This is a slightly modded Interfax Harmonic Percolator that I made today. It's an odd little circuit - it can run from clean, a bit overdriven, then into distortion, and finally to overly compressed fuzz madness. The range of sounds are so broad, it's almost worth setting it up as a dual pedal (which I might do).
In terms of mods, it's kind of a mix of a couple of schematics, the Land Devices HP-2 and a bit of my own thing. It's still very much a Harmonic Percolator, that much is sure. If you want to use the layout for a stock Harmonic Percolator (whatever that may be), there's fairly extensive discussions on the usual forums, including schematics and of course many and varying opinions as to what it should be.
INTERFAX HARMONIC PERCOLATOR - MODDED VERO LAYOUT
When it was on the breadboard I tried a few different transistors. The differences were noticeable, and I'd recommend testing before soldering, to make sure that you have a combination that you're happy with.
If you have a breadboard, it's worth getting it on there to play with - there are so many variations on this circuit, and they do make a difference.
The silicon transistor had a hFE of around 240 and the germanium was about 70 with almost no leakage (like many soviet transistors). The type of germanium and silicon diode is not important.
ON THE SCOPE
With the input trimmer down, it's a pretty clean signal (called Harmonics on the pedal). I run a 200mv 440hz sine wave into my scope.
