Increasing the gain on a marshall shredmaster

[gystratis]

Hi, I'm more or less ignorant when it comes to all things electronic so I was wondering if anyone on this forum would be kind enough to walk me through a simple mod to a Marshall Shredmaster to increase the gain? I use the pedal for a heavy scooped mids distortion and there's something unique about it's sound that I'm very fond of, but it could use a bit more drive. I was hoping there would be a very simple component switch that could be made?

I'm also curious as to what gives the pedal such an idiosyncratic sound as as far as I understand it the distortion is the result of a very simple one-stage diode clipping, yet it has that toothy roar that I tend to associate with tube amps rather than the fuzz/buzz that most pedals produce.

[WGTP]

Show us the schematic you want to work from, and we will figure it out.   

[gystratis]

Much appreciated, how about this:

http://www.harpamps.com/schematics/shrdmstr.pdf

[WGTP]

That will work.  Look at the R6 3.3k resistor to ground and C4 47n cap.  This controls the amount of negative feedback that is drained to ground and does not get back to the non-inverting op amp.  It creates a filter that theoretically rolls off bass at 1000hz at 6db/octave.  This keeps things from getting to bassy, wooly, fuzzy.  

Substituting a 2.2k resistor for R6 allows more negative feedback to escape to ground and increases the gain.  It also raises the bass roll off to 1600hz, so it acts as a treble boost.  To restore the original bass roll off to 1000hz you will need to change the C4 cap to 70n.  These 2 parts are the key to the "character" of the distortion.  I would use a socket for both.  You may want to increase the bass or treble and these 2 parts will do so pre-distortion.  It sounds like you are OK with it, so I would start with R6 2.2k and C4 at approximately 70n.  

If you want more try a 1.5k resistor and an 82n cap.

This same process applies to most op-amp based distortions.  Hope that helps.  

[gystratis]

OK, that doesn't sound too complicated.

Here's a question that may well be too vague to answer, I'm wondering what it is in pedals that's responsible for the 'toothy' roar/growl that is synonymous with a modern, high-gain sound, and whether there's a way to augment that aspect of the shredmaster's sound (perhaps increased gain will have that effect, orthe opposite?). I'd thought that in tube amps it was the multiple gain stages that produced that sound but the little I know about diode clipping in pedals seems to contradict that idea. It appears to be the quality that's hardest to model in the digital world, i.e. digital simulations tend to lean towards fuzziness/fartiness. I've just been looking at a lot of fairly new pedals such as the Barber Dirty Bomb and they seem to do that high-gain gnarled roar really well, but there's still something about the tone of the Shredmaster that makes me want to continue to use it, I think it's something to do with the way the upper mids get saturated, it's a sort of 'chiming' sound. But I've no idea what's responsible for these qualities. Anyhow, if there's any way to improve on the shredmaster's 'toothiness' then that would be great.

[WGTP]

That's a good question.  Part of the Mojo.  I suspect it is both EQ and Dynamics.  I saw an older schematic of the Shredmaster that had LED's rather than the 1N4818 silicon diodes in the schematic you posted.  There are many possible diode combinations that would work in this circuit and effect the dynamics, compression, distortion part. 

It has lots of filtering after the distortion that can also be tweaked if needed.  I think the EQ part is underrated and should be addressed more in many designs.  Many folks have suggested reducing the bass before the distortion and boosting it afterwords (Mr. Barber for one) and I find this helps create a better distortion IMHO with less of the fuzz/farting you mentioned.

The 2 op-amps in this design are probably distorting before the clipping diodes giving you 3 stages of clipping, especially if you increase the gain.   

[gystratis]

That's interesting as I've often thought that the Shredmaster I currently have doesn't sound as good as the one I had back in 1991, but I put that down to nostalgia/rose-tinted specs. I didn't know there were variations in componentry.

[WGTP]

http://www.gmarts.org/index.php?go=217  scroll to the bottom.  I made a mistake.  It was the DriveMaster that had the LED's.  Sorry.  It's not uncommon for parts values to change over time, plus other variables like guitar or amp, so it's hard to say.   

An easy way to make this mod is to get another 3.3k resistor and 47n cap and solder them in parallel to the ones on the circuit board, automatically halving resistance and doubling the capacitance.   

[gystratis]

Would those two approaches to the mod result in the same alteration of the sound? Or is the first likely to cause some sort of slight timbre change due to the frequency roll off involvement?

[WGTP]

They should be about the same tonally.  Due to rounding error and visual error simulating with the Duncan Tonestack, the values are off a little...   

[gystratis]

The 2 op-amps in this design are probably distorting before the clipping diodes giving you 3 stages of clipping, especially if you increase the gain.   

Am I right in thinking that the character of the distortion will then depend on the amount of clipping occurring at each stage. . . i.e. if I want to maximise 'gnarled roaring toothiness' or something like that I might achieve that by increasing the clipping at a particular stage rather than increasing the gain overall? Or perhaps, like you say, by removing some of the very low frequencies prior to clipping?

[WGTP]

Now your getting into "tuning" territory that doesn't have standard answers.  The answer is yes, but how much difference you can hear and the effect it will have, is hard to say.  I have messed with tweaking the distortion level of the different stages a lot and and used different types of stages and at bedroom levels its hard to make any definitive statements. Depending on bass content and treble roll off, type of stage, the stages will sound different.  How that contributes to the overall tone is hard to say.  You could boost the second op amp by changing the 680k resistor to 1M, but that will also roll off more treble because of the relationship between that resistor and the cap in parallel.  I usually try to make sure the first op amp has enough gain to distort on it's own, so the second op amp doesn't have to do all the distortion itself.  But, I also usually have diodes in the FBL.  You just need to experiment and see what you like...   Have you searched for other mods?  I suspect there are many.