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Sunn 200S Power Supply Help

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  • #16
    How do you know if the fundamental of the hum is 120Hz not 60Hz?

    If you have a scope, please post a picture of the hum signal.
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    • #17
      Decent 6550's should be able to handle the B+. I may have missed it, but I didn't see bias current measured in the thread yet. Not voltage, but current. If bias is extremely high, any noise/hum that might normally be considered negligible might become annoying. I would start there. It's an easy thing to check and should be checked anyway.
      "Yeah, well, you know, that's just, like, your opinion, man."

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      • #18
        Ripple hum generated in the power stage is mainly caused by unbalanced idle currents (not so much by high idle currents themselves).
        Another cause could be poorly filtered bias voltage, but that would result 60Hz based hum.
        Last edited by Helmholtz; 04-30-2021, 10:30 PM.
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        • #19
          I agree, but I'm not sure it's ripple hum at this point. We haven't seen scope pics.
          "Yeah, well, you know, that's just, like, your opinion, man."

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          • #20
            Originally posted by The Dude View Post
            I agree, but I'm not sure it's ripple hum at this point.
            Neither am I.

            We haven't seen scope pics
            As requested above (post #16).
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            • #21
              And just for the OP's benefit. Hum is not generic. There are numerous places hum can come from. Each has its own cure. The cure for one may have zero effect on another. For example, working on your grounds will not help weak filter cps, And replacing all the filter caps will do nothing to help poorly grounded input jacks.
              Education is what you're left with after you have forgotten what you have learned.

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              • #22
                I'll add: There may be more than one source of hum simultaneously.
                "Yeah, well, you know, that's just, like, your opinion, man."

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                • #23
                  Thanks folks, always learning and love the feeling of accomplishment.

                  I did something different to determine the hum cycle. I used my smart phone guitar tuner app to listen to the speaker output. It reported 120 hz.

                  Using my Bias Tester adapter, I measured current at the output tube at 58 mA. This was with 530V at the first filter cap.

                  I measured the amp output on the scope. Not sure I did this right (please advise). Below are pics. I have to dial up the volume to see this. At zero volume there is not much happening on the scope.

                  Click image for larger version

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                  • #24
                    The bias is on the hot side, but maybe not your problem. Still, I'd adjust for about 48mV instead of 58mV on this amp with that B+
                    "Yeah, well, you know, that's just, like, your opinion, man."

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                    • #25
                      And are both power tubes idling at that 58mA, or did you just check one? The balance could be too far off and that can cause hum as Helmholtz mentioned.
                      "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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                      • #26
                        Adjusted variac to line level. Seeing 554 V at the first filter cap. Amp is drawing .9 A

                        Adjust output tube to 48 mA, other tube reads 40 mA. Swapped output tube positions, 40 mA lower reading stays with the tube, not the tube socket. Still see noise on the scope if I dial up the volume.

                        Too high difference in tube current?

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                        • #27
                          Originally posted by misterc57 View Post
                          I measured the amp output on the scope. Not sure I did this right (please advise). Below are pics. I have to dial up the volume to see this. At zero volume there is not much happening on the scope.
                          First picture shows a 45kHz signal riding on some low frequency, confirmed by second picture. This looks like parasitic oscillation.

                          To read the fundamental frequency set time base to 5ms/div or a value that allows displaying at least 2 positive peaks (or 3 zero crossings).
                          Last edited by Helmholtz; 05-01-2021, 01:27 PM.
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                          • #28
                            Originally posted by misterc57 View Post

                            how can I tell a good from bad choke?
                            Measure ripple voltage at each end of the choke. There should be a significant difference before and after the choke.
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                            • #29
                              Originally posted by Helmholtz View Post

                              First picture shows a 45kHz signal riding on some low frequency, confirmed by second picture. This looks like parasitic oscillation.

                              To read the fundamental frequency set time base to 5ms/div or a value that allows displaying at least 2 positive peaks (or 3 zero crossings).
                              Did I get it?

                              Click image for larger version

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                              • #30
                                Originally posted by misterc57 View Post

                                Did I get it?

                                Click image for larger version

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                                Please set time base to 5ms/div as advised.
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