There are a host of Big Muff clones and variation, but one of the most sought after clones is the Ibanez OD-850. Like all vintage Ibanez pedals, the OD-850 is just a rehoused and rebadged Maxon design–in this case, the D&S OD-801. While this shares a lot in common with a Ram's Head BMP, it differs greatly in the tone section. Here's Kit Rae's description: Maxon opted to alter the tone stack for this clone, fixing the bass value across the tone pot sweep... The whole high pass and low pass tone section is incorporated into the recovery stage. What this did was to make the bass and treble sides have a more useful range of sounds. The treble side retains some bass across the sweep, so it is not as ear piercing and trebly as a standard BMP when dialed completely to the treble side, nor do the highs seem to be completely removed when dialed fully to the bass side. It kind of makes finding the 'sweet spot' a bit different sounding than the traditional BMP tone stack, but that is what makes this version unique. I've laid this out for more common CBE pinout transistors, where as the C828s of the originals are BCE. This follows the first version released in 1974, and will fit in a 1590B with board mounted pots. Here's the schematic for reference.