Simple Guitar Distortion Box
Intended modifications for final design:
I would finish this, but I gave the prototype to a friend's son some years ago. If I had to describe the sound I would say with the gain at 2/3 it was harmonicly very rich and with the gain on full it sounded like a square wave synth. My best guess at the original circuit diagram is below. It's very simple.
If something a bit weirder is desired one could have the led and switch (removing the series resistance R9) in series with the 9V rail. At certain gain settings this will produce a pleasant octave effect. It works by virtue of fiddling the operating conditions for the transistor stages and more importantly the LED such that it's in just the right region of its characteristic to produce the effect. It's pure luck and will require a bit of time tweaking the controls to find anything worth hearing.
Lots of guitar effects of this type (loosely called timbre modifying circuits) make use of the nonlinear properties of active devices. Most analogue designers would consider them the height of poor design. Still, it depends what you aim to achieve.
This project is the height of bad design for several other reasons most notably the high output impedance and low input impedance. As I have already stated it is unfinished.
I can't remember the value of RV1 and RV2 but I suspect they are 4k7 linear types.