68 Custom Princeton Reverb - hum

Discussion in 'Glowing Bottle Tube Amp Forum' started by NateD81, Sep 15, 2019.

  1. NateD81

    NateD81 Tele-Meister

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    Hi all—

    I’ve read what feels like every post I can find about the CPR having him when turning the reverb past 3 on the dial. I too am having this issue and looking to get some advice moving forward.

    First of all, I got the amp used and really love it sans the reverb hum so I can’t return it.

    What I’ve done:
    Retubed the amp with TAD, Gold Lion and Mullard tubes (except for the rectifier tube).

    Bought a better Reverb cable
    Swapped the tank 180 degrees
    Took the tank out of the unit to see if it was being affected by anything in the amp

    When I unplug the tank, I get no gum at all.
    When I use the footswitch to disengage the reverb, then turn the nob, no hum happens.

    My thinking is that this seems like a Reverb tank issue since the hum is not present when on 10 and the tank is disengaged.

    Would you guys suggest a different Reverb tank? If so, what brand?
    Should I have a tech look at modifying something?
     
  2. Uncle Daddy

    Uncle Daddy Tele-Holic

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    Have you checked the nuts on the reverb plug sockets?
     
  3. NateD81

    NateD81 Tele-Meister

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    I tightened them up, in addition to the input sockets - should I pull them out to make sure they’re soldered correctly?
     
  4. jimash

    jimash Friend of Leo's

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    Oops you already did that.
     
  5. stefanhotrod

    stefanhotrod Tele-Meister

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    The reissues run with a 12ax7 in V2, better swap for the correct 12at7.
     
  6. 1300 E Valencia

    1300 E Valencia Friend of Leo's

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    Here's what the tubes in my '65 PRRI do:
    V1 (12AX7) input preamp, post-tone-control make-up gain

    V2 (12AT7) both triodes are used to drive Reverb

    V3 (12AX7) one triode boosts the output of the Reverb tank, other is used to mix the dry and reverb signal and add some gain

    V4 (12AX7) one section is the trem oscillator, other section is the phase inverter

    V5,V6 (6V6) power tubes

    V7 (5AR4) rectifier
     
  7. NateD81

    NateD81 Tele-Meister

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    I’ve got the same set up tube wise, but getting the him regardless. I’m wondering if it’s something on the board, or maybe the tank?
     
  8. NateD81

    NateD81 Tele-Meister

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    When I retubed, I put a TAD 12at7 in and maybe not perceivable difference unfortunately.
     
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  9. stefanhotrod

    stefanhotrod Tele-Meister

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    I would try a different Accutronics reverb tank. They aren‘t expensive.
     
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  10. knockeduptele

    knockeduptele Tele-Meister

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    on the export amps Fender wind the reverb cables through a choke ring 5 times to meet RFI spec - it seems to make a considerable difference for this specific problem
     
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