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Canít Stop the Hum!!

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  • Canít Stop the Hum!!

    I am working on a friendís Ď75 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 wasnít 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

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    • #3
      The hum doesnít stop exactly, it just gets less noticeable. But gets loud again when I engage the boost. Itís 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

        "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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

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                  • #10
                    I do have a schematic in my photos but I’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?
                      "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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                      • #12
                        100watts

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                        • #13
                          Is this the same schem. as the one you are using?
                          Attached Files
                          "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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                          • #14
                            I canít 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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