I have been trying to get the sound Frank Zappa gets on his "Penguin in Bondage" solo forever. That's one of my favorite recorded guitar solos of all time, it's on his "Roxy and Elsewhere" CD, during the "golden age of the Mothers" era....he always had great bands, but this was the greatest: George Duke on keys (say no more!), Ruth Underwood, Napoleon Brock, the Fowler brothers, and on and on.... Anyway Frank has a bodacious lead sound on that tune, a sort of liquidy envelope follower sound, and a guitarist I know said that Zappa used a Mutron III to get that sound. I remember messing with one at UC Santa Cruz in college (back during ancient times) and loving the damn thing, but not having the dough to buy it.
So here I am a few decades later (!!) and still not willing to part with the hundreds of dollars to get a vintage Mutron III. Besides, you can't get the exact opto it used, so better hope that priceless component doesn't blow up, lest the unit will be rendered useless. And those cool Mini-Moog type switches wear out, I think those can't be replaced either. And like all vintage effects you take to your beer-soaked bar band gig: you'd better hope and pray the drummer doesn't barf and nail your Mutron III....say he does, then you open anything by Musonics, being that the drummer barfed in it and you need to pick out all the puke, the thus the vintage gear loses it magic mojo; whatever, you get the idea. It sounds like a good candidate for cloning. So I bought the board from general guitar gadgets and off we go.
To get right down the meat of it, I never could get the thing to sound the way I remember it, especially in "down" mode, where the envelope is inverted. It just doesn't have the beefy sound I remember the Mutron III's of old have. The "Low Pass" setting is OK, but the bandpass and hipass are very thin and raspy sounding, to where they are not that useful.
So I spent about $70 in parts (not a big deal) and about 12 hours building it (big deal--I could be playing instead of messing with a clone project). The clone works, but sounds really bad. Marvey. After 12 hours I wasn't going to just throw the unit away. Get out those jumpers and junk-baggie of resistors and start modding!! What I tried: I tried all different values for Rx and eventually used a 10K trimmer. Rx interacts with R18/R19 for the "down" bias, so I ended up putting a 100K trimmer in instead of R19 and tweaking it so the "down" and "up" pretty much matched, sweep wise. And the box had a horrible "smoothness" problem during the envelope decay--the cutoff frequency was unacceptably jittery during long sustained notes. Putting in a 22uF cap for C9 which made the decay smoother but the attack longer. The "down" feature, even when biased up right, doesn't sound as good as I remember the original sounding--it's not very useful musically to my ears anyway. Finally, for me, the gain part of the circuit (first op amp) has a lot more gain then I needed--even for my guitars and whatnot with wimpy pickup power-- and thus the interesting sounds are with "gain" between "off" and "noon", so the box definitely did not go to 11. I thus put a 47K resistor in between the center lug of the gain input pot and ground, which effectively lowered the gain-making capability of the first stage of the circuit, which made the "gain" knob a bit more usable.
I used the "9V conversion" option; as per email I got from RG Keen, I got rid of the 7660 power converter, used a pin-for-pin compatible MAX1044 instead, and tied the MAX1044 pin 1 to V++. Otherwise I got a hideous 10K whine that was especially annoying when the box was set to "high pass" mode.
What next? Well I guess I should look at how well matched the resistors are in the filter stage--I used the $6-odd single LED/dual Photo can from smallbear for the opto. Perhaps there is a big non-linearity in this particular can, I don't know. But I feel I am out of time on this one. I need to spend more time doing music and less time soldering. Better for the lungs.
Even though it's not all I hoped, I think the Mutron III clone came out OK--not great, but OK. If you want to hear how it sounds, check it out: I created a couple of demo tunes using Ableton in my crappy little project studio. The clav, synth, and guitar sounds are all processed through the Mutron III that I built. Here is one; here is the other.
So would I recommend building a Mutron III clone? Dunno. Looking at the original schemo and the one on GGG, the clone, at least parts wise, is almost identical to the original. But it just doesn't sound as good as I remember. Definitely not ready for ripping into "Penguin in Bondage". So probably not. Better to find an autowah you like, sound wise, and just buy it, IMO.