with Tango transformers and a yet improved power supply. Best regards Thomas

Hi! As mentioned in my previous post, I finished development of a new entry level line preamplifier to match the Octal Phono Preamplifier. In this post I will present the circuit of the signal section. For many people the cost of a preamplifier with transformer volume controls is too high. So I came up with this rather classic and simple circuit, which keeps the parts cost down. This means it uses a regular resistive potentiometer for volume control and no transformer coupling. The signal is routed from the inputs through a source selector switch and from there into the volume control. In order to keep the input impedance high, a 100k pot is used. From there a single triode section provides some amplification and low output resistance. The goal was to avoid feedback circuits, cathode followers and the like, yet the output impedance should be well below 1k. This is the circuit: I chose the 6GL7 tube for amplification. This is a dissimilar duo triode with a high mu and a low mu system in one bottle. In this preamp only the low mu system is operational and the other section is left unused. Grid, cathode and plate of the unused system are tied to ground. The reason why this tube was chosen is because of the properties of the low mu section. It has a mu of 5 which will result in an amplification of about 4 (12dB) when RC coupled. This is not too high, so the volume control can be used in a sensible region, yet provides some gain which is needed to bring the output of the Octal Phono Preamp to suitable levels to drive a power amp. The plate resistance of the section which is used is a low 780 Ohms. It runs at about 25mA. So it provides plenty of drive for longer cables or power amps with low input impedance. Down to 10kOhm load should not be a problem for this preamp. Let's go through the circuit in detail. Input selection and volume control should be pretty obvious. The 1M resistor from grid to ground provides a ground path also in case the wiper of the pot looses contact. The cathode resistor of 1k generates the bias. It is capacitively bypassed to keep the low output impedance and to avoid any loss of gain. With the 4.7k plate resistor the tube operates quite linear. In the schematic a 1uF output coupling capacitor is shown. This is fine for power amps with high input impedances of 47-100k, as is typical for tube amps. If a solid state power amp is used which typically has an input impedance of 10-20k, the coupling cap should be increased to 4.7uF. The coupling cap should have a voltage rating of at least 450V. I used a 630V MKP. The 1M resistor from output to ground avoids any static charge build up if the preamp is not connected to a load. This avoids loud plops if the preamp is plugged into a power amp which is turned on. Also shown is the B+ decoupling cap of 47uF and a bleeder resistor. The 1k to B+ functions as decoupling from the other cannel, since the preamp uses a common power supply. B+ voltage requirement is 300-350V. The tubes can be AC heated without any hum. The heater should be referenced to ground with two resistors across them and their joint connected to ground. A simple and easy to build circuit which does it's job well. I have two variants of power supply circuits for this which will be shown in part 2. The 6GL7 which is used in this circuit will get it's own coverage in the Tube of the Month post of September. Stay tuned! Best regards Thomas

Hi! In the poll most visitors voted for single ended amplifiers. Most of them for a low budget amp, but many also for a cost no object SE amplifier. So let's see how we can cover both with a single concept which can be adapted to various budgets and will deliver exceptional sound quality within a given cost range. 'Low budget' has quite a different meaning for different people. So let's discuss this first. In a DIY project there is a wide range of possibilites to influence the budget. The more time you are willing to put in to search for cheap parts, the lower you can go. Since this concept needs to be reproducable we need to resort to readily available parts. The foundation of a good amplifier is the iron: output transformers, interstage transformer (if any), power supply transformer and power supply chokes. As I mentioned already in the post about the 6CB5A, the idea for this concept was born on the german tube forum Röhren und Hören. There was a thread about a DIY amplifier concept which should be fairly easy to build, affordable and provide excellent and hum free sound. The desired cost range was determined through a poll. The result was a budget of 1000 Euros for all electronic parts, including tubes but without chassis material. This should be for a stereo amplifier. Of course there was a wide spread in the votes for the budget, so some flexibility was desirable to be able to scale the cost down to 500 Euros by selection of cheaper parts or a simplified concept and also to scale it up by using better parts, building mono blocks, external power supply, etc. But the requirement was that the base concept for the cost of 1000 Euros should deliver exceptional sound quality and come with very good parts. There was a clear preference for a single ended amplifier concept. But not too low in power output, something which gets closer to 10W than to 1W. Many people would have liked the 300B tube but that would have taken out a big portion of the available budget. Especially if there is the desire for some spare tubes. And no way to even think about original manufacture Western Electrics with the given budget. And in my opinion: If you want 300B sound go for original WE 300Bs (not the reissues of the 90ies) but that will be covered in a later post. Most people are too focussed on just the output tube anyways. It is the whole concept which determines the sound quality. Driver stage at least as much as the output stage. And of course the power supply. Most designs have 4-5 parts in the signal path per stage. Well optimized designs get it down to three. All of them have an equal influence on the sound quality. This is a very simplified view, but if you look at it this way, the output tube maybe contributes a third to the overall quality of the output stage. Power amps have 2-3 stages, so best case, the output tube makes up one sixth of the overall sound. And this is not even counting the power supply! Therefor equal effort was spent in this concept to get all parts on a comparable level. I'd rather listen to a well designed amplifier with a lesser tube than the 300B but with solid iron and capacitors, than a 300B amp with cheap output transformers, electrolytic caps and a marginal design. There was another reason not to go for a directly heated triode like the 300B. The amplifier should be as hum free as possible even on sensitive speakers. A directly heated triode would have required DC filament supplies, except maybe the 45 or 2A3 which run on 2,5V filaments. But these were ruled out due to their low output power. A DC filament supply would have added cost and complexity. An indirectly heated triode would be as simple to use as it can get with regard to heating. How that lead to the choice of the 6CB5A was described in the tube of the month post from last week. Besides exceptionally low cost, the 6CB5A has another advantage. It's operating points and requirements to the output transformer are very close to that of the 300B. So the same concept could be very easily changed to the 300B output triode and a comparison between the too would be very easy, even allowing the comparison in the same amp, with the same parts, except for an additional filament supply for the 300Bs. With the tube cost beeing so low, that left almost the entire budget to spend on high quality parts, especially the iron. But before we come to the choice of parts, the basic architecture needs to be defined. The number of stages in an amplifier has a major impact to the complexity (and also cost). This is also dependent on the gain requirements. If you read my post about gain, headroom and power, you'll remember that my philosophy is to use only as much gain as is necessary, with as much headroom as possible. This lead to the choice of a two stage concept (driver and output stage). To keep complexity low, the driver should be supplied from the output stage B+ via a separate decoupling circuit (RC or LC). Interstage transformer coupling would yield good headroom from a given B+, better than RC coupling. Also transformer coupling was not very widely used ta that time in Germany, so such a solution would bring some new concepts into the scene. Of course also because I always got the best sonic results from transformer coupling. The requirement for the concept to be fairly easy to build naturally leads to cathode bias as the method to maintain the operating points. This avoids additional supplies for bias, and any complexity to ensure the right sequencing order of the supplies during turn on. Just a single B+ supply for both channels. Tube rectification will take care of delayed and slowly rising high voltage. In order to use something better than average, a nice, classic choke input filter approach was selected for the HV supply. Again to keep it simple also for beginners to build, no regulation in the pwer supply just good solid passive filtering. So the basic architecture was defined. A two stage transformer coupled concept, using indirectly heated tubes, cathode biased with a single tube rectified and choke input filtered B+ supply and AC heating throughout. No silicon at all in the entire amplifier. Where does this leave us with the budget? Here is a raw calculation: A good choice for excellent sounding transformers at moderate cost is Lundahl . They have a wide range of suitable tarnsformers. For the primary impedance requirement (3-5k) of the output transformer the LL1663 or LL1664 would be suitable. That is about 250 Euros the pair. The LL1660 interstage tarnsformer is about 180 the pair. A heavy duty power transformer for the PSU would be around 100-120. Chokes come at 50-75 each, depening on supplier. At least 3 chokes would be required. One for each channel for decoupling. One in the common PSU, better two since additional smoothing might be required due to choke input. That is 200-300 Euros for the chokes. This sums up to 750 Euros max. For the iron which leaves 250 Euros for the rest. Since tube cost is low, this allows even for some nice oil caps. Here is a sketch of the schematic of the concept so far: Straight forward circuit, a separate choke in each channel which allows the use of a common supply with minimal interaction between the channels. The driver stage B+ is derived from the same supply via it's own RC filter segment. In a more elaborate implementation this could be upgraded to LC, but since we are on a moderate budget, let's stick with RC here. But wait, there is one unusual aspect which is not commonly seen: The capacitors from B+ to cathode in both output and driver stage. This is the so called 'ultrapath' concept. The origins of this approach go back to the engineers from Western Electric. Lynn Olson covered this on his website in an article Western Electric - Rosetta Stone for Triodes. As far as I'm aware the first person to mentioned this approach again in 'modern' times and who re-introduced it to vacuum tube audio is Jack Elliano of Electra Print. He is also the one who named it 'ultrapath' in an article in the magazine Vacuum Tube Valley. What ultrapath basically does is to provide a 'shortcut' for the signal path. Normally the signal would traverse from the tubes plate through the primary winding of the coupling transformer to B+. From there through the power supply (usually the last cap in the filter chain) to ground. From ground through the cathode resistor and/or the cathode bypass cap (which usually is an electrolytic) to the cathode of the tube. The ultrapath cap is usually fairly low in value and a high quality cap can be chosen. It bypasses the cathode circuit with it's electrolytic alltogether. Depending on the circuit, tube and output transformer, often the cathode bypass cap can be left out with the ultrapath connection. For clarity it is left in the above scheme. There is quite a lot of misunderstanding out there about the purpose of the ultrapath cap and it also got some bad press recently. In my opinion it is a very effective and cheap way to boost the performance of any transformer coupling stage by reducing the components in the signal path. It can also be used to reduce powers supply rejection, since it couples residual ripple from B+ to the cathode. The ratio of ultrapath and cathode cap can be chosen such that ripple is cancelled out. But this is not the purpose of this approach here. We only use it to control the signal path. Especially if the cathode cap is omitted, ultrapath will require a very well filtered and hum free B+ supply, since ripple is coupled to the cathode. Hence the provision for the second choke in the PSU, which will be 3 LC stages if the separate decoupling chokes per channel are counted. If the available budget is much lower, the circuit can be significantly reduced, by abandoning the ultrapath concept and changing the interstage coupling to RC. For further cost reduction, the individual decoupling filters can be replaced by a single electrolytic with high capacitance. Of course this will have an impact on the resulting sound quality. The change to RC coupling will also reduce the ehadroom in the driver since the driver tube will operate on about half the voltage. The other half will be consumed by the plate load resistor. Here is the conceptual schematic of the low cost version: Such a concept only needs 4 pieces of iron: 2 output transformer, power transformer and one choke. The choke could even be left out, but let's not make it to primitive it should still sound good. If a less oversized power transformer is used, the iron set would be around 250 for the output transformer (let's still use something very good like the Lundahls), 50 for the choke and 100 for the power transformer. That's 400 Euros for the iron set. Another 100 - 150 should be enough for the tubes, sockets, resistors and capacitors. And the beaty of this: You could start with the simple RC concept and later upgrade to transformer coupling. In the next parts of this series we will fill the concept with a bit more flesh. I will write about the driver tube selection and sizing of the resistor and capacitor values. After that I will present some power supply concepts for this amp and will also show how the design can be easily converted to use directly heated triodes like the 300B, 45, 2A3 or 801A in the output stage. But even there the journey will not end, the concept can be enhanced to an all DHT amplifier with directly heated triodes in the driver stage as well. Stay tuned! Best regards Thomas