Marshall GV2 boost mod

[Silvio55]

I have a Marshall Guvīnor 2 pedal and Iīm not using it much since I got my first tube amp, as I use the overdrive of the amp, I was thinking if it can be modded so I can add a "clean boost switch", like the Bluesbreaker 2 pedal for boosting the amp for more gain.
Iīve already tried lifting the diodes but it didnīt make any significant difference, it sounded almost the same.
Hereīs the schem:


Thanks in advance!

[mozwell]

There is a high gain around the op amp IC1-B, change R11 from 680k to maybe 100k or lower.
There will be volume drop in the tonestack, so you will need to compensate for this by adding gain around IC1B.
maybe set gain to maximum, fit a 500k trimpot instead of R11, and adjust it so you can get a boost of the level you want. Then remove the trimmer, measure its value & install the same value fixed resistor.

[Mark Hammer]

The simplest way to eliminate the clipping and have it be a clean booster is to simply disable the pair of clipping diodes. That can be done at either end of the diodes. You don't have to cut the connections at both ends; one end will do what you want.

Note however that sometimes, high gain circuits will clip in spite of the absence of clipping diodes. And in this case, Mozwell may have a sound point in reducing the gain of IC1B, since at max gain the circuit asks that measly little 9v battery and op-amp to increase the amplitude of the input several thousand-fold: something a 9v battery is unlikely to be able to do with an input often on the order of 100-200mv.

However, with the diodes lifted, you should be able to turn the gain down to a point where the headroom limitations themselves do not impose clippping, but the clean output is hotter than it might be with the diodes in place.

[J M Fahey]

Certainly it can be done, but it was designed from the ground up as a high gain channel simulator, so even lifting diodes you might still clip the Op Amps against the rails easily.
Lowering gain will help, but itīs still a wild animal.
Above suggestions are excellent, of course.
The Bluesbreaker is quite the opposite, barely clipping even when asked to, so the mod is much easier.
Why don't you just build a clean boost?
They are simple and fun.
Or even a Bluesbreaker clone

[Silvio55]

Well I was just thinking of modding this pedal since I donīt use it now and I can use the enclosure, I have a bluesbreaker clone I built a couple of years ago (it was my first pedal project), it works fine but never built an enclosure for it, I tried to put it in the guvīnor enclosure but the pots didnīt fit in there, and the bluesbreaker doesnīt seem to boost the volume much, just clips the signal. Maybe I should try to lif the diodes with a switch and see what happens.

[Mark Hammer]

Here's another idea.

Remember that gain is multiplicative. One stage with gain = 10x, followed by another gets you a combined gain of 100x.

The Gain control essentially redistributes resistance such that it simultaneous affects gain in the first two stages by taking feedback resistance away from IC1A as it adds input resistance to IC1B, reducing the gain (and the product) of two stages at once. At max gain, the gain of IC1B is 680/9.4 = 72, and the gain of IC1A = (100+2.2)/2.2 = 46. That is a LOT of gain, and in fact far more than the supply and chip can provide.

Let's say that R10 (presently 2k2) gets raised to 10k. That sets the maximum gain of IC1A at 11x, and the total max gain at just under 800x. That's still considered high, but certainly not as high as 3300x. More importantly for us, it does a few things.

1) It lowers the low-frequency rolloff in stage 1 from 720hz down to 160hz, giving it less of a Tube Screamer tone.

2) It raises the point on the Gain pot rotation where the gain becomes high enough to produce clipping from the diodes. Note that with the gain pot set halfway (50k in the feedback loop of IC1A, 50k placed in series with C10) the cumulative gain is 6 x 11.4 = 69, which will get you some LED clipping, but not wads of it as it probably does now. The same position with R10=2k2 would get you a gain of 271x, which is higher than the max gain of a TS9 or a Distortion+.

Raise R10 to 12k, and you get even more gain reduction and shift in the bass rolloff.

Turn your attention to IC1B, now. The combination of R11/C11 gets you a treble rolloff starting around 1060hz. It's not steep, by any means, but it is lower than what one might want for a clean-ish booster. Dropping that cap value down to 47-68pf gets us into more reasonable territory, where SC pickup sparkle is retained, just not in excess.

So, the change in R10 and C11 would move the pedal into something much more in the range of a booster with just a touch of coloration (if desired) and plenty of toneshaping - certainly more than you'd have with a Micro-Amp, SHO, otr even a Klon Centaur.

The ideal, in some respects, would be to replace R10 and C11 with the aforementioned values, and wire up a DPDT toggle to strap a 150-180pf cap in parallel with C11, and a 2k7 resistor in parallel with the 12k unit. That would switch back and forth between stock Guvnor values, and our new semi-clean booster.