Wednesday, 2 March 2016

Electro Harmonix '75 Unknown Muff

Here's a layout for a big muff from a schematic from October 15, 1975. It's known as the mystery muff. Should be a nice addition to any muff collection.


The only deviation from the schematic is that I added the mids switch, I changed the 1uF caps to polys instead of electrolytics, and added a polarity protection diode. The transistors are unknown so I looked at the schematic for muffs of a similar vintage and they used BC239C's, but you can and may use more common Si transistors like 2N5088, 2N5089, etc. The diodes are also unmarked, as they were unknown from this vintage so I marked them as the common 1N914/1N4148's, but you can try other Si diodes.

30 comments:

  1. Ah, nice, Zach,... other than the addition of the switch is the parts list the same as the Tone Report one we were discussing? What kind of switch? Thanks.

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    1. Looks like a DPDT on-off-on switch would work.

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    2. Mahouts have been in the build notes. It's a dpdt on-off-on. I'll add tahat when I get home.

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    3. Would you not want a dpdt on on? An on off on would have no capacitor in the middle position. I could be wrong however...

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    4. As it should be. Center off would be the stock Muff scooped sound. Either of the two on positions would put a 2nd cap in parallel to the 4nf cap in the tone section. Creating the flat and boost settings. I LOVE that switch. It literally is all the difference between barely being heard in a band mix and cutting through like a knife. Although I rarely have the need to go beyond flat.

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  2. Sounds like a big muff. It's hard to say how it exactly sounds since it's a schematic for a mystery big muff model. Some of the values make it look like a tweaked violet ramshead. Also, even with the schematic no two big muffs of the early generations sounded the same due to constant subbing of parts when they were built.

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  3. With all those 1uf caps though chances are it will be a bit on the bassier side of things. I doubt it will be as crisp and clear as a Violet Ram or a Triangle. Probably closer to the V9 reissue. The only way is to build it and find out. Which I look forward too.

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    1. True. From what I see in different schematics it looks like a cross between a violet ramshead and a V6 big muff, not so much the newer V9. But that's just from looking at the schematics real quick over at kitrae.

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  4. Built it and I'm getting a VERY weak output signal. Effect when engaged sounds like 25% volume of the clean bypass signal. Anyone else having this issue? (I used BC550 transistors, and yes they have the correct pin-out, and I skipped the scoop/boost switch and went right to the Tone pot. Otherwise no mod's and built like the above layout.)

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    1. not sure if anyone else built this yet so i have no idea. when you say you skipped the scoop/boost switch and went right to the tone pot, do you mean you took wire the should run to the switch on the board, SW1, and ran that to tone pot lug 3?

      i looked over the schematic and the board, and they match. i also checked the schematic and compared it to another generation muff, and there's nothing strikingly different that would lead me to believe it will have lower output. also, check for any build errors, scrape the tracks and make sure there are no solder bridges. look for other common errors.

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  5. I traced it down to the 8.2k res. Not sure why but it's fine before that and then drops off after. Tone stack section is super quiet. Final recovery stage is still quiet. But if I prove it after the second gain stage it sounds fine.

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    1. *probe it

      And yeah I've built a bunch of tagboard effects and stuff on PCB, so I checked all the usual troubleshooting stuff like 3 times before commenting here. Also tried with BC338 and BC550 with same results.

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    2. hey matthew. i'm still thinking there's an error somewhere in your build. if you compare the schematic for this "unknown" muff with any other muff like the green russian muff, what's an 8.2k resistor in this build is a 10k. the lower emitter resistors, 100r in this layout, should increase gain and output.

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    4. the 22k resistor is correct. if you look at the schematic you'll see it's in series with the 1uF cap into the base of Q2. that's not the problem. the 8.2k is in series with a 1uF cap that connects the collector of Q2 to Base of Q3.

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    5. There's only 1 22k res on the original schematic, and the schematic calls for 2x 8.2k res, and your layout only has 1. That's at face value. I swapped out the first 22 you called for in the layout for an 8.2 like it says in the schematic and there's still the same significant volume loss. But that schem is hand-written and I could see how it'd look like a 22. Anyway, I'm still having the same issue with it, I've gone over it like 4 or 5 times now, so I'm just going to build the schematic out on a Mudbunny board once it shows up and see how that fares.

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    6. hey man. check the schematic again there are 2 22k resistors and 1 8.2k. there's a 22k from tone 3 to ground, marked R5, and the 8.2k resistor is the one mentioned above and marked R12 on the schematic. i see what you mean that it can look like 8.2k as well as 22k, but i don't think that's the problem.

      you mentioned that you have good signal up to the tonestack and the gain recovery is also low. i think there's an issue with your build in the area with those components, not earlier. i still think there's an error in your build not the layout, as it's just one of the verified muff layouts with the values modified to fit the the '75 unknown muff schematic. it could also be that the values in the schematic could be wrong.

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    7. For what its worth in the Mystery Muff vero that was in Tone Report the 22k resistor in series with a 1uf cap into the base of Q2 was a 8k2.

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    8. I'll make the change in value, it's no biggy. Like I said I can see how it looks like either value. I still can't see that being the issue mentioned above, do to the place where the signal is lowered.

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  6. If one were to wish to split the tone section into separate treble and bass controls, how might that be accomplished?

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  7. You'll have to remove the tonestack and replace it with one that's treble and bass, like a bandaxall tonestack. There's no may to really do it by keeping the stock tonestack.

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  8. Well, right. I guess I mean to ask how would you replace it the existing tonestack with something like a baxandall...

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    1. exactly as i described. you're going to have to remove the components of the tonestack, check the schematic to see where they are, and put in the components for a bandaxall. you want to leave the gain recovery stage, Q4 and it's assorted parts.

      i can't just tell you where to place them cause it would require a making a new layout.

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    2. No problem. Thanks for the reply! I'm new at this and wouldn't have a clue how to change the vero layout even though I can identify the section in a schematic. I'm sure I'll figure it out eventually.

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  9. I built this and have exactly the same issues as matthew. Really low signal. Matthew did you sort your problem out?

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    1. Found the problem on my build, the Gain pot was dodgy. This a great sounding muff.

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    2. great. i'll mark it verified. good job man.

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