Rhythm & Detonation


I built this pedal for Chad Miller from Cincinnati, Ohio, who wanted a Rat for bass with the 4th knob mod.

The original ProCo Rat was a very popular distortion pedal that came out in the late '70s and has been used by everybody, but it especially found favor with grunge musicians in the mid '90s for it's cutting distortion.

It's an equally popular pedal for bass and guitar, most famously (to me) used by Krist Novoselic of Nirvana. There's always room for tweaks and improvements, though, to tailor the sound for bass, and there is a popular mod called the Ruetz mod that removes one of the resistors from the circuit, keeping the low end cleaner and distorting the mids and highs. This is great for bass because it keeps the low-end integrity, with no loss of detail or bottom end of the bass.

Even better than removing the resistor for the Ruetz mod, though, is putting it on an external knob, hence the four-knob Rat (FKR).

The Ruetz knob is very interactive with the gain and the filter controls, and while  it takes some tweaking, you can tailor it beyond a regular Rat circuit with one trick.

The other mod I did for the bass included increasing the input capacitor to 47onF from 22nF, which lowers the amount of low-frequency content that goes into the circuit. I played around with the clipping diodes before settling on MOSFET clippers in a symmetrical arrangement.

I used the Four Knob Rat PCB's available at The board shop.

Chad is also a graphic designer, and he provided his own artwork based on a photograph of the band SUNN O))) in silhouette with a circle halftone pattern. I painted the enclosure in Lime Ice and used white knobs with a bright green LED.