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Vintage threads from the first ten years

Re: MuTron III schematic? Transistors??

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2/8/1997 6:04 PM
R.G. Keen	Re: MuTron III schematic? Transistors?? I guess this question is really about your busted Big Muff pedal. If you're just starting, you might go read the Guitar Effects FAQ on my web page at http://www.eden.com/~keen.

1/24/1997 2:31 AM

Re: MuTron III schematic?

i think the colors on the cap are like the resistor code. the yellow is 4, the violet is 7, and the green is 5 and is the multiplier. it is probably either a .047, .0047 or .47uf. i think it's the .047uf. the op amps are duals and the circuit may not work if these are substituted w/ op amps w/ dissimilar input transistors (fets, bipolar) and frequency compensation. i don't know what the other cap might be but there is a cap meter made by Phillips that Antique Electronics in Tempe Az. carries. it's around 100 bucks, maybe less but a great bench tool. good luck.

2/1/1997 12:00 PM

Given the nature of this particular beast, I wouldn't be surprised if the
metal can device is a CA3080 transconductance amp, which is often used for
envelope and voltage-controlled filters that do more "tricks" (which the
Mutron does). Manufacturers rarely use metal-can op-amps when they don't
have to, and my own buying experience at the time that Mutron III's were
in production (mid-1970's) was such that it was hard NOT to get metal
can 3080's. You can identify if it is such an entity in a relatively
reliable manner as follows:
- if pins 1 AND 8 (the little "tongue" protruding from the can points to
pin 1) are unused, then the device is more than likely either a 3080 OR
a 741 single-op amp equivalent (741, 301, 308, etc.), since pins 1 and
8 would be used on a dual op-amp
- if pins 1, 8, *AND* 5 are unused, then the odds are high that it is a
single op-amp of the 741 type
- if pins 1 and 8 are unused but pin 5 *is* used, then this is likely a
3080, since pin 5 is used to control the gain of the device

A second way to verify that this is a 3080 is that they often employ
small trimpots situated near them for fine adjustment of distortion
characterics.

Am I *absolutely* confident? No. But I'd be very surprised and
enlightened if it wasn't what I suggested.

Good luck with the schematic. I'd sure love to have one.

2/1/1997 1:06 PM

Hey, thanks alot for the reply; very helpful info. I'll let you know when I feel confident that I've got a good schematic, and make sure you get one.

I am somewhat dubious at this point as to whether I will be able to pull off recreating this effect, after having talked with the owner of a local music shop that carried the "officially licensed" reissue. He claimed that it sold poorly, due to the mfg.'s inability to get original parts, or close effective equivalents. He also said that the guy who attempted it had the oppurtunity to acquire some of the more significant parts (I believe he referred to the op-amps...), but only if he bought in huge quantities, which he decided against. The net result was that players were disappointed by the fact that they didn't sound like the original - so much so that this dealer had to send many back to the mfg., and severely discount the rest.

Apparently the failed entrepeneur then went on to successfully market dog collars that had electronic tracking devices. The music store owner suggested that he apply the same concept to musical instruments. Who knows, maybe it'll fly. I know I've had enough stuff stolen from gigs...

Anyway, thanks again, and I'll let you know how it's progressing.

Craig

2/3/1997 5:01 PM

Actually, the original MU-Tron III's didn't exactly fly off the shelves at music stores either. It just wasn't the sort of thing that appealed to most guitar players in the 70's, being more of a "novelty item" than "something everyone had to have."

Don't be discouraged because of what the music store guy said. There are no "magic parts" in the MuTron III. You may not be able to obtain the exact same opto-isolator they used, or "vintage 4558 op-amps" but you can get close enough with presently available parts. In fact, you can build a better one if you have a mind to. The original has some serious flaws, not the least of which is a very low input impedance that loads down guitar pickups. But then maybe that's considered a part of the "vintage sound", in which case you could always make it a switchable option.

There are three basic circuits in the MuTron: A preamp, a 2-pole state-variable filter, and an envelope follower (full-wave rectifier with filtered output). The follower drives a dual opto-isolator, who's varying resistance is used to sweep the filter frequency. Examples of these circuits can be found in any good book on op-amps. If you can find a copy of Craig Anderton's "Electronic Projects For Musicians", just look at his "Super Tone Control" project, and his notes later in the book on how to combine it with an envelope follower and opto-isolators to make an "envelope_controlled filter", which is what the Mutron III is.

2/3/1997 5:14 PM
Craig	Thanks for the de-mystification on the MuTron; I actually have the Anderton book, and will definately give it a look. Thanks for the suggestion on the mod too.

2/3/1997 3:05 PM
Steve Morrison	I don't know how many revisions there were on the Mu-Tron III, but I've never seen one that used ota's. The ones I have seen used a dual opto-isolator. This is the black nose-cone-shaped thing that I mentioned in an earlier post. I drew a complete schematic of one many years ago, and if I ever run across it again I'll make sure you guys get copies.

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