Darrell rants about the Proco Rat and the ruetz mod.
It’s no secret that the rat is my favourite pedal to ever exist. It’s only drawback is the fact that it can cut a bit too much bottom end for use with low tuned instruments such as baritone guitars, heavily detuned guitars, or bass. The most common mod to remedy this is the ruetz mod. It was the first pedal mod I ever did and the thing that got me hooked on messing around inside pedals. I’ve played around with this mod and the whole rat circuit a ton and am obsessive about getting as much usable low end as possible out of every pedal.
The ruetz mod basically consists of messing around with the 47ohm resistor (R6) that is part of one of two rc (resistor/capacitor) filter networks from the opamps feedback loop to ground. Sometimes this resistor is literally just snipped leaving only the 560ohm/4u7 rc pair to set the gain and low end roll off. Other times that 47 ohm resistor is replaced with a pot. Usually around 1k.
My problem with both versions is that bass is never actually increased. Instead the gain of treble frequencies is actually being decreased by varying degrees. This does give the perception of more low end but in my opinion the decrease of gain in the high end actually robs the rat of its true character. The are tons of “boutique” rat derivatives and rat inspired pedals out there that incorporate some version of the ruetz mod and it boggles my mind.
So what’s the solution?
Well if you know anything about rc filters and especially rc filters in opamp based dirt circuits, you know that the capacitor in that rc pair has an equal impact on the frequencies amplified by the opamp which are then clipped by the diodes. Typically the resistor sets the gain and that resistors relation to the capacitor sets the frequency roll off.
If we look into the rc filters in a rat we’ll see that the 47ohm/2u2 rc pair sets one high pass filter roll off at around 1500hz while the 560ohm/4u7 pair sets a second roll off point around 60hz although with much lower gain.
There are tons of popular mods for equally classic pedals (tubescreamer, distortion plus etc) that involve tweaking the value of the cap in the opamps rc filter to get more bass out of the circuit while leaving gain levels alone.
So why does that seem uncommon in a rat circuit? Why do we ignore the capacitors?
Because I don’t think anyone has written it on the internet yet and a large part of the boutique/DIY pedal community can’t do anything until someone else does it first and tells them how it’s done. The idea of understanding concepts applicable across the board is just beyond some.
So back to these rc pairs…
My favourite version of the ruetz mod was always just replacing the 47ohm resistor with something around 400 ohms which gives a roll off about 180hz while sacrificing a fair bit of gain but If you increase the value of the 2u2 cap instead you can get your low end back without sacrificing ANY gain. 22uf will give you a roll off of around 150hz which is low enough to let most bass frequencies through without getting muddy.
If you want a tweakable version you could wire up a capacitor blend pot (usually used at the input or output of a circuit) to blend between the stock 2u2 and a much larger cap or any 2 values you want. I’ve used this in several of my own builds and it is massively usable.
Leaving the amount of gain in the circuit alone also makes diode changes much more obvious. Put a big cap (or cap blend pot) in there and a 3 way diode toggle (or even a diode blend pot) and you’ve got a super flexible rat that still has all the amazing qualities of the best rats.
Also check out the comparative rat schematic demonstrating why the “vintage” rat circuit isn’t all that special (with the exception of the lm308). It’d take anyone who can solder 20 minutes or less to covert any rat to vintage specs. My rat 2 was already “vintage spec” other than the missing lm308.