By Robert Witmeyer (Blue Skool Records)
Jeff Beck, Metallica's James Hetfield, REM's Peter Buck, Pink Floyd's David Gilmour, The Arctic Monkeys, Blur, Nirvana, and The Foo Fighters all share a "secret sauce" pedal in their tonal tool kit, and it is the often-overlooked ProCo Rat.
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As folklore goes, the pedal was developed by accident in the basement of the ProCo building in Michigan, in 1978, when Scott Burnham and Steve Kiraly attached the wrong resistor... and the Rat was born! Today, the Rat is still a popular distortion box which has spawned many offspring including: The Rat 2 (1988), Turbo Rat (1989), Vintage Rat Reissue (1991), BRAT (1997), Deucetone (2002), You Dirty Rat (2004), and the Whiteface Reissue (2010). Despite being a classic pedal with many usable tones, players have some valid complaints, which, as always, have led to a slew of modifications.
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1982 ProCo "The Rat"
Peter Buck (Guitar Player for REM)
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At its core, the Rat circuit is very similar to a Boss DS-1, but utilizes an LM308 op-amp integrated circuit (IC) with such a poor slew ratio that the pedal acts like an overdriven tube, despite being solid-state in design. While it's a really solid distortion pedal, the Rat lacks tonal nuance, despite the circuit being well suited for a huge array of tones. Ironically, ProCo understood this flaw, as well as the value of changing, or modifying the clipping diodes in their own circuit. As a result, many of the spawn of the rat circuit are basically the same design with different clipping diodes. For instance: the Rat 2 features silicon clipping diodes, where the Turbo Rat has LEDs for clipping the signal. You Dirty Rat uses germanium diodes for clipping, while the Fat Rat has the ability to choose between Silicon, Mosfet, and Germanium Clipping Diodes. Basically, the diodes are the main difference between these iterations of the Rat pedal, as just that one mod can vastly change the pedal's sonic palate.
Different Iterations of the Rat
Duectone Rat
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The Rat 2
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Turbo Rat
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Common Mods
The most common mod performed since the birth of the Rat is changing the pesky 1/8 inch mini power jack to the traditional Boss/1 Spot style 2.1 mm center negative style jack. This mod alone makes the Rat infinitely more compatible with most players' rigs. Another common mod is the Ruetz Mod, where one can disengage half the drive circuit, reducing the gain and getting rid of the bass attenuation. This mod gives the pedal a thicker, fuzz-like bass response at the expense of losing the meaty distortion. While we can perform said mod, we do not suggest it.
More commonly, a DPDT (double pole/double throw) on/off/on switch is added to select between three clipping textures: option 1 / no diodes / option 2. Our favorite version would be Mosfet on one side, and LEDs on the other, though we can put any combination on the switch, be it germanium, LED, traditional silicon, MOSFET, or Schotky diodes. Moreover, we can even add a diode lift to one side, or both, for asymmetrical clipping.
More commonly, a DPDT (double pole/double throw) on/off/on switch is added to select between three clipping textures: option 1 / no diodes / option 2. Our favorite version would be Mosfet on one side, and LEDs on the other, though we can put any combination on the switch, be it germanium, LED, traditional silicon, MOSFET, or Schotky diodes. Moreover, we can even add a diode lift to one side, or both, for asymmetrical clipping.
Rat Circuit Board w/ LM308 Chip
Another common Rat Mod is swapping out the op-amp, or engine of the pedal. The original Rat featured a magical (actually super shitty - so shitty it sounded great!) LM308 chip. We have a stash of magical NOS LM308 chips, and can swap your newer chip for an original. That said, this mod normally isn't the same game changer as swapping clipping diodes, so we suggest checking out the traditional clipping diode mods before resorting to this level of cork sniffing. Likewise, we can also swap the stock chip for a MOSFET CA3130, high-quality NE5535, LM741 (used in MXR Dist + Circuit), or the common TLO71 or 81. All of these op-amp mods will have subtle changes in the amount of distortion on tap, the headroom of the pedal, and the amount of noise generated. Finally, should the Rat you own be a bit harsh or shrill, we can perform the common mod of "softening" the tone with silver mica or metal film caps for better fidelity.
If you are interested about any of the aforementioned Mod's or would like to talk shop give us a call at (408)-395-0767 or email us at robert@khguitars.com
About the Author
Obsessed with building and modding pedals for the past 10 years, Robert Alan Witmeyer is a local professional musician and multi-instrumentalist in the South Bay Musical Community. He received his first pedal as a gift during high school, and instantly became a pedal addict. While Witmeyer recently won the KFOX Riff Off, earning him the title of "Best (unsigned) Guitarist in the SF Bay Area", he is also proficient on 163 other instruments including piano, bass, mandolin, banjo, vibraphone, organ, and various percussion. After releasing multiple albums on itunes, amazon, spotify, etc, Robert found his live tone lacking, and began researching what big name acts did to achieve their legendary tone. Upon learning about the art of "modding", Witmeyer began altering all his pedals, and later, started building his own effects using his label name, Blue Skool Records. Throughout the years, Blue Skool worked with other companies to create unique circuits, including designing and promoting an original Fuzz and Octavia pedal, mini Amp, and other circuits for Peacekeeper Guitars at the infamous NAMM show. As a music journalist, Robert reviewed pedals for Guitar Player magazine, and was a contributing writer with pedals as his forte. Today, Blue Skool Record's stompboxes are on the boards of many local pros including Simon Santiago (the Houserockers), Rome Yamilov (Aki Kumar Band), and Jim Thomas (The Mermen).