DIY ProCo RAT: The True Vintage Distortion Pedal

Hello, and welcome to my guitar effects workshop! This circuit has existed for 46 years and is not that popular today. But if you want tons of vintage mojo, you should consider buying or making a ProCo RAT yourself.

This is one of the world's first full-fledged distortion pedals. Do you know why they stand out among other overload effects?

Fuzz is a highly high-gain pedal; it distorts the signal significantly and gives it a very specific sound, completely different from tube amplifier overdrive. However, fuzz is not technically an amplifier. The fuzz needs an amplifier with a cabinet to create the timbre of the sound. Adding fuzz to the line input will be a mistake; the sound will be terrible.

Overdrive, on the other hand, is a moderate gain pedal. Combined with the clean channel of a Fender-like amp, it gives the sound-breaking blues vibe. If connected to the input of an already overdriven amp like Marshall or Orange, the overdrive pedal not only pumps up the gain but also focuses the sound, emphasizing certain frequency bands and suppressing others.

Overdrives have built-in signal limiters on diodes, LEDs, or MOSFETs. Still, setting the gain so high that the limit triggers is unnecessary. It is assumed that we're clipping an overdriven amp plus the internal clipping of the overdrive pedal.

Boosters should also be mentioned; when the headroom is large, they do not limit the signal but simply increase its amplitude. Today, they are often built into electric guitars, especially bass guitars.

Some pickups, like the EMG and Fishman Fluence, have built-in boosts. And for electro-acoustic guitars with piezoelectric pickups, a built-in preamplifier is a must, taking into consideration the high output impedance and weak signal of the piezo pickup.

Transparent boosters have an amplitude-frequency response close to linear, like the 1968 Electro-Harmonix LPB-1 Linear Power Booster does.

A variation called Screaming Bird was released in 1970, and it was a treble booster, a much-needed item for many vintage tube amps.

Thanks to the Screaming Bird, the clean sound became crisper, individual notes were clearer, and the presence of the guitar in the mix was more obvious. The amplifier did not choke on the bass when overloaded, so the result was not fuzz but overdrive or distortion.

Here is the 1965 diagram of the British Dallas Rangemaster Treble Booster. I won't claim that Mike Matthews copied it when developing the EHX LPB-1; it's just a transistor stage with a common emitter. Such stages are found in many pedals, radios, and so on. But I won’t rule out this possibility either; we learn from each other.

Guitar amplifiers were also produced under the Dallas Rangemaster brand.

Besides the Linear Power Booster and Screaming Bird Treble Booster, as you might have guessed from the diagram, EHX released the Mole Bass Booster.

The pedal-style version of the bass booster was called Hog's Foot.

Finally, some buffers do not only change the frequency spectrum of the signal but also its amplitude. Simply put, they have high input and low output impedance, thus helping to mitigate signal losses in the effects chain. The loss of high frequencies due to the influence of cable capacitance is the primary candidate for such an application.

The amplification devices between the electric guitar and the amplifier input can differ. Then what is distortion?

A full-fledged distortion is already a preamplifier, equipped with clipping and timbre shaping. Theoretically, it can be connected to the input of a power amplifier with a guitar cabinet for a fair level of sound quality.

Not every distortion pedal is an amp in a box. And most amps in a box sound even better when connected to the input of a tube amp rather than a power amp. However, each distortion pedal has its own character and recognizable sound.

So here we have the 1978 ProCo RAT circuit diagram. If you're familiar with guitar pedal circuitry, you know that most distortion circuits look similar or have similar building blocks. Because it was ProCo RAT that laid these circuitry foundations.

Many filters will catch your eye if you know what to look for in the diagram. First, an input filter consists of a 22 nF series capacitor, a 1 kΩ resistor, and a grounded 1 nF capacitor.

The 30 pF capacitor is a standard component to compensate for the frequency characteristics of the good ol' LM308 op-amp.

It is believed that only the LM308 can provide authentic RAT sound. That's true; the circuit is very dependent on the specific characteristics of the op-amp, namely frequency response and slew rate.

Since 2005, ProCo RAT pedals have been using OP07 operational amplifiers, almost a complete analog of the LM308. With the LM741, the circuit also works and sounds fine, but nothing like the original one.

A 100 pF capacitor between the output and the inverting input is found in many pedal circuits. It prevents the op-amp from self-oscillating and generating unpleasant high-frequency sound components called "sand."

There are as many as two RC circuits between the op amp's non-inverting input and ground: 4.7 uF 560 ohms and 2.2 uF 47 ohms. From previous articles on guitar pedals, we already know that such a filter stage determines the frequency structure of the overdrive.

Pedals like the Hermida Zendrive and Horizon Devices Precision Drive feature a reconfigurable filter. And ProCo RAT has two of those. This is one of the secrets to the pedal's famous sound character.

Next, a limiter on two back-to-back diodes is included, followed by another filter acting as a tone regulator.

Finally, the pedal's output includes a JFET buffer and volume control.

The Landtone kit I've assembled has an original vintage LM308 with signs of re-soldering. RAT distortion sounds the way it should with this chip.