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Silverface Bassman AB165 Bias Mod

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  • Silverface Bassman AB165 Bias Mod

    So i just picked up a AB165 and i'm getting a slight hum out of it. When i had it hooked up to a Ampeg 6x10, playing bass through it, it wasn't really noticeable. But once hooked up to a 4x12 cab and playing guitar through it. its really noticeable. i've opened it up and everything looks good inside, including the filter caps. Im guessing that it has something to do with the way that bias works in this amp. So i was thinking about re-wiring it to a real bias. But my question is, what should the bias be set to and where should i measure off of?

    Also side note. In the filter caps the big 2 70uf have been changed but the others are mallory's. they look completely fine. But, could they cause problems even if they're not physically damaged?

  • #2
    Unplug the guitar from the amp. Just play the amp through the speaker with nothing plugged into the front. Does it still have the hum?

    Turn your controls all to zero. Hum stay or go?

    If the bias control is wired as a balance, then twiddle it for least hum - simple as that. If it is wired as a bias level control then there in no correcting any imbalance, all you can adjust is the heat.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      It's easy as pie (Pi) to convert the bias in these amps so you have the stock bias balance but also adding a bias amount pot so you have the best of both worlds. I usually use a small 10 turn cermet pot that I glue to the bias balance control. There are various ways of doing the bias balance in Fender amps; check the schematics for a good balance circuit.

      Bob M.

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      • #4
        IIRC the AB165 had a weird bias arrangement with one side of the push pull at a non adjustable fixed bias and the other side adjustable.
        That is: the bias adjustment is a balance control.
        Set the volume to 0 and then adjust the bias ("Balance") control for minimum hum.

        Cheers,
        Ian

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        • #5
          And to add to the above good advice/suggestions,..

          Inquiring about mods and supposing that the hum may be a flaw in the initial design is EXTREMELY short sighted. Fender didn't approve a design that was prone to too much noise floor when they already had so many flagship designs in their history to offer. When the amp is working right it won't hum offensively. I can't argue with changing the bias arrangement to something more user friendly, but the stock arrangement, adjusted correctly, isn't the source of the hum because of poor design. The amp did work correctly without offensive hum when it was sold.
          "Take two placebos, works twice as well." Enzo

          "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

          "If you're not interested in opinions and the experience of others, why even start a thread?
          You can't just expect consent." Helmholtz

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          • #6
            Originally posted by Bassist001 View Post
            So i just picked up a AB165 and i'm getting a slight hum out of it. When i had it hooked up to a Ampeg 6x10, playing bass through it, it wasn't really noticeable. But once hooked up to a 4x12 cab and playing guitar through it. its really noticeable. i've opened it up and everything looks good inside, including the filter caps. Im guessing that it has something to do with the way that bias works in this amp. So i was thinking about re-wiring it to a real bias. But my question is, what should the bias be set to and where should i measure off of?

            Also side note. In the filter caps the big 2 70uf have been changed but the others are mallory's. they look completely fine. But, could they cause problems even if they're not physically damaged?
            most probably the hum is caused by one of the old filter caps, try parallel them one at a time with a new one. Other things are a bad tube, or a bad soldering especially on chassis.

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