1988 ProCo RAT (LM308) | Gain Appreciation

classic 80s filth…

The ProCo RAT was first introduced in the late 70s and quickly cemented itself as a classic. The mid focused distortion pedal was able to produce an excellent approximation of the cranked amplifier stacks popular in rock music at the time.

The unit we look at in this video dates back to the mid part of 1988 and is technically a RAT2, the revised model from the mid 80s onward which features a smaller enclosure and indicator LED which was absent in the original RAT. There is practically no difference to the circuit other than these two factors.

An important feature of the RATs from this era is that they utilised Motorola LM308 opamp chips to perform the signal amplification.

By modern standards the LM308 isn’t a great opamp, failing in a few important ways:
LM308 has a poor slew rate (how fast the chip is able to swing its voltage from one value to another) making it unable to recreate frequencies much above 5kHz.
It also has a non linear frequency response when amplifying. At high gain levels the opamp is unable to amplify high frequencies to the same level as low frequencies.

This leads to an interesting result where the higher the distortion is set, the less prominent high frequencies become. The speed limitation of the chip also eliminates much of the high frequency harmonic content that cause fizz in distortion pedals.

All of this leads to a pedal that never gets fizzy, has a strong mid focus around 1kHz, and gets thicker and darker the more distortion gets added.
The opamp collapsing under duress leads to some filthy, spitting tones which may be exactly what you need if you want to get nasty.

If you want something a bit smoother however, setting the RAT to moderate distortion and feeding it with a compressor results in a smooth, saturated response which makes neck humbucker lead lines sing.

While the modern production RATs lack the LM308 in favour of modern opamps, they are still as nasty and gritty as the 80s version.
You can grab one for yourself at the following links:
Thomann (EU) ProCo RAT
Sweetwater (US) ProCo RAT

You can read up further about the RAT and its LM308 opamp here:
Electosmash RAT circuit breakdown
LM308 Datasheet