Sunday, January 01, 2006

Pro-Co Rat Distortion Pedal Modification

As a guitarist and an engineer I've always been facinated with guitar effects pedals. I've built my own and modified a bunch of them over the years. This is a recent modification to a Pro-Co Rat distortion pedal that I was quite pleased with.

The Rat is a great distortion pedal but one of my annoyances with it is that it doesn't have an LED (like most other pedals) to let you know when it is on. Yeah, if the distortion is turned up high you can tell but I often turned down the distortion and cranked the volume on this pedal to slam the input of my tube amp. Anyhow, this modification fixed the no-LED problem and made the output much hotter at the same time.

The Rat uses an operational amplifier (op-amp) in its distortion circuit. Two diodes are used in the feedback loop in parallel (and in opposite directions) to clip the signal at about 0.7 volts. LED's clip at a higher level (about 3.5 volts depending on type and color). So by replacing these feedback diodes with LED's the pedals output is much hotter (great for driving that tube amp). A side effect is that the LED's light up when the circuit is clipping. The brightness of the LED's is porportional to the amount of clipping. So when I did this I mounted the two feedback LED's on the case so I would know when the pedal is on (and have a cool light effect to boot). Here is another picture:

1 comment:

Guitarshawn said...

Hey jim - I have been a long time user and enjoyer of RAT distortions, and since you are an electronics guy I wondered if you might answer a question for me. I was oblivious until recently that there are all of these arguments about how older RAts sound better than the new, and how the lm388 chip or whatever has been replaced with something else and that has changed the sound... blah blah blah. Do you know what chip that Pro Co changed to - and what if any changes that made to the sound of the distortion? Thanks - if you don't mind helping me understand this please email me at