cap mod for turbo rat

[jimmy462]

can anyone explain to me how to do a cap mod on a ProCo turbo rat for the purpose of retaining some low end? thanks!

 

[tplyons]

Identify the input capacitor, replace it with one of a higher value. I do not have a Turbo Rat in front of me, but I can tell you that I've done it successfully in the past. I do not recall the values I used.

 

[bongomania]

Jimmy, check out the "Big Muff cap replacement" thread going on now. Also, go to geofex.com and diystompboxes.com for more info.
Good luck!

 

[jimmy462]

thanks bongo-am i correct in my understanding that i need to replace both the input AND output capacitors? how do i know the correct values for this particular pedal?
thanks!!!!!!!!!!!!

 

[tplyons]

thanks bongo-am i correct in my understanding that i need to replace both the input AND output capacitors? how do i know the correct values for this particular pedal?
thanks!!!!!!!!!!!!

More often than not, I find that it's only necessary to replace the input capacitor, but you may find that you may want to replace the output cap as well. IMO, the best way to do this is by guess and check. Start with a slightly higher value than what's there, and proceed from there. There is no perfect value for modding these, that I have found. I've done quite a many pedals with this method and don't recall using the same value more than a few times.

 

[seanm]

Looking at the schematic, I would be tempted to try upping R1 from 220k to 1M rather than changing the cap. It should give you a higher input impedance and reduce the rolloff.

However, the current input should start rolling off the bass at 33Hz or so. Not bad really.

Hmmm, and there seem to be a couple of other places with more rolloff than the input cap. For example, the cap/resistor to ground in front of the output buffer is closer to 72Hz. I would try changing the resistor to ground in front of the FET to 1M first. According to the schematic, this is what the original rat had.

 

[bongomania]

thanks bongo-am i correct in my understanding that i need to replace both the input AND output capacitors? how do i know the correct values for this particular pedal?

It's not that you necessarily need to change both (or either), it's just that they are both places that commonly are worth looking at. Those two points regulate which frequencies get IN and OUT of the circuit. As seanm points out, actually knowing something about the circuit can change your whole approach. But with any given pedal, if don't have the schematic, and I hear the lows getting rolled off, the in and out caps are the first place I look. Like Tim says, the "right" values are found by experimentation. Aside from the values I mentioned (.22uF to .47uF), you can try doubling the value of whatever is in there currently. For experimenting, carefully remove the stock cap, and stick in a test new cap. Just poke the cap's legs in a little ways, and apply a very tiny dab of solder, just enough to make a connection. That way you can desolder and try out a few different caps with less risk of damaging the board or the caps.