What have I done!? (Proco Rat 2 ROT mod)

[apotheodaimon]

So I was playing around with the guts of my rat2 today and got a wild hair to just start sticking caps in random places to see what might occur.

I stuck one lead of a 0.022uf cap onto lug 1 of the gain pot and the other onto the middle lug of the filter pot. Gain went down slightly but volume increased slightly, I found that by turning the filter knob thereafter I got a sort of notch filter or presence control that sonically reminds me at times of a %^&*ed wah, but the filter knob no longer attenuates treble, so I popped a similar 0.022uf cap onto lugs 2 and 3 of the level pot.

I know in doing this to the filter I bypassed... things, but what exactly? I dig the effect but I'd still like the original filter as well. My question is what did I do exactly? Also can I use a concentric pot to get this effect as well as the original filter back somehow?
I'm not sure.

Somebody with a real brain help me figure this out. I'm terrible at math and I'm not an electronic engineering major.

I'm on break at work, will add a pic of the guts when I'm off work later.

[acehobojoe]

Check out the schematic man. Some guys were showing it to me earlier.




It looks like you may have bypassed quite a lot! haha, but maybe not, I'm still not sure as to what may have happened. Some other guys probably have some ideas... I find the best ideas sometimes come from "doing random stuff"

[acehobojoe]

One other thing to note is that you may be leaking DC out of your pedal now. If you ended up liking this mod or wanted to make it switchable, you could add in another decoupling cap. If you don't know what those are, they are just circuit's ways of cleaning out the dc that is often necessary in different effects. I am no expert at all, but you can read all about this stuff on the forum, and on AMZ, GEOFEX, Beavis Audio, or Mark Hammer's page, those guys love teaching newbs. So much good info on those sites.

[apotheodaimon]

I use the beavis audio rat 2 schematic for my reference tool on this pedal all the time. I've been doing mods on it for a year or so, most end up back to stock, the only thing that's stayed in it so far is a chip socket, an LM308AN IC op amp, an extra diode and a switch to lift the negative side of the clipping loop. This new cap mod came about because I was going to raise the values of the input and output caps but it did nil in testing so I moved on to some shenanigans and found what I've done. I'll post a pic around 4am central when I'm off work.


Sorry my camera is a potato. The blue lines are the lead for the cap between the gain pot and the filter pot and the red lines are the leads for the cap on the level pot. Nothing is clipped down and soldered yet, just shoved the leads into the holes in the lugs.

[acehobojoe]

Do you like the way it sounds? And does it pop when you click it on or off?

[GGBB]

The cap between gain-1 and filter-2 creates a second negative feedback loop (gain-1 is op-amp pin 2), which affects overall gain and is limited by the RC filter of that loop formed by the 1k-4.7u-R_filter-0.022u series.  Since part of the filter pot is in the loop, adjusting it affects the range of frequencies as well as the gain.  Since the clipping diodes are also now in the feedback loop, they also have an impact, but I'll need to leave that to someone more knowledgeable to explain.

Because the RAT already has low end roll-off in the gain section (R4,R5,C5,C6), this new gain section interacts with that and is probably the reason you hear a wah-like effect.  The minute details are beyond me - hopefully someone else can add more info.

The cap across the level pot will just be a treble filter that varies with the level control.  It shouldn't cause any problems as long as you like what it does.

[antonis]

The cap across the level pot will just be a treble filter that varies with the level control.  It shouldn't cause any problems as long as you like what it does.

As I understand apotheodaimon's connection, he put another cap of 22nF in parallel with C3..

So he lowered the cut-off frequency of C3*R3 by 220 times...!!!
(73Hz instead of 16kHz - that's the WHOLE guitar frequency range..)

[GGBB]

so I popped a similar 0.022uf cap onto lugs 2 and 3 of the level pot

As I understand apotheodaimon's connection, he put another cap of 22nF in parallel with C3..

I read "level pot" as volume pot not gain pot, but I could be wrong.  If he put a cap across lugs 2 & 3 of the *gain* pot, that would cut highs inversely variable with gain.  In parallel with C3 would cut highs across the board fairly drastically as you noted.  Neither makes much sense so I think it probably was the volume pot.

[apotheodaimon]

Yes I do quite like what it does for the variety of tones I can get. No popping or funny stuff in transition between off and on. In fact  the usual hiss and hum this pedal always had seems gone by a large degree. Any thoughts on how I could have both the original filter and this new filter simultaneously?I'm thinking pull the cap off the filterpot, connect a new pot with its input and output parallel to the filter pot and use that one for the cap? Would that work or would I still lose the filter?

And yes, the volume pot is the one with the cap on 2&3.

[GGBB]

I'm thinking pull the cap off the filterpot, connect a new pot with its input and output parallel to the filter pot and use that one for the cap? Would that work or would I still lose the filter?

That should work fine I think.

EDIT:

You might want to try splitting after R6 and adding a second 4.7u cap before the new pot.  This will remove the clipping diodes from the feedback loop so it should sound a little different (I would expect more gain from that loop) - your call as to whether its better or worse.

Another idea would be to leave out that pot and connect the loop back to the gain pot lug 3 (not lug 1) so that the original gain control now controls both loops.

[ashcat_lt]

Gain went down slightly but volume increased slightly...

This is just plain not possible!  Maybe you got less distortion, but that is not the same as gain.  No, it's not a matter of semantics, is a matter of accuracy.  You might get away with it talking to your guitarist friends, but it only leads to confusion in a technical forum like this.

The diodes to ground in the feedback loop should kind of force the opamp to add more gain for louder signals - like an expander kind of effect.  It would only really affect the frequencies that pass the cap, though, which is probably not much, and would be rolled back off by the other things happening in there.  The action of the Filter pot kind of undoes itself in this situation.  As you turn it toward more resistance, the highs freqs are killed, which causes the opamp to want to gain them back up...

[apotheodaimon]

Fair enough. I got a tiny bit less distortion.

With the cap in place as the filter is rolled off you can hear a frequency hump begin to to shift. At the lowest settings it is an undesired tone and at the highest it is just the same as simply having the filter unmodified, however there is a happy middle area that seems to affect presence greatly. I can now tune in a sound which is actually great through my Mesa amp, which before was incompatible with this pedal. It also works wonderfully with my svt as a nice gritty bass tone in the lower settings.

Edit: I now would highly recommend this modification to anyone looking for either something new to do with their RAT or as a much better (although completely different) alternative to the ruetz mod. The tones go from standard rat to completely rotten and back. Very fun.

[antonis]

I now would highly recommend this modification to anyone ...

If you post a schematic with the final modification, it would be nice...

(speaking for myself, I'm totally confused about the caps connections..)

[apotheodaimon]

There is a picture up there. I can modify an existing schematic in awhile to show what I've done.

A word of caution the mod won't sound nearly as good unless you set your amp a little dirty.

0.022uf cap from Innermost lug of the distortion pot to middle lug of filter pot is the main bit of the mod. A second cap can be added from the innermost lug of the volume pot to the middle lug of the volume pot.