Fender Twin Squeal

Discussion in 'Amp Tech Center' started by aaronftnm, Apr 4, 2013.

  1. aaronftnm

    aaronftnm TDPRI Member

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    So I went with my buddy to buy a Silverface Twin yesterday for quite the steal, and its having a crazy reverb issue. I can hear the reverb working up until about 3 on the knob, then it just starts to squeal. Super high pitched and incredibly obnoxious, it's so loud that I can't even hear the guitar. I tried every obvious fix, new RCA cables, different reverb tank, all new tubes (pre-amp and power) all to no avail. I am now 100 percent positive that its something with the circuit because it even oscillates when the cables are unplugged, and pressing the footswitch to turn off the reverb doesn't stop the squeal. Other than not having working reverb, everythig sounds great, and the amp is every bit as loud as it should be. Oh, also, turning the reverb knob will adjust the frequency of the oscillation. Surely this has to be some type of feedback loop, but where do I start looking?
     
  2. aaronftnm

    aaronftnm TDPRI Member

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    Just gonna bump this once, maybe someone can even suggest a better place to go ask about this. Also, i just replaced the reverb transformer, and still the same problem.
     
  3. BobbyZ

    BobbyZ Doctor of Teleocity

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    Ok you might get shocked or killed doing this. (my attorneys make me say that)
    Have you tried moving wires around with the amp on and squealling? Use a chop stick or Sharpy pen NOTHING METAL. (dammed attorneys) Might just be a lead dress problem.

    After that start checking caps,resisters and every ground and solder joint.

    Speaking of caps all the cathode bias caps are new right?
     
  4. aaronftnm

    aaronftnm TDPRI Member

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    We do plan on changing out all of the caps, I would just like to try and have the amp working a little better first before we spend the cash. I did do some poking around with a large wooden knitting needle, and could find no obvious dress lead problems. I did however notice that if I change v4 to an ecc81/12at7 I can turn the reverb knob a bit higher with no oscillations, and if I pull the tube all together, no oscillations at all, but the amps volume has dramatically decreased. Maybe this is just because it is a lower gain tube, so the problem isnt being amplified? Im not sure, but I think I may poke around inside of this amp for a few more days, and if nothing turns up ill probably have to throw in the towel.
     
  5. aaronftnm

    aaronftnm TDPRI Member

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    Problem fixed, i changed all of the electrolytic capacitors that were on the board this morning. I did it one at a time just to see which one specifically, and the capacitor coming off of v4 headed over to the reverb jack was the culprit. Thanks!
     
  6. BobbyZ

    BobbyZ Doctor of Teleocity

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    Glad to hear it's working !
     
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