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Gibson GA-30RV Off/Standby/On switch

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  • Gibson GA-30RV Off/Standby/On switch

    Customer asks if I can install a Gibson style on/standby switch on his 5E3 Deluxe. I understand how the primary side of the of the DP3T switch works, but I'm a little unsure how the upper standby side of the switch is working. Is it connecting the two grids of the power tubes, cancelling them out? Is this the basic theory behind the post PI volume control?

    And does anyone see any reason this could not be done to a 5E3?

    http://www.ampwares.com/schematics/gibson/ga30rv.pdf
    It's weird, because it WAS working fine.....

  • #2
    Yes. That's how it works- phase cancellation. I see no reason you couldn't do the same with a 5E3. If your 5E3 has a ground polarity switch, you could remove the death cap and use the existing switch for a standby switch and save some trouble and parts cost.
    "Yeah, well, you know, that's just, like, your opinion, man."

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    • #3
      The term "phase cancellation" makes no sense to me.
      The standby switch shorts 2 opposite polarity signal voltages. So we have differential signal cancellation.
      - Own Opinions Only -

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      • #4
        Originally posted by Helmholtz View Post
        The term "phase cancellation" makes no sense to me.
        The standby switch shorts 2 opposite polarity signal voltages. So we have differential signal cancellation.
        The term is used a little loosely. Opposite polarity signals being 180 degrees 'out of phase'.
        "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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        • #5
          Frankly, I don't see what's so ambiguous about it. IMO, it seems to fit the situation pretty well.

          The definition of phase cancellation:

          Phase cancellation occurs when two signals of the same frequency are out of phase with each other resulting in a net reduction in the overall level of the combined signal. If two identical signals are 100% or 180 degrees out of phase they will completely cancel one another if combined.

          "Yeah, well, you know, that's just, like, your opinion, man."

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          • #6
            Originally posted by g1 View Post

            The term is used a little loosely. Opposite polarity signals being 180 degrees 'out of phase'.
            Sure, but connecting 2 "out of phase" signals means a short. And a short cancels voltages, not phase. (Zero phase wouldn't mean zero voltage.)
            More exactly a short cancels any differential voltage.

            BTW, a short always causes increased currents, without changing their phase.

            I think the misleading term "phase cancellation" is used to describe signal cancellation/attenuation caused by 180 phase difference of the signals when adding/mixing.

            But shorting two (out of phase) signal voltages is not a constructive way of mixing signals.
            Last edited by Helmholtz; 01-07-2021, 12:59 AM.
            - Own Opinions Only -

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            • #7
              We're into a semantic argument here and I'm sure everyone understands how the circuit works, so it's a bit of a silly detour, but I'll say this. As you say, the short cancels voltages, but only because the two signals are out of phase. If they were in phase they would add together when shorted, so the more important point for the given objective is the phase relationship- not the short, IMO. And, again, by definition, this is phase cancellation. I'll move on now.
              "Yeah, well, you know, that's just, like, your opinion, man."

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              • #8
                In a 5E3, with it's Concertina type phase inverter, the two phases won't cancel completely, but because of the short, the same signal is applied to both power tubes and that will look like a common mode signal. So the result is a pretty quiet output depending on the match of the tubes and the output transformer.
                WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
                REMEMBER: Everybody knows that smokin' ain't allowed in school !

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                • #9
                  Originally posted by The Dude View Post
                  the short cancels voltages, but only because the two signals are out of phase. If they were in phase they would add together when shorted, so the more important point for the given objective is the phase relationship- not the short, IMO.
                  Sorry, no.

                  A short cancels any (differential) voltage, regardless of phase relationship of components.


                  The difference when shorting between in phase and out of phase components would only show in the common mode signal.
                  While shorting 2 balanced out of phase signals will result in zero common mode signal, shorting 2 in phase (100% common mode) signals would not change the common mode signal level.
                  (Paralleling 2 equal voltages does not add them.)

                  In other words, shorting the signals kills any differential voltages but doesn't change common mode voltages.

                  - Own Opinions Only -

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                  • #10
                    Originally posted by Helmholtz View Post

                    Sorry, no.

                    A short cancels any (differential) voltage, regardless of phase relationship of components.


                    The difference when shorting between in phase and out of phase components would only show in the common mode signal.
                    While shorting 2 balanced out of phase signals will result in zero common mode signal, shorting 2 in phase (100% common mode) signals would not change the common mode signal level.
                    (Paralleling 2 equal voltages does not add them.)

                    In other words, shorting the signals kills any differential voltages but doesn't change common mode voltages.
                    I think when the dude says they "add" together, he is not using the term in a strict mathematical sense. I think in the context he is simply saying that the 2 AC voltages will "combined".
                    But, to your point, is it not the superposition of the two AC voltages which causes the nulling effect??... actually, I see it now. Yeah, I think you might be correct in this example.
                    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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                    • #11
                      Originally posted by SoulFetish View Post

                      I think when the dude says they "add" together, he is not using the term in a strict mathematical sense. I think in the context he is simply saying that the 2 AC voltages will "combined".
                      But, to your point, is it not the superposition of the two AC voltages which causes the nulling effect??... actually, I see it now. Yeah, I think you might be correct in this example.
                      Not sure if there is a question involved.
                      Combining two voltages is not a precise technical term. Results depend on how the sources are wired and where the reference point is.

                      Maybe these explanations help:

                      Differential voltage is the voltage difference between the PI output/ power tube grid signals. If the PI is perfectly balanced, the differential voltage is twice the signal output (wrt ground) of each PI plate as the 2 opposite polarity voltages appear wired in series.
                      With a perfectly balanced power stage (tubes, OT), only the differential voltage can produce speaker output.

                      Connecting the power tube grids shorts/eliminates the differential voltage ( there can't be any voltage between directly connected nodes) but maybe leaves a small common mode signal at both grids.
                      Common mode signal is caused by imperfections of the PI and is measured between both (connected) power tube grids and ground.
                      Last edited by Helmholtz; 01-07-2021, 09:04 PM.
                      - Own Opinions Only -

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                      • #12
                        Originally posted by Randall View Post
                        Customer asks if I can install a Gibson style on/standby switch on his 5E3 Deluxe. I understand how the primary side of the of the DP3T switch works, but I'm a little unsure how the upper standby side of the switch is working. Is it connecting the two grids of the power tubes, cancelling them out? Is this the basic theory behind the post PI volume control?

                        And does anyone see any reason this could not be done to a 5E3?

                        http://www.ampwares.com/schematics/gibson/ga30rv.pdf
                        did it to mine
                        Jack Briggs

                        sigpic
                        www.briggsguitars.com

                        forum.briggsguitars.com

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