First time poster here with a quick Q about ye venerable proco rat layout.
I'm interested in building a sort of vero test board up with the rat's power/clipping/output section, but that replaces the filter/eq section with input and output jumper points. The idea being that I'd like to try a couple of different active EQ circuits and it would be simpler to just build secondary boards for FMV/baxandall/HM2/etc and keep patching them into something that already exists.
All that said, I'm still at the bottom of the mountain when it comes to this stuff, so while I can follow a layout pretty confidently, knowing how to succesfully move things around is a little beyond me.
My understanding is that the rat's filter stage is just a simple passive LPF made up of the pot, the 1.5K resistor and the 3.3nF cap, so leaving the filter knob fully ccw would be pretty much the same things as leaving it out altogether?
If so, would I replace the 1.5K resistor w/ a jumper, then use the leads to Filter 1 and Filter 2/3 as in and out points? Would I still need the 3.3nF going to ground or would I just omit that altogether?
Basically, I think I just need to stick two jumper points between D2 and C9 and then plug some junk into it... am I on the right track?
I guess, alternately, would it work basically the same way to just insert the in/out points between filter 1 on the board and filter 1 on the pot? Again, if it's just a passive circuit, then leaving the knob fully ccw would effectively be the same thing.
(ie, can I just cut one wire on this confirmed-to-be-functioning rat that I already have sitting here and commence partying?)
It would be a lot easier to advise you about modifying a specific layout if you actually linked to the particular layout you are referring to: There are two layouts for the original rat on tagboard fx, and neither have C9 marked. C9 may well be on some schematic for the Rat, but you haven't linked to that either.
If you want to hook up some different tonestacks so you can evaluate how they sound, you need to connect them directly between the 4u7 output cap (this node also has a pair of diodes to ground) and the gate of the JFET.
The 1k5 resistor, 3n3 cap AND the 22n cap of the original circuit will all influence the tone and should therefore not be included if you want to get a true representation of the effect of a new tonestack on the sound, e.g. the 22n limits the bass response, so if you try out a bax active bass+treble stack with the 22n in place, you will not hear the full response of the bass pot.
Using the layout below, you can use the filter1 wire for the "send", but the return needs to go direct to the gate - There is a space on the last/far right-hand column, above the 9v wire, where you could bring this return, but that also leaves a connection to ground through the 22n and 3n3 caps which will reduce the high end slightly, even if the original filter pot is not in circuit. You would need to snip off one or both of the caps (just the 22n would do). You must check that all of the EQ circuits you test have an output cap and omit any ground connections on the output of any EQ circuit that would end up in parallel with the JFET's 1M ground resistor.
I can totally understand the desire to tinker: However, it is MUCH easier and more reliable to experiment on a small breadboard where no soldering is involved. Testing out circuits by modding a working vero always carries the risk that it will become non-working.