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Reverb Hum in Gibson GA-19 RVT Falcon

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  • #1

    Reverb Hum in Gibson GA-19 RVT Falcon

    Several months ago, a friend brought me a basket- case Falcon to repair.I believe it was made in the early/mid 1960s.

    http://www.webphix.com/schematic%20h...ps/ga19rvt.pdf


    Among other things the reverb transformer was blown so I replaced it with a standard Fender reverb transformer.I also changed the cathode resistor in the reverb recovery triode to 1K.
    While I was working on it I also replaced all electrolytic caps,beefed up the power supply and did general routine servicing.Changes were made to the input stage to make it more like the earlier tweed model and the dreaded PEC network was removed.
    The amp ended up sounding very good and everybody was pleased.

    The amp has developed a hum that is present only when the reverb circuit is engaged by the footswitch which is hard-wired. Otherwise, the amp is very quiet.
    I've swapped tubes, substituted a known-good tank and checked the footswitch. The reverb send and return cables are also hard-wired.
    The noise goes away when V2 (6EU7) is pulled but stays when V3 (6C4) is pulled leading one to think the problem involves the reverb recovery circuit.Pulling V1 (6EU7) has no effect on the hum.

    Any suggestions or insights sincerely appreciated.Thanks,

    SG
    Tags: None
  • #2
    Have you checked continuity of the send return grounds from the tank jacks to the inside of the amp? That'd be my first go-to.

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    • #3
      Hum won't travel through the pan, so we expect pulling the drive tube not to help.

      Isolate the problem. The reverb FS is a long wire that connects directly to the signal path and trails outside the chassis. Disconnect the hot lead of the footswitch right at the circuit. Now it cannot act like an antenna. Any difference?

      I don't know why you changed R28 from 2.2k to 1k, but the grid has a 100k R19. Tack a piece of wire across it. We are grounding the grid. DO it right at that resistor, not a clip wire to chassis from the grid pin. Kill the hum?

      0.01uf C23 is the output from the reverb recovery, is it leaking ANY DC voltage at all?

      Does this schematic match your amp exactly? I have several different GA19 schematics.
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        Lowell and Enzo,
        Thank you for your helpful suggestions .
        Things have gotten busy around here so I haven't had time to work on the amp. Will report back later. Again, thanks.
        SG

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        • #5
          Lowell and Enzo,
          I went through your suggestions and the amp now works beautifully. No more hum.
          I found slight leakage on C23 .01 (ceramic), so I replaced it.
          All relevant grounds and connections were checked and reflowed in the process of performing the tests you suggested.
          When I was finished the amp worked.
          Sure beats throwing parts at it.

          Thanks again, SG

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