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Ampeg SVT-3 Pro with bad hum

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  • Ampeg SVT-3 Pro with bad hum

    Client said his SVT-3 Pro is making crackling noises after warming up. When I plugged it in and played some music thru it, I found the output low and distorted, and a little hum present, even with master volume at zero. I measured less than 10mV across each the ceramic source(?) resistors, so I turned the bias up until I get ~20mV across each. Now hum is much louder. I pulled ALL five of the preamp tubes out, and hum is the same.

    Next troubleshooting steps?

    Anyone have schematic? Having trouble sourcing one.
    --
    I build and repair guitar amps
    http://amps.monkeymatic.com

  • #2
    Did you check this one at ET? https://elektrotanya.com/scm_electro.../download.html

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    • #3
      Thank you. Did you know the amp won't run with the fan disconnected? Now I know. With no fan, the heaters for V4 and V5 don't work, and there is a bad hum.

      Amp is running fine now, but I'm playing some music, waiting to see if owner's original complaint, crackilng noises, happens.
      --
      I build and repair guitar amps
      http://amps.monkeymatic.com

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      • #4
        No issues, back to owner. In looking at my records I see I had another SVT-3 Pro in 2017 that needed a new 12AU7. I guess the cathode follower config is hard on those tubes!
        --
        I build and repair guitar amps
        http://amps.monkeymatic.com

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