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Solid state problem - Fizz? Static? Distortion?

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  • Solid state problem - Fizz? Static? Distortion?

    I've just acquired a 70s Ampeg GT-10 - it's a small, champ-like 1x10 solid state amp with reverb. Pretty decent for a solid state. But this thing has a faint... fizziness on top of the notes. It's tough to describe (and therefore tough to google), so I thought I'd ask the experts.

    It's kind of a faint distortion sound on top of notes played. Almost fizzy. Static-y. Kind of like distortion, but the note itself is still clear as can be. It's like this sound is separate, or "on top" of the note. Does this sound familiar to anyone? If so, can you recommend some troubleshooting steps, or at least some more helpful google search terms?

    As for me, I've got experience doing simple repairs on tube amps, and I have a pretty general grasp of how tube amps work. This is the first time I'll be taking on a solid state project, so I figured this small (and hopefully simple) amp would be a good place to start. If this were a tube amp, I would check the tubes, clean all the sockets and pots and jacks, etc.

    My (mostly blind) guess is this will turn out to be a leaky cap somewhere. If that's the case, what's a good way to check for that, without a scope? If it helps, when I have the reverb all the way up, the fizz dwells as long as the reverb. So I would tend to think the fizz is being introduced somewhere after the reverb, right?

    I've attached a photo of the amps guts, if that's of any help. Thanks very much in advance.
    Attached Files

  • #2
    Originally posted by yaryaryar View Post
    It's kind of a faint distortion sound on top of notes played. Almost fizzy. Static-y. Kind of like distortion, but the note itself is still clear as can be. It's like this sound is separate, or "on top" of the note. Does this sound familiar to anyone?
    Possibility 1: Bad speaker. What's the amp sound like with a known good speaker? or 1A: foreign object in speaker frame, if you're lucky. Maybe something from France.

    Possibility B: insufficient bias current to output transistors. Sounds like bad speaker but problem will follow to known good speaker.
    This isn't the future I signed up for.

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    • #3
      Amen, first disconnect its speaker, and connect the amp chassis to some other speaker. Is the noise still there?
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        Here is the schematic.

        GT-10.pdf

        If the fizz is in the reverb circuit too, then the problem lies before the point where the reverb signal starts.

        The very first thing I would check is the power supply.
        Are the voltages correct.
        Is there very little ripple riding on the dc voltage.

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        • #5
          Thanks for the suggestions. I did have a chance today to try this through a known-good speaker. Same issue.

          So, if we've eliminated the speaker as the culprit, what do think is the next step? Leo Gnardo mentioned "insufficient bias current to output transistors". How do I check for that, and how is it remedied?

          Likewise for "ripple on the dc voltage", as Jazz P Bass suggested - how do I test for / fix that?

          Thanks again!

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