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A100 hums like crazy during warmup.

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  • A100 hums like crazy during warmup.

    Hey, y'all.

    My Hammond A100 starts humming like crazy, I mean like a amp with the guitar chord plugged in and a thumb on the tip of the other end. type of humming. But right about the time the tubes really begin to heat up, the humming fades away to nothing!! Anybody have any idea what my problem is?

    -Mike

  • #2
    High ESR in the power supply filter capacitor, most likely.

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    • #3
      The A100 I take out to gigs hums on startup,first start is noticeable.Second start no hum.
      It's all about length of time between use.A week or a month,no worries usually.
      Ten years? You got issues.

      I've been in 'gotta recap' mode for twenty five years.These get started and played once a month at least.
      Installed power supply caps on a few others that really needed it more,et al,didn't work at all.....

      I work,and gig,and if I have to? Repair this stuff. Not lazy by any means.
      It's recommended the power supply caps get replaced,not expensive at all compared to what's at stake.
      Sooner than later I'll heed my own advice.
      sigpic A100/251 A100/147 A102/222 B2/147 BCV/122 M3/145 M102/145
      Various modern keyboards and modules.

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      • #4
        The power amp in an A100 has a push-pull output stage. (Actually, the whole amp is balanced.) Thus, it relies partly on balanced/matched EL84s and 12AX7s with decent section balance for minimal hum. As tubes age, they don't always age exactly alike, so if one EL84 starts to conduct significantly earlier than the other, they will be temporarily out of balance until the slower one catches up. You will hear this as hum through the speakers. With two matched EL84s, hum from the power supply will be canceled. It would be more unusual for the problem to come from the 12AX7, but it's not inconceivable.

        As KC9UDX says, it may also be a sign of worn power supply capacitors that are no longer filtering out power supply ripple as well as they should.

        For whatever reason, people run the EL84s in A100s and PR40s until there's practically nothing left of them.
        I'm David. 'Dave' is someone else's name.

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        • #5
          Hello,

          You can try shorting the GG terminals (black and red wires); right side of the A039 amplifier wiring panel.
          This will cancel it's input signal.
          So you'll know if the problem is in the amp or in the preamp.

          JP

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