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Ampeg Gemini G-12 Reverb Hum

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  • Ampeg Gemini G-12 Reverb Hum

    Hey y'all, I'm trying to see if I can reduce the hum in the reverb circuit on my Ampeg G-12 Gemini. It's always been there and is live-with-able, but I'd be great if it wasn't there.

    Here's how it behaves:

    -Reverb pot directly affects hum volume, 0 = no hum, 10 = lots of hum
    -Disconnecting reverb tank eliminates hum
    -Removing V7 eliminates hum

    -Toggling footswitch does NOT affect hum
    -Removing V6 does NOT affect hum

    One thing I noticed is that the schematic appears to show a ground lead from both sides of the reverb tank. One of the leads from my reverb tank has the shielding wired to ground, but on the other one the shielding is just trimmed off. There appears to be a through hole to ground adjacent to the reverb signal line that was never soldered? However, when I tried running a jumper from the cable shielding to ground it didn't have any impact on the hum.

    Image of footswitch and reverb tank connection to board:
    https://firebasestorage.googleapis.c...2-9484293a158f

    Full schematic:
    https://www.drtube.com/schematics/ampeg/g12-jp.gif

  • #2
    Oh, I should add -- I've also tried a bunch of different 12AX7's in the V7 position and this hasn't made a significant difference.

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    • #3
      Try disconnecting the reverb leads one at a time with the Reverb control at zero.
      Is it a Send or a Return problem?

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      • #4
        Actual Ampeg schematic attached.
        Have you tried moving the tank around to see if the hum is affected?
        Attached Files

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        • #5
          Originally posted by Jazz P Bass View Post
          Try disconnecting the reverb leads one at a time with the Reverb control at zero.
          Is it a Send or a Return problem?
          Thanks for the suggestion - there's no hum with the reverb control at zero, regardless of whether the tank is fully or partially connected with either lead. With the 'Out' lead disconnected there is no hum, even when the reverb knob is turned up. The 'Out' lead is runs through some circuitry to pins 7 and 8 on V7, if that helps identify it.

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          • #6
            Originally posted by g1 View Post
            Actual Ampeg schematic attached.
            Have you tried moving the tank around to see if the hum is affected?
            Thanks! Actually, I think that the Piazza schematic is more accurate to my exact model... or this one: https://ampeg.com/support/files/Sche...matics%202.pdf . The amp's a late 60's 'G-12,' with early V-4 / SVT style blue lettering / black tolex, printed circuit board, etc., not the earlier diamond-check 'G-12 Gemini I'. It looks like there's a bunch of minor differences in the circuit.

            I haven't tried moving the reverb tank around, but moving the chassis around relative to the position of the reverb tank doesn't seem to do anything to the hum.
            Last edited by keganheiss; 06-27-2022, 01:48 AM.

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            • #7
              If the footswitch does not kill the hum, but disconnecting the 'out' connector does, then it seems the hum must be being picked up by the tank.

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              • #8
                Huh, yeah, that makes sense. Just read up a bit about different reverb tank grounding / isolation schemes... it seems from the schematic that this amp should have the input and output grounded, but the original circuit left the input isolated, even though there is a spot for a ground lead. I tried wiring the shielding from the input lead to the empty grounding post but it didn't make a difference. I'll try unscrewing the tank and moving it around next.

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