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135 watt Twin Reverb loud hum

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  • #31
    Originally posted by lo-watt View Post
    I actually used a massive soldering iron to make sure all wires from the PT were freshly soldered directly to chassis instead of via mechanical connection like a transformer screw. All of the grounds from fiberboard to brass strip were checked too.
    I'd wirebrush the brass strip and star washers and the pots it all other efforts fail and make sure the brass strip is well grounded.
    If you have a scope you can build this an check your grounds.
    http://tone-lizard.com/high-resistance-ground/
    nosaj
    Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

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    • #32
      So how does the hum respond to adjusting the hum balance and output matching pots?
      That might help to at least determine if we are dealing with 60hz or 120hz hum. Does one of those pots introduce more hum of a different frequency, etc.
      "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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      • #33
        The hum balance pot decreases and increases hum mildly,as expected on AC heaters. Tube balance pot does adjust as well,I set the tubes at 35mv. Both are factory and stock

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        • #34
          Originally posted by lo-watt View Post
          The hum balance pot decreases and increases hum mildly,as expected on AC heaters. Tube balance pot does adjust as well,I set the tubes at 35mv. Both are factory and stock
          Does Tube balance pot affect hum at all?

          nosaj
          Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

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          • #35
            Originally posted by nosaj View Post
            Does Tube balance pot affect hum at all?

            nosaj
            If I recall,no. But it does change the “bias”.

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            • #36
              Originally posted by lo-watt View Post
              Yes, I pulled all six preamp tubes. It does not hum without the power tubes. I alligator clipped in new F and T caps under the dog house one by one,to make sure it was not a bad filter or bias cap. No change there. I verified all the screen and grid resistors were within spec and voltages on the power tubes were within spec.
              I just want to go back over this one. Did you pull them one at a time until you got the power tubes?

              Thanks,
              \nosaj
              Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

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              • #37
                Originally posted by nosaj View Post
                I just want to go back over this one. Did you pull them one at a time until you got the power tubes?

                Thanks,
                \nosaj
                I did starting with V1(far right). Virtually no change in hum through all six pulled one at a time.

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                • #38
                  Originally posted by lo-watt View Post
                  I did starting with V1(far right). Virtually no change in hum through all six pulled one at a time.
                  Do you have a scope? still need to scope the bias supply.
                  nosaj

                  Also does the line out jack have hum on it?
                  Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

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                  • #39
                    I wanted to update: the customer decided to live with it as-is. I called to check on him and he quoted me some YouTube videos(did you do this or do that?) when I gave him the price on a new transformer only available from MM he was shocked. So the amp is gone and maybe he will bring it back one day. Thank you for all your help, on to the next repair...

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                    • #40
                      Just a thought here ...

                      You never mentioned any of the control settings and if they affect the hum or not. What about the reverb control? Take a 1/2" nut driver and loosen all 4 of the RCA connections on the rear panel where the reverb tank and footswitch connect. Clean underneath with wire brush if necessary. Then tighten them all good and snug, which will ensure the grounds there are solid.

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                      • #41
                        Originally posted by Tone Meister View Post
                        Just a thought here ...

                        You never mentioned any of the control settings and if they affect the hum or not. What about the reverb control? Take a 1/2" nut driver and loosen all 4 of the RCA connections on the rear panel where the reverb tank and footswitch connect. Clean underneath with wire brush if necessary. Then tighten them all good and snug, which will ensure the grounds there are solid.
                        Amp is gone,i dont think ill see it back for a while.

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