Blue on Black

Since I first stepped on Proco’s RAT distortion pedal sometime in early high school it has remained a favorite of mine. In my early years my guitar playing was largely rhythm, and I loved how the pedal’s compression and treble sensitivity to cut through in a band situation.

Later on, I discovered how much fun it was to play lead with this pedal. Palm mutes, pinch harmonics, pick scrapes and the not-infrequent clam all sounded a little more exciting with the Rat on. The pedal’s tones fared well in the angst-riddled post-punk, new-wave and college-rock sounds of the 80’s. It was capable of menace, which I found scant in bluesier boxes like the Tube Screamer.

The Rat’s sound is dense and gritty. It is not everyone’s cup of tea, but being close to my heart it was one of the first distortions I explored internally to find out what was going on. Much has been written already on ways to expand and alter the sound of the Rat so I will attempt to repeat none of that here. If you want to tweak a distortion though, the RAT platform is a fun one to use. Its op-amp, the LM308, and its tone stack lend themselves well to circuit bends and modification.

One of the most basic mods to the pedal, removing the two clipping diodes, reveals a great overdrive pedal with none of the Rat’s typical “hash” noise. The tone is cleaner and more spacious. It still alludes to the grit and fury of the Rat’s sound but presents the tone of the instrument more openly. The pedal takes on the character of an overdrive or dirty boost. This can be useful if you have dirt coming from elsewhere in the signal chain (such as your amp) and just want to goose it a little without adding the turmoil of full-fledged Rat distortion.

A consequence of removing the diodes from the Rat is that it makes the pedal’s output appreciably louder. This is not a problem per se, but if switch between diodes-ON and diodes-OFF with, say, a stomp switch, you will encounter two decidedly different output levels. This is great if you want the boost but not so great if you don’t.

Players with lots of pedals in the signal chain value predictability so we searched for a way to reconcile the volume differences. The search led us to a mod that not only addresses the volume differences but adds a little tonal interest of its own: FET clipping.

The Rat’s output buffer uses a FET (transistor) as a unity-gain (clean) amplifier. By “browning down” the operating bias of the FET in the non-diode mode we can match the output level perfectly AND introduce a decidedly different type of dirt than the op-amp’s distortion, or the diodes. FET clipping has a warm open and tube-like sound—bluer—like the sound of a 12AX7 pre-amp tube when it’s starting to get pushed.

Knowing that FETs can vary a bit, we decided to create a tunable bias point using a potentiometer so that the user can dial in just the right amount of volume drop (and clipping) to match the output level of the two modes of operation.

Switching between the Rat’s diode-clipping mode and the warmth of the FET changes the color of the breakup from the aggressive tones of the original to a warmer, bluer sound akin to amp overdrive. This flexibility is nice particularly if you play multiple musical styles and/or change instruments during performances.

The beauty and curse of the original Rat is its way of taking anything you give it and producing more or less the same distortion profile. The harmonic structure of the diode clipping is severe enough that past a certain point it masks the underlying harmonic structure of the instrument. Modified however, the pedal can reveal the voice of the instrument you’re playing much more vividly: a Strat sound like a Strat, a Paul like a Paul, etc. You can take the pedal’s tone from of the shrieking howl of the Rat, into full, warm, amp-like overdrive. You can have your black tones and blue tones side by side.

There is obviously no substitute for having multiple pedals to precisely nail certain types of dirt: Blues Drivers, Tube Screamers, Rats, Fuzzes—they’re all different. However, with pedal board real estate being at a premium these days it seems a worthy endeavor to develop the range of tones offered by a single pedal architecture.

As a builder, I generally rely on my experience playing guitar in live situations for deciding what to build and how. I am not a great tap-dancer; I value flexibility without the overhead of more buttons and knobs, dead batteries and dirty jack contacts, not to mention the cost of two pieces of gear vs. one. Not every pedal out there can be expanded as readily as the Rat, but for all the repetition and tweaks of classic pedal designs available today, there still exists great potential for new uses and sounds.