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67 Ampeg Gemini I static

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  • 67 Ampeg Gemini I static

    Hello,

    1967 Ampeg Gemini 1 G-12:


    Im getting static noise through my speaker. Its usually not until the amp has been on for a while. Its definately static, and NOT hum.

    Any ideas of how to debug as to where its coming from? All the tubes are fine. The static will occur when volume is off, and all knobs are turned off.

    Would this indicate a leaking cap, where DC is leaking and becoming amplified through the speaker?


    All suggestions welcome

    thanks

  • #2
    Originally posted by tonefiend View Post
    Hello,

    1967 Ampeg Gemini 1 G-12:


    Im getting static noise through my speaker. Its usually not until the amp has been on for a while. Its definately static, and NOT hum.

    Any ideas of how to debug as to where its coming from? All the tubes are fine. The static will occur when volume is off, and all knobs are turned off.

    Would this indicate a leaking cap, where DC is leaking and becoming amplified through the speaker?


    All suggestions welcome

    thanks
    Check the 1M grid leak resistors in the phase inverter. If they are the grey film type resistors, i’ve found these to be particularly problematic(particularly in V4 models).
    You can isolate the problem by taking a HV rated film cap, 0.47uF of so, and shunting the plates and grids of each tube to see if you can isolate the problem.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

    Comment


    • #3
      To clarify, you’d be using the .47 cap as a probe. Due be careful not to short any nodes or shock yourself. One hand behind your back, etc
      If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

      Comment


      • #4
        I had the same issue on an old Supro amp, turned out to be... You guessed it, a grid resistor.

        Replaced it, and all is quite normal now.

        I believe a bad plate resistor can also make a very similar noise. Not sure in the Ampeg how things are laid out, but I would measure the resistance of both is what I would do, and while your at it, the other resistors. Anything more than 20% off spec you probably should replace, as it could be more than one thing at the same time, and it's better to only go in once to fix it.
        " Things change, not always for the better. " - Leo_Gnardo

        Comment

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