Project 10 - Pro Co Rat Distortion

If you were to ask a group of guitar players what the most popular Distortion Pedal is, I would suspect many would say it's the Pro Co Rat.

The Rat distortion pedal has been available to guitar players since the mid-to-late 1970's and artists who have recorded with it include Dave Grohl, Kurt Cobain, Jeff Beck, James Hetfield, David Gilmour, Thurston Moore, Joe Perry, Joe Welsh, Thom Yorke and many others.

Over the years Pro Co has made a number of changes to the original design and have also released several different models - each having their own unique sound and functionality.

The history of the Rat pedal is well documented, so I won't include that information here; but if you would like to learn more, I recommend starting with this informative Wiki article.

How The Circuit Works:

The Rat Distortion pedal has a fairly simple design that uses a variable gain circuit with a pair of diodes that short the output-to-ground at a certain voltage, resulting in hard clipping.

For those of you not familiar with diodes, or hard vs soft clipping, my previous post on the Hemmos Bazz Fuss will tell you all you need to know - read about it here.

My Build:

Looking back, I have always loved the distortion generated by Rat pedals. The current "Rat 2" model has a distinctive sound that can be described as gritty and crunchy, with a warmer tone than most other distortion pedals.

To generate this unique distortion, the Rat 2 uses a pair of 1N4148 clipping diodes which, to my ears, sound almost identical to the 1N914's found in the original design (even after several days of A/B testing, I'm still not entirely sure if there is any noticeable difference between the two types).

Initially I was just planning to build the Rat 2, but then I discovered the only difference between a Rat 2 and a Turbo Rat is the clipping diodes. Instead of 1N4148 diodes, the Turbo Rat has a pair of Red LED's, which generate a brighter, less compressed sound with more volume, headroom and overall gain. Since I could easily add both sets of diodes to this build and include a toggle switch to choose between them, I decided this was the better option.

For the Op-Amp chip, unfortunately, I wasn't able to source the Texas Instruments OP07DP or Motorola LM308, but was able to find some LM308N chips on eBay that are manufactured by National Semiconductor.

To keep this build as flexible as possible I also added sockets for each diode, the transistor and the Op-Amp chip, allowing for easy removal should I decided to try different component values in the future.

Controls:

A standard Pro Co Rat pedals has 3 control knobs: Distortion, Filter (which is the Tone control) and Volume. For my build I included these controls, but also added a fourth which allows me to decrease the integrity of the audio signal. Turning the Integrity knob all the way clockwise is the "normal" operating position for this control. As you turn it counter-clockwise, the signal starts to break up, giving it a dying Fuzz type of sound.

As you can see in the photo below, the Vero Board (aka Strip Board) layout for this circuit is rather large, but I did manage to fit it (just barely) into a Hammond 1590B enclosure.

How does it sound?

Unless you've owned a Rat 2 and/or Turbo Rat, then you might be wondering what these pedals sound like. If so, check out this short video I made...

More Information:

If you would like to learn more about the Pro Co Rat Distortion pedal and gain a better understanding as to how each stage of the circuit functions, I recommend watching this video by veteran pedal builder Brian Wampler...