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Can’t Stop the Hum!!

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  • Can’t Stop the Hum!!

    I am working on a friend’s ‘75 Twin Reverb with Master Volume/Boost circuit. It has an annoying 120hz hum that can only be stopped by grounding the grid or plate of V4b. I have replaced that grid wire with a shielded lead, replaced the socket, and of course, the tube. The main filter caps were replaced about ten years ago but I parallel in new caps one by one just to make sure it wasn’t one of those. I also replaced all of the cathode caps, the bias caps, checked voltages and grounds, and even replaced the reverb transformer. The hum comes down to an acceptable level when I unplug either of the reverb tank cables but is loud again when I engage the boost. I have also disconnected the boost and the MV but the hum is still there. I am at a loss so any advice would be appreciated!

  • #2
    I'm not familiar with 120Hz being associated with a ground loop, but when you report that unplugging EITHER of the reverb cables (I assume you mean in or out?) that the hum stops the first thing I think of is a ground loop. Isolating one of the jacks such that the shield is still grounded at one end (but no longer both) may be worth a try.
    "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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    • #3
      The hum doesn’t stop exactly, it just gets less noticeable. But gets loud again when I engage the boost. It’s also present and loud with both cables unplugged. But I will definitely try your suggestion. Thanks

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      • #4
        Does the amp exhibit the same hum reduction when no guitar is plugged in?
        "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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        • #5
          Use a termination plug on the input. Hum still there?

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          • #6
            The hum is there with the channel volume turned to 0. It seems to be picked up by pin 7 of V4. I can ground pin 1 of V4 and the hum is still there. But if I ground pin 6 or pin 7, the hum stops.

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            • #7
              The hum reacts the same regardless of channel volume or a guitar plugged in or not.

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              • #8
                Does that one have both the 'output tubes matching' and 'hum balance' pots? Do either get rid of it?
                There are several versions, can you post the proper schematic?
                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


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                • #9
                  Output tubes match and balance pot has been adjusted. But hum stops when V4 is pulled so itÂ’s not the output section.

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                  • #10
                    I do have a schematic in my photos but IÂ’m not sure how to post it on this forum.

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                    • #11
                      It has master & boost, down in the lower right of schematic, does it say 100W or 135Watts?
                      Originally posted by Enzo
                      I have a sign in my shop that says, "Never think up reasons not to check something."


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                      • #12
                        100watts

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                        • #13
                          Is this the same schem. as the one you are using?
                          Attached Files
                          Originally posted by Enzo
                          I have a sign in my shop that says, "Never think up reasons not to check something."


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                          • #14
                            I can’t open the attachment for some reason

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                            • #15
                              What happens when you ground either side of your 3.3M reverb mixing resistor?

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