OK, this is not a Fuzz, but as far as I know it was intended to sound like one. I have heard that somehow the ProCo Rat was designed to imitate a Fuzz Face with more gain and more reliability. Well, this did not work out, but like often when a copy failed, something new was rising up.
This is a distortion pedal, this is THE distortion pedal. Many musicians, some you wold expect, others not, used this thingy to shape their tone.
It can go right in your face, but also very smooth and warm, thanks to the great lo-pass filter knob. Even some Jazz guitarists use this to fatten up their tone in musical territory where you wouldn’t expect a Rat. It’s a bit better for humbuckers than for single coils, contrary to its origin, the Fuzz Face. Mine has a mod, I think it’s the so called Ruetz mod, which can be switched on and off and gives you a bit more possibilities to shape the tone, but if it hadn’t I would not bother. I bought it used for very few bucks, considering it has the sought after LM308 Chip.
When Catalinbread came out with their Katzenkoenig Fuzz pedal I took the on ball and stacked my Rat with a Tonebender, and indeed guys, this is a nice, mean, over the top tone! No wonder you guys at Catalinbread combined parts of the circuit of both into a single pedal.