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Fender '68 Custom Twin Reverb Hum

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  • Fender '68 Custom Twin Reverb Hum

    I've deferred starting this thread hoping to find an answer somewhere online and while it seems to be a common problem, I could find no suggestions that made a difference. Here is what I know:
    • This is a brand new Fender '68 Custom Twin Reverb
    • It has a low level hum (more like a buzz) that is only affected by the reverb pot.
    • Removing V4 eliminates the hum.
    • Replacing V4 with another tube does not make a difference.
    • The wires coming off of Anode 1 & 2, and Grid 1 (pins 1,6 and 7) on V4 are microphonic.
    • R26 is slightly microphonic.
    • The wire leading to the + of the white RCA jack (output from reverb tank) is microphonic.
    • Hum persists whether reverb tank is connected or not.
    • Clicking the reverb off with the footswitch does not eliminate the hum but makes it not respond to the reverb pot anymore.

    All of this seems to point to a reverb recovery problem but I'm at a loss on what to do. It sounds like a ground problem. There is a wire coming off the board with the RCA jacks on it that goes to ground but it doesn't seem to make a difference whether it's grounded or not.
    Last edited by jeffects; 05-27-2015, 08:47 PM.

  • #2
    The hum balance pot quieted it down quite a bit. I didn't think it would make that much difference.


    • #3
      It is a brand new amp, so it has a brand new warranty along with it. If this is really a problem, then take it to the nearest fender warranty shop for repair.

      The sensitive reverb recovery stage is amplifying some hum. Removing the tube that does the amplifying will kill the hum.

      If you wish to experiment with grounding things, ground them to the bottom of the cathode resistor of the reverb recovery stage, NOT just to chassis.
      Education is what you're left with after you have forgotten what you have learned.


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