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Wednesday, July 8, 2015

Electro Harmonix Germanium OD

The EHX Germanium OD offers 60s overdrive tones using a combination of germanium and silicon transistors (all PNP). All 3 pots control the voltage is some way. The Bias pot controls the amount of voltage that reaches the Gain control, which in turn controls the remaining voltage that gets to the germanium transistor. The Volts control acts like a sag control on the overall voltage the circuit is running at, so you can emulate a dying battery. I've laid this out for on board pots mounted from the solder-side of the board using right angle PCB mount pots.



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14 comments:

  1. bassybeatsJuly 9, 2015 at 1:52 AM

    Love your layouts. Do you take requests..? Would be keen to see http://tagboardeffects.blogspot.co.nz/2013/02/madferret-germageddon.html

    I seem to bugger up big tagboard layouts like that haha

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    Replies
    1. dtgehringJuly 9, 2015 at 9:20 AM

      Thanks! I do take requests. Use the Layout Request form on the left and if you can, include a link to the schematic.

      Delete
  2. Marcello M.May 19, 2016 at 12:14 AM

    Hi. I tried this one but doesnt work. I used bc108 because I didn't find germanium transistor. Also I don't understand the led on the schematics. I link the positive on the board and another part? Sorry my poor english. And I love this blog.

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    Replies
    1. Effects LayoutsMay 19, 2016 at 12:44 PM

      It may not work because of the BC108s. You'll need a lower gain transistor. If you don't have access to germaniums something like the PN2369 or 2N5550 might work, but no guarantees. You would connect the LED pad to the positive (anode) side of the LED, and the negative (cathode) side to the footswitch, assuming you're using typical 3PDT bypass wiring.

      Delete
    2. Marcello M.May 19, 2016 at 12:54 PM

      Thanks for the answer. I´ll try with one of this transistors. If I don't plug the led for the tests, should it work? Also I used a B pot for the bias because don't find the C pot too.

      Delete
    3. Effects LayoutsMay 19, 2016 at 1:24 PM

      Yes it should work. It's just for on/off indication, and not part of the audio path.

      Delete
  3. milubitFebruary 24, 2017 at 10:22 AM

    does ac128 work instead nkt275?

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  4. andrea83April 24, 2017 at 11:44 AM

    Hi and really thanks for the interesting project! the 10k resistor connected to led is for a low current one?

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    Replies
    1. Effects LayoutsApril 24, 2017 at 11:47 AM

      Yes, though you can adjust that 10k value for whatever LEDs you have on hand.

      Delete
  5. andrea83April 26, 2017 at 7:27 AM

    Hi! The layout is working, I verified it! a good crunch overdrive but you can turn it to something experimental or to a very 60s fuzzy sound! cleans up nicely with guitar volume pot.

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  6. andrea83April 26, 2017 at 7:30 AM

    I've tried with several transistors. ac128 sounds nice to me, but HFE can really make a big difference (and transistor too obviously)

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  7. RodrigoMay 27, 2018 at 6:48 PM

    Thank you very much for this pedal, its working for me. I tried a few japanese germaniums, also an ac 121 and an ac 188. I also used a 100k lineal(b) for the gain pot. With this configuration, i can only turn the bias and gain knobs till 2 oclock, because beyond that point the sound fades away... do you happen to imagine whats happening? I will continue trying other transistors if i found any. Thank you very much again!

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