Pettyjohn Electronics Pettyjohn ROUS

The Pettyjohn ROUS ("Rodent of Unusual Size") is an analog distortion pedal from Pettyjohn Electronics, part of the builder's CORE series of compact pedals¹. Pettyjohn describes it as "a highly evolved member of the RAT family," a take on the classic Pro Co RAT circuit that blurs the line between overdrive and distortion². Its gain range moves from low gain overdrive to what RockBoard's PedalPedia calls unusually large sounding distorted tones quickly with the turn of the Gain knob¹.

The ROUS, "Rodent of Unusual Size", is described by Pettyjohn as a highly evolved member of the RAT family²¹, an analog overdrive and distortion pedal whose gain range moves from low gain overdrive to unusually large sounding distorted tones quickly with the turn of the GAIN knob¹. Much like the original RAT, the gain amp drives an array of diodes in shunt position, and a Fire Swamp switch adjusts that diode clipping array between two settings for a marked change in tone.

A RAT-family distortion with sweepable filters

The RAT references are not disguised: the name spells it out, and Delicious Audio noted that "the references to the RAT aren't disguised, ROUS stands for 'Rodent of Unusual Size'"³. Much like the original, the ROUS's gain amp drives an array of diodes in shunt position, the same basic arrangement Guitar World describes in the Pro Co RAT as an op-amp based distortion circuit combined with diode clipping³⁴.

Where the ROUS departs from a straight RAT is in the filtering. Instead of the RAT's single tone control, the ROUS puts a sweepable low pass filter before the gain stage and a sweepable high pass filter after it, so you shape what feeds the clipping and what leaves it independently³. This approach has a documented lineage: an independent technical analysis of the earlier Blakemore Effects R.O.U.S. on the Guitar FX Layouts blog found that pedal to be "basically a Rat with the pot between the 47R and 2u2 in the pair of high pass filters at the inverting input to control the frequency, with passive high and low pass filters after the opamp stage replacing the simple tone control"⁵. Guitar Player's Dave Hunter counts the Pettyjohn ROUS among the numerous clones and homages that "contain a good dose of ProCo Rat DNA," alongside the Joyo Splinter, Jam Rattler, Mooer Black Secret, JHS Pack Rat and ThorpyFX Warthog⁶.

The controls

The ROUS carries a four knob control set plus a clipping mode switch. RockBoard's PedalPedia entry lists the set as Level, Gain, Lows and Highs¹; Guitar Pedal X's coverage lists the same four knobs as Level, Drive, Lows and Highs, matching the knob set of Pettyjohn's RAIL fuzz⁷⁸.

The two filter knobs are the working end of the design. Because the Lows control sits before the gain stage, it changes how hard the clipping stage is pushed in the bass; the Highs control then trims the top end after the distortion has been generated³.

Fire and Swamp modes

The Fire/Swamp switch adjusts the diode clipping array between two settings that Delicious Audio described as making "a remarkable change in tone"³.

• Fire: Pure silicon hard clipping, the setting that most closely resembles the original RAT design³.
• Swamp: Adds germanium diodes into the clipping array, increasing headroom and softening the clipping effect in a vintage, musical way, with softer clipping and vintage overtones³.

Power, switching and build

The ROUS runs on 9V DC with a center negative plug and draws 100mA¹. It is true bypass, using a relay based switching system that Delicious Audio described as being "for minimum pop when engaging the effect"³. The pedal is hand made in Oregon, USA, and shipped in a new enclosure by GORVA design³. Custom mod options were available only through the Pettyjohn online shop⁹.

What reviewers said

Guitar Pedal X's Stefan Karlsson covered the pedal twice. In his 2020 roundup of the year's new fuzz and distortion pedals he noted it was still incoming at the time, writing "This is one that is still incoming, I was hoping to have it by Christmas, but it's no doubt been held up somewhere along the way"⁸. By his 2024 roundup of compact RAT-style distortions he had spent time with it, and his verdict was positive:

In SWAMP mode the pedal adds germanium into the clipping array, which increases the headroom and softens the clipping effect in a way Pettyjohn describes as vintage and musical³. This contrasts with FIRE mode's pure silicon hard clipping, giving players two distinct voicings from the same circuit.

FIRE mode is pure silicon hard clipping, and Pettyjohn notes this setting most closely resembles the original RAT design³. Paired with the sweepable LOWS and HIGHS controls, it gives the pedal a versatile overdrive and distortion range with deep nostalgic origins.

Versions and status

The circuit follows the original RAT topology: the gain amp drives an array of diodes in shunt position³⁴. Tone shaping is split around the gain stage, with the LOWS control placed before it and the active, fully buffered HIGHS control after it³. The Fire Swamp switch then adjusts the diode clipping array between the FIRE and SWAMP settings.

Videos

Click to load

Click to load

Click to load

Questions