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Loud, high-pitched buzz from my Bassman

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jpervin, Feb 2, 2006.

  1. jpervin

    jpervin Supporting Member

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    I've got a David Allen-built Bassman combo amp that I've had for about a year. Up until now I've had no problems. The other night, after connecting a guitar into it, I plug the amp in and hit the power switch to let it warm up for a few minutes. When I hit the standby switch I immediately got a loud high-pitched buzz (which turned into a squeal). I turned the standby off right away and it stopped. Checking the power and rectifier tubes, I found them to be glowing normally, though I didn't take the shields off of the preamp tubes to check them. I also checked to make sure that all the tubes were seated properly.
    Tried it one more time and got the same loud buzz/squeal.
    I emailed David for some advice. He said it could be a bad phase inverter tube (last tube before the power tubes).

    Anybody here have something similar happen to a tube amp?
     
  2. Chiba

    Chiba Gold Supporting Member

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    I had a similar problem with a Carvin Bel-Air - but I doubt it would help you out, as my problem was caused by two of the EL84's melting through the PCB :)

    Swap out your preamp tubes and see what happens. If you've been using it for about a year, using it regularly, it could simply be ready for new tubes.

    --chiba
     
  3. jpervin

    jpervin Supporting Member

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    Ouch!! :(

    I'll try that. Thanks, Chiba!
     
  4. donnyjaguar

    donnyjaguar Member

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    Had this happen when one power tube melted down. Blew out a couple of resistors in the process which in retrospect helped me nail down the culprit very quickly.
     
  5. jpervin

    jpervin Supporting Member

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    Well, I pulled the preamp tubes and had a look at them. The Chinese 12AT7 in V2 (?) has a blue-ish, cloudy film around the inside, on the 9-pin end. That doesn't look normal. The other preamp tubes (3-JJ 12AX7s) looked fine, though that probably doesn't mean anything.

    I need to order some spare tubes today.
     

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