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Signal leaking through with volumes off

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  • Signal leaking through with volumes off

    Hello friends I need some guidance please.

    I've recently built an AB763 kit. It works really well, sounds great, it's dead quiet. I'm 99% very pleased with it.

    The last 1% that's doing my head in is the guitar signal faintly leaking through to the speaker even with the channel volumes turned all the way down. It happens on both channels. I changed the volume pots, didn't help. They were very stiff and wonky so I thought that had to be the problem. But no. New CTS pots didn't cure it.

    How is this happening on both channels?

    I've pumped a sine wave into it, chopsticked the wiring, watched it on a scope, swapped tubes, tapped around the connections, checked solders, I can't tell how that little bit of signal is getting through. Please give me some ideas to try. I'm still new to amp building and finding this issue is eating at me.


  • #2
    Try directly grounding the grid that the volume pots connect to on each half of the 7025. If that kills the signal you may have some resistance between the pot wiper and ground when the pot is turned right down.

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    • #3
      In theory, if the volume control is turned all the way down, then the signal is at ground potential.
      Off.
      In reality, the controls may not physically reach 'zero' ohms.
      Ground the middle leg (the wiper) of the controls.
      If the signal is dead quiet, then the issue is a quirk of the controls.

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      • #4
        Please post schematic of your amp.
        - Own Opinions Only -

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        • #5
          Originally posted by Helmholtz View Post
          Please post schematic of your amp.
          Click image for larger version

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          • #6
            Thank you guys. I will try the ideas posted and report back shortly.

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            • #7
              The last 1% that's doing my head in is the guitar signal faintly leaking through to the speaker even with the channel volumes turned all the way down.
              Then you do NOT have a problem.
              That is an amp designed for ear splitting LIVE use.
              When improperly used in a bedroom you may notice tiny "problems" such as that one.
              Juan Manuel Fahey

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              • #8
                The likely source is what we call crosstalk. The controls are not at fault. The high gain input stages radiate some signal. Sensitive circuits later in the amp can pick that up, and that is what you hear.

                Think about it, how often do we play with the controls at zero?
                Education is what you're left with after you have forgotten what you have learned.

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                • #9
                  Originally posted by Enzo View Post
                  The likely source is what we call crosstalk. The controls are not at fault. The high gain input stages radiate some signal. Sensitive circuits later in the amp can pick that up, and that is what you hear.

                  Think about it, how often do we play with the controls at zero?
                  Well never at zero, but it just doesn't seem that this should be happening. Am I nitpicking? I have a virtually identical build that doesn't exhibit this situation.

                  And to be fair, it's faint. Really faint. But it's for a friend and I'd hate to ship it to him like this.

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                  • #10
                    Originally posted by J M Fahey View Post
                    Then you do NOT have a problem.
                    That is an amp designed for ear splitting LIVE use.
                    When improperly used in a bedroom you may notice tiny "problems" such as that one.
                    Ha ok. I get that, but I have way louder truly ear splitting amps that don't have this little issue.

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                    • #11
                      Some other amp has differences, so be it. Could be due to wire dress, could be positioning of components, there are lots of reasons one amp does something another does not.
                      Education is what you're left with after you have forgotten what you have learned.

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                      • #12
                        Originally posted by Enzo View Post
                        Some other amp has differences, so be it. Could be due to wire dress, could be positioning of components, there are lots of reasons one amp does something another does not.
                        Okay. I think my lead dress and component layout is good. I think my grounding is good. I get no noise, hiss, hum, or weirdness. The scoped signal is very clean and the voltages are in spec and the amp really does sound great. It functions perfectly...except for this slight input signal leaking into the phase inverter.

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                        • #13
                          Since I myself don't consider it a problem, I can't fault your lead dress. Crosstalk can be an artifact of lead dress (or a lot of other things), but not be "incorrect".

                          If it really bugs you, you can apply systematic troubleshooting to find what is picking it up and from where. Then I am sure it would be possible to cure. But it won't be a bad part.
                          Education is what you're left with after you have forgotten what you have learned.

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                          • #14
                            Originally posted by Enzo View Post
                            Since I myself don't consider it a problem, I can't fault your lead dress. Crosstalk can be an artifact of lead dress (or a lot of other things), but not be "incorrect".

                            If it really bugs you, you can apply systematic troubleshooting to find what is picking it up and from where. Then I am sure it would be possible to cure. But it won't be a bad part.
                            It happens on both channels, normal and vibrato, so could it be getting picked up as far downstream as the phase inverter?

                            To the best of my ability I've checked the B+ rail and grounding and it doesn't seem like the signal has gotten in there somehow. With the channel vol all the way down the weak signal is present at the .047 cap after the plate of V1b which then goes into the grid of the phase inverter. So it seems like whatever is happening is happening in the preamp of that channel. But....it exists equally on both channels.

                            Even if it's not a big deal I'd like to at least learn how to chase it down.

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                            • #15
                              I'd start grounding grids from the input on to see if you can determine at what gain stage it's starting.
                              "Yeah, well, you know, that's just, like, your opinion, man."

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