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vintage VOX AC30 1979 - mystery switch & pcb pots

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  • vintage VOX AC30 1979 - mystery switch & pcb pots

    I recently got some beer to clean my both guitarists' AC30's - that both are from 1979, Rose Morris era.

    It's always fun to look at those guts inside. I'm wondering... there's a switch that is completely covered by a back wooden plate. You can see it only when you remove the back. Looks original to the amp. Is that a voltage switch or polarity switch or....? (pic attached). Second question. The two big pots on the main lower PCB - I assume they are not bias pots, as Voxes are cathode biased, what could those be for? Any thoughts? We need to replace the tubes, so maybe also clean the amp and set it right way.

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  • #2
    From a contemporary schematic https://el34world.com/charts/Schemat...x_ac301978.pdf I guess that the switch is to select between 240 and 220V primary taps on the PT.
    The trimmer near the mess of HT ecaps may be the heater circuit humdinger, the other is probably the trimmer on the output of the modulation oscillator.
    My band:- http://www.youtube.com/user/RedwingBand

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    • #3
      Allright, so the trimmer near caps is acrting like "hum balance" (in e.g. old ampegs)? and the left one controls strenght of the modulation of the vib-trem circuit? Right?

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      • #4
        Yes, though humdingers are typically more of a ‘buzz balance’ than ‘hum balance’.
        My band:- http://www.youtube.com/user/RedwingBand

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        • #5
          Originally posted by pdf64 View Post
          Yes, though humdingers are typically more of a ‘buzz balance’ than ‘hum balance’.
          How this? What's the difference (language barrier)? What would be a real hum balance?
          - Own Opinions Only -

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          • #6
            Originally posted by Helmholtz View Post
            How this? What's the difference (language barrier)? What would be a real hum balance?
            I think he was referring to the 'sound' of the hum and the harmonic content, rather than the circuit type.

            As far as the language thing, I'm an english speaker and even my reading comprehension is not so good. I was wondering why boroman was going to clean the amp with beer.
            "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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            • #7
              In my experience the AC30 humdinger is very effective regarding compensation of 50Hz hum. The position of minimum 50Hz hum may not provide optimum cancellation of more audible higher line harmonics, though. Requires compromising.
              One of the reasons is direct hum injection/coupling between PT and OT in some versions.
              - Own Opinions Only -

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              • #8
                Yes, it’s a higher harmonics thing; the interference from an out of balance heater circuit seems more like the ‘buzz’ from a bee, rather than the ‘hum’ of someone that knows the tune but not the words
                My band:- http://www.youtube.com/user/RedwingBand

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                • #9
                  effective regarding compensation of 50Hz hum
                  I think that is the problem. Over here we have 60Hz, which is like 10 more Hertz, innit?
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #10
                    But this one (in my plane) goes to 400Hz...

                    Click image for larger version

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                    "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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                    • #11
                      Originally posted by Enzo View Post
                      I think that is the problem. Over here we have 60Hz, which is like 10 more Hertz, innit?
                      So what's the point?
                      - Own Opinions Only -

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                      • #12
                        They're just being silly
                        My band:- http://www.youtube.com/user/RedwingBand

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                        • #13
                          Helmholtz. It was just a joke. I was implying the line from the movie SPinal Tap, where all their amps "go to eleven... Well that's one louder, innit?" ("innit" being the written form of "isn't it?" when spoken unclearly.) And I was trying to nudge that into the buzz versus hum thing. Obviously the hum abatement pots work equally well at both 50Hz and 60Hz.
                          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
                            Helmholtz. It was just a joke. I was implying the line from the movie SPinal Tap, where all their amps "go to eleven... Well that's one louder, innit?" ("innit" being the written form of "isn't it?" when spoken unclearly.) And I was trying to nudge that into the buzz versus hum thing. Obviously the hum abatement pots work equally well at both 50Hz and 60Hz.
                            Thanks for explanation, Enzo. Don't forget we have much more line volts in 50Hz countries

                            Actually there are some advantages of a higher line frequency, e.g. PTs have less core losses and thus can be built smaller which additionally reduces copper losses. Also supply filters are more efficient at higher frequency.
                            - Own Opinions Only -

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                            • #15
                              Actually there are some advantages of a higher line frequency, e.g. PTs have less core losses and thus can be built smaller which additionally reduces copper losses. Also supply filters are more efficient at higher frequency.
                              Yes indeed. and that is why military and similar use 400Hz instead of 60Hz. And I suppose ultimately that is why we use SMPS with frequencies as high as MHz.
                              Education is what you're left with after you have forgotten what you have learned.

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