|Ol' Dirty started out as a feedback experiment with an LM386 which ended up as a
dirty fuzz/distortion effect. No (real) calculations where involved in the design of the ciruit, just some experimental breadboarding.
The circuit has an input level control pot. followed by an inverting amp (U1). And an inverting output amp (U3) which mixes the original sound with the distorted sound, followed by a volume control pot. In between these two amps sits the main distortion/fuzz/feedback circuit build around the LM386 (U2). The 100K pot. controls the feedback level and you can set it from little distortion, bursts of oscillation noise, up to a point where the LM386 goes in to complete self oscillation. The 4K7 pot is the gain control and it controls the amount of fuzzyness. I added the light because it just happened to lay on my table (left over from my Swinging Santa) and I don't think it really effects the sound (but it sure looks great!). The diodes are used for hard clipping to create the fuzzy sound and they also make sure that the ouput level doesn't get too high.
I used an old steel box, which I had welded a very looong time ago, added a wooden base and used steel nuts as knobs (I tapped M6 thread on the pot. axis). To get the new shiny nuts to rust for a dirty look, I first tried putting them in a saltwater solution on top of a radiator. After a couple of days they looked dull and I got some nice salt crystals but hardly any rust. Then I found a recipe on the internet: 50% vinegar, 50% hydrogen peroxide (H2O2) and a bit of salt. The result was amazing! instant rust in a couple of minutes. I also used this mixture on some of the other parts and finished the look with a bit of red paint.