The Four Knob Rat

A Proco Rat with mods

The Proco Rat is probably one of my all-time favorite pedals. Nothing comes close, as far as I'm concerned, to the raunchy irreverent growl that comes out of that box. A few months ago I built a Rat clone and started studying the circuit. What I discovered is that the Rat topology is an ideal laboratory for mods. It uses a cheap nasty LM308 opamp with a pathetic slew rate, and bog-standard 1n914 diodes that clip the output. So why does it sound so good? I think a combination of things like the interesting feedback loop to ground and the fact that the LM308 adds a lot of its own clipping.

I've long wanted to try and create a sort of distortion pedal synthesizer. No single pedal meets all of my needs. Wouldn''t it be swell if I had a distortion stompbox that I could flip switches and turn knobs so as to create a whole array of sounds. And if the Rat is a great lab for mods, maybe that's a good place to start? Indeed! That how the Four Knob Rat (FKR) started. The FKR started out as a clone of the Rat, but I deleted the input buffer stage and added true-bypass switching. From there, I've moved the stock 2-diode clipping components off to a separate circuit stage that allows switching between:

  • Stock: 2 x 1N914 diodes
  • Stock/Asymmetric Clipping: 1 x 1N914 + 2 1N34 Germanium diodes
  • LED: 2x red LEDs for much more intense clipping (incidentally, this is the difference between the Rat and the Turbo Rat)
  • MOSFET: 2xIRF520 MOSFETS + 1N34A Germanium diodes for boutique clipping as found in various pedals including the Fulltone OCD and Hermida Mosferatu.

Additionally, I implemented the Ruetz Mod whereby you swap one if the fixed resistors in the feedback loop with a pot. This allows you to mellow out the Rat--you can go from slightly dirty boost, to mild overdrive to full out face-shredding tone pownage.

Finally, I added an additional clipping section to the feedback loop of the LM308 opamp. This clipping section allows the choice between no loop clipping, red LED, or MOSFET clipping. So in essence you have a new section that is more like a Tube Screamer overdrive. And you can have it turned on at the same time as you have any of the other clipping mods turned out. Outrageous!

The FKR Mark II

After the fun of the Mark I, I designed the FKR Mark II.This adds a few interesting things to the original Mark I. First it adds a switch to put the Ruetz mod (called Lube on the FKR) in or out of circuit. This makes it easy to switch between stock and modded Rat.Additionally, there is an effects loop which is a silly idea if I do say so myself.

What does it Sound Like?

Here's a wankage clip from your's truly.

Can I Build My Own?

Absolutely! See the History page for schematics and other info.

FKR Family Foto Fun

The Mark One was the first production version. Here are some photos of various pedals and batches.

The First Production Batch

Back when I built pedals for other folks: Ready to Ship

That's a Lot of Wires

Here's the Standard Knob Rat--A straight clone with true bypass in a small box. This one now belongs to JJ Pistols of Austin Texas.

SKR Guts

Messy Lab

Special Thanks To

There is no way I would have come up with the FKR without the huge pool of wisdom and ideas out there in the interwebs. Jack Orman's awesome article on the Rat was good starting place. The folks at DIYStompboxes also had a big impact on the design with their ideas and contributions. And thanks to Philip Ruetz of for his awesome pot mod.





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