Duocast - Transistor?

P

playedincanada

Member
Alright,

So I've built this pedal multiple times. I've never had an issue like this, so I've come here to see if anyone can think why the pedal would act a particular way.

Everything works as it should.

Until you put it into 27V Mode.

Then, if the gain is up pretty high for the 1st side (which if I'm not mistaken is the Germanium Transistor), it will self-oscillate, sounding as if a "glitch mode" has been activated.

I think I have read elsewhere that if the hfe is too high for the transistor this causes these kinds of problems. It sounds fine in the regular 9V mode and doesn't do the osccillation thing then.

It's not present if you have the other channel engaged (including the silicon transistor)

Any insight would be greatly appreciated!

- Matt
 
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I also saw somewhere some specs were posted.

Do we have a definite answer as to what they are? I've always just worried about the hfe being somewhere between like 60 and 100, but I think someone had mentioned leakage as well?
 
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So indeed, it most likely would be the transistor. Which also makes sense because I believe it was my first time USING that specific Germanium Transistor. At 9V, it's fine....at 27V, not so much. I have others, so I will just use those. The page you provided indeed is a great resource, I just have a feeling I'll be scouring pages and pages of the datasheets depending on what the transistor is.

Thank you for your help, it puts a very old problem/question to bed, but it makes sense.
 
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What chip/IC for the charge pump? I had some noisy ones I bought cheap and they brought other problems, 7660SCPAZ, I think. That's probably not your culprit, but if you're not using the LT1054, it might have an effect.
 
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