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I’ve been wanting to buy a proco Rat for a while and ended up acquiring a plague rat clone in a trade. The issue I’m having is that there’s a huge amount of feedback, beyond what I’ve ever experienced with any fuzz or distortion I’ve owned. It’s a constant high pitch as soon as the gain is above 12 o clock and it also seems to be affected by the low and high cut knobs. It happens across all clipping options and it goes up and down in tone when adjusting values. Any help would be appreciated. I know rats are very prone to feedback, which is something I want, but this makes it unuseable on higher gain.

I believe I’ve had the same issue you’re describing with a drive pedal (not a Rat though). It turned out to be the powersupply.
Changing this solved the problem.
You could try it first with a battery. If the problem goes away then you could take it from there and try a different powersupply.

It happened when i modded one with a pot on the pulldown resistors (ruetz)

But seemed to go away with buffer or turning down a knob on guitar. I dont know if the knobs on this are similar mods? But it was there on certain settings very consistently, i think these are resistors that purposely make a feedback loop in circuit?