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Gibson GA-15RVT oscillation? Strange one!

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  • Gibson GA-15RVT oscillation? Strange one!

    Ok. Gibson GA-15RVT. Schematic attached, and it seems to actually match! Rare for these!

    concern. When playing with volume and reverb on. sound cuts out after hard strumming and a min or so… disconnecting reverb tank no change, changing 12au7 and swapping the two 6EU7 (didn’t have any spares on hand so just checking obvious stuff, or trying to. So, while feeding a signal.. bringing up the volume towards max and slowly down will start to make the sound go away.. it’ll keep coming back, if you keep rolling it down slowly the sound will continue to be cut out, always trying to come back though. I can only assume a parasitic oscillation as I’ve never experienced anything else quite like it. I can probe with the scope and watch the signal diminish. I can get the signal to cut out by either leaving volume on amp at max and adjusting amplitude of signal generator up and down, or leaving generator up and using amp volume. I’ve chopsticked all the socket wires, trying to ensure grid and plate wires are apart etc. And thought before I go any further down the rabbit hole.. I better ask the legends on here that may have already had this experience and can shed some light on this for me.
    I had to step away from it this evening as I wasn’t seeming to make any progress. And that’s always a good time to step away and ponder I find.
    Attached Files

  • #2
    My first thought would be an intermittent vibration sensitive connection. I'd try chop sticking around in the unit, see if you can duplicate the symptom, and maybe narrow it down to a particular connection.
    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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    • #3
      Originally posted by The Dude View Post
      My first thought would be an intermittent vibration sensitive connection. I'd try chop sticking around in the unit, see if you can duplicate the symptom, and maybe narrow it down to a particular connection.
      i would think so… but this is doing it into a dummy load.. reverb tank disconnected.. only when reverb is on. so no vibrations to deal with.
      (odd speaker driven setup and transformer PI)
      crosstalk was first guess. Initial design is a bit of a mess anyway layout wise from the board… but still. Shouldn’t be doing this.

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      • #4
        Originally posted by Indyryder02 View Post
        reverb tank disconnected.. only when reverb is on. so no vibrations to deal with.
        (odd speaker driven setup and transformer PI)
        I know Gibson did some speaker driven reverbs, if that is what the above "speaker driven setup" means.?. But the schematic is not that circuit. Since the problem seems associated with the reverb an accurate schematic would help. This if the above schematic is not actually accurate for the reverb circuit.
        "Take two placebos, works twice as well." Enzo

        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

        "If you're not interested in opinions and the experience of others, why even start a thread?
        You can't just expect consent." Helmholtz

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        • #5
          Originally posted by Chuck H View Post

          I know Gibson did some speaker driven reverbs, if that is what the above "speaker driven setup" means.?. But the schematic is not that circuit. Since the problem seems associated with the reverb an accurate schematic would help. This if the above schematic is not actually accurate for the reverb circuit.
          my mistake.. confusion when reading about these. The schematic above does match what I have though as far as I’ve seen.

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          • #6
            Originally posted by Indyryder02 View Post
            odd speaker driven setup...
            Could you post pics?

            - Own Opinions Only -

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            • #7
              Originally posted by Indyryder02 View Post
              I can only assume a parasitic oscillation as I’ve never experienced anything else quite like it. I can probe with the scope and watch the signal diminish.
              If there was oscillation you should be able to see it on the scope.

              - Own Opinions Only -

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              • #8
                Originally posted by Indyryder02 View Post

                When playing with volume and reverb on. sound cuts out after hard strumming
                So it doesn't happen with reverb switched off?


                Last edited by Helmholtz; Yesterday, 01:02 PM.
                - Own Opinions Only -

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                • #9
                  It WAS indicated that this was ONLY with the reverb on. I assume that means it doesn't happen with the reverb off since the word "only" was used.

                  I'm suspecting a poor ground or failing/bad decoupling cap causing interaction between circuits. But it's too early to call it.
                  "Take two placebos, works twice as well." Enzo

                  "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                  "If you're not interested in opinions and the experience of others, why even start a thread?
                  You can't just expect consent." Helmholtz

                  Comment


                  • #10
                    The reverb footswitch grounds the node between R15 and C15 for reverb off.
                    So the question is what does this change for the dry signal path.
                    If for instance C15 is leaky shifting the V2A bias, turning the reverb off would stop that.
                    In that case V2A cathode voltage would change with the reverb switch setting.

                    Also if V2A doesn't have a good grid leak path, R15 would provide that with reverb off.
                    - Own Opinions Only -

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