The Proco Rat has become a classic pedal that has a legion of fans among guitar players. I have 3 of them, two Rat2 and a Turbo Rat.
The Rat is designed to have a large amount of gain and a significant treble boost. This is part of what gives it a characteristic sound, but also makes some players dislike the tone it produces.
The resistors R6 and R7 along with the capacitors C7 and C6, are the main determining factors for the low frequency rolloff.
The C6/R7 pair has a 60.5Hz cutoff.
We can see that the first easy mod is to remove C7, which will leave only the C6/R7 network determining the frequency response and gain. R6 is not in the circuit once C7 has been removed.
The capacitor is easy to find. Locate pin 2 of the opamp and follow the trace to the 47 ohm resistor. It will have bands of yellow-purple-black. The 2.2uF will connect to it. Desolder the connections and remove the capacitor. You could also remove the resistor.
Gain is now lower and the frequency response is flat across the band of guitar frequencies. This will give it a tone with much less treble boost, and somewhat like a Big Muff.
If the Rat has now lost too much bite for your rig with this mod, then it can be fine tuned by changing the value of C6. Remove the 4.7uF (C6) and replace it with a 0.47uF capacitor rated for 25v minimum (50v, 63v or any higher rating is acceptable too). This will give it a low pass corner of about 600Hz.
Make C6 smaller to roll off more bass – 0.22uF will have a 1292Hz corner. Use the AMZ Filter Calculator to work out a combination that you like.
Customize your Rat pedal for the best sound with your guitar and amp!
More custom mods for the Rat are available in the AMZ Muff-Rat eBook.
Update: You can apply these mods to the BYOC Mouse kit. Here is the parts cross reference:
- RAT : BYOC
- R7 = R4
- R6 = R5
- C6 = C5
- C7 = C6 (remove this one)