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It wasn't the tubes!

Discussion in 'Amps and Cabs [BG]' started by shatner, Jan 11, 2005.

  1. shatner


    Sep 22, 2004
    Isle Of Wight, UK
    Was having problems with my SVT3pro. When I cranked up the treble and pulled back on the neck pickup I got this insane wobbling whistle. Very loud! When I turned down on the gain, treble or put both pickups back up to maximum it stopped. I assumed it was the tube microphonics since I hadn't changed them since buying it second hand a few months ago. The guy before me had used it for about ten years without changing them.
    Anyway, I routed my backup Trace Elliot preamp through the power amp stage and it worked fine (I also got a ton more power out of the Ampeg). Yesterday I replaced all the 12AX7s with new Electro Harmonix tubes and the thing is still making the whistle noise.
    The tweeter is not to blame coz I turned it off and the Trace Elliot allowed me to do all the other tone things without a whistle. The weirdest thing is how the pickups can affect whether it does it or not. The bass is a Marcus Fender with stock single coils.
    Any ideas??
  2. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Have you tapped on all the tubes (one at a time) with the amp on and listened? Power tubes too...Maybe one of the new tubes is microphonic...it happens.
  3. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    A couple of things.....it might not be in the amp at all.

    If the settings on the bass affect it, what about a different bass, and different cable?

    Possible that there is a bad cable, or bad wiring in bass etc, although usually a bad cable will give other symptoms as well (hum or buzz, noise if you wiggle connections, etc).

    If by chance you are running at a pretty high gain, the bass might not be well shielded, so it picks up some feedback from the speaker wires. Pickups can actually receive feedback from the speaker voice coils. If it changes when you walk around carrying the bass, that would almost nail it as that kind of "system feedback" problem. I have seen that once or twice before, but on other gear, not Ampeg.

    I know, the other preamp didn't whistle, but there are reasons that could account for that.

    First thing would be to try another bass and cable. Even a regular 6 string guitar. Then see what happens at same type amp settings.