Reverb problem - 12AX7 Tube help, please

Discussion in 'Ampage Area' started by Btr6120, Apr 4, 2019.

  1. Btr6120

    Btr6120 Electromatic

    84
    Feb 19, 2016
    Vancouver, BC Canada
    The reverb on my Princeton Reverb Reissue was overpowering when I got it. Never played it above 3. The amp had a 12AT7 in V2 and 12AX7's in the V1, V3, and V4.

    When it came back from the tech after some hum-reduction improvements, I put a 12AU7 in V2 (I think my reasoning was to tame the reverb a bit). Also replaced the JJ 12AX7's with modern Tung-Sols. Played with it like that for a year. Then I noticed how weak the reverb was getting. In fact, full out at 10 it was downright puny. So I put the original 12AT7 back in V2, but there was hardly any change -- still a really weak reverb, so I'm thinking the tubes aren't the problem. The other 12AX7's in the amp are all near-new. I checked the reverb tank and cables and all looks good there.

    As an experiment, I put a 12AX7 into V2, and now the reverb is back to about 80% of the original depth, so I am happy. But will that 12AX7 in V2 cause any problems inside the amp? Thanks for any help.
     
  2. wabash slim

    wabash slim Friend of Fred

    Age:
    69
    Feb 10, 2010
    lafayette in
    All of the 12A_7 tubes are in the same family, and are all interchangable. The higher in the alphabet the letter, the higher the gain, The 12 AT7 should be the lowest gain of the three.
     
    mrfixitmi likes this.
  3. Hammerhands

    Hammerhands Country Gent

    Aug 26, 2011
    Winnipeg
    The 12AT7 can do something the other tubes can’t, it can handle more power.
     
  4. Btr6120

    Btr6120 Electromatic

    84
    Feb 19, 2016
    Vancouver, BC Canada
    Thanks for the info. I'll keep the 12AX7 in V2 and ignore the mystery as to why the reverb isn't super-extra-deep compared to the original.
     
  5. TeeDub

    TeeDub Synchromatic

    624
    Jun 21, 2011
    Colorado
    I believe all Fender amps specify a 12at7 for the reverb driver. Amplification factors for the 12a_7 tube family:
    • 12ax7 - 100
    • 12at7 - 60
    • 12ay7 - 44
    • 12au7 - 17
     
  6. Rckt-Brew

    Rckt-Brew Electromatic

    17
    May 25, 2017
    Trabuco Canyon
    Check your springs.

    I received a VibroVerb reissue a year ago. I thought the reverb was weak/pathetic. Problem was 1 of the springs had broke off in the tank. I replaced the tank and all is well.

    OK, just reread OP and saw you checked the tank.
     
  7. Ricochet

    Ricochet Gretschified

    Nov 13, 2009
    Monkey Island
    Isn't V3 the reverb tube, and isn't it supposed to be an AT7?
    V1 and V2 are AX7...
     
  8. Btr6120

    Btr6120 Electromatic

    84
    Feb 19, 2016
    Vancouver, BC Canada
    V2 is reverb send, and V3 is ½ reverb recovery and ½ reverb mix, according the the PRRI chart.
     
    Ricochet likes this.
  9. somebodyelseuk

    somebodyelseuk Country Gent

    Jan 22, 2013
    Birmingham, UK
    Saw an 'Uncle Doug' video on 'the 'tube' a couple of weeks ago where he diagnosed and fixed this kind of issue.
    If I find it, I'll post the link, but from memory, it involved a resistor replacement, not valves.
     
  10. Ricochet

    Ricochet Gretschified

    Nov 13, 2009
    Monkey Island
    And right you are... :)
     
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