Announcement

Collapse
No announcement yet.

Vox AC30 6TB hum problem; bad power transformer or something else?

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Vox AC30 6TB hum problem; bad power transformer or something else?

    Hello all,

    I'm having an issue with a humming Vox AC30 6TB. It's one of the reissues from the 90s with a tube rectifier and vib-trem switch. The amp has several known design issues which increase its noise floor. I've performed the modifications described in the following video:

    https://www.youtube.com/watch?v=Kfoj4ft_yuY

    except I haven't elevated the heaters, I left the center tap connected to ground as in stock. So I have rewired the heaters with tightly twisted wire; cut the traces to the grids of V8 (the first stage for the normal and brilliant channels) and replaced them with shielded wire under the board, and moved the input grid stopper resistors (R30, R31, R33, and R34) to the end of this shielded wire and tied directly to the pins of V8; and reworked the grounds such that the input section is grounded separately from the power section. I also replaced a very noisy tube; I think it was V8. All of these improvements have decreased the noise floor of the amp, but it still hums very loudly with everything turned down and nothing plugged in.

    I think it's a 120 cycle hum. It hums with all preamp tubes except V6 pulled. Pulling V6 stops the hum, as does switching the amp to standby. I've tried moving around my new heater and ground wires and tapping on things to no effect. I can definitely hear the power transformer physically humming as well. A known issue with these amps is that the power transformers start to hum and eventually fail, in some instances injecting that hum into the rest of the circuit which then becomes audible through the speaker. I wonder if that's what is happening to mine as well? I'm posting to see if anyone has any ideas for other things I can check before I replace the power transformer, and if not to make sure that theory makes sense.


    Click image for larger version

Name:	ac3093pr.jpg
Views:	143
Size:	210.9 KB
ID:	928191

    Click image for larger version

Name:	ac3093pa.jpg
Views:	102
Size:	192.9 KB
ID:	928192

  • #2
    What makes you think it's 120Hz hum?

    Do you have a scope?
    - Own Opinions Only -

    Comment


    • #3
      Is 120 Hz hum existed before your intervention on the wiring, or was created after your intervention?
      Removing V6 stops the hum, because interrupt connection the preamp with the power amp.
      Who does not know and knows that he does not know - teach him Confucius)
      Who knows and does not know that he knows - wake him Confucius)

      Comment


      • #4
        My scope isn't working so I'm trying to gauge by sound. On listening again, it may actually be 60 cycle. I'm not sure, I've always had trouble telling the difference by ear. The hum in the speaker sounds to be the same frequency as the hum from the transformer, of that I'm much more confident.

        The amp was even noisier before I did the modifications. It was humming away even on a current limiter. After removing the other sources of noise and putting it on wall power, I noticed this problem with the transformer and the amp humming.
        Last edited by The Jonald; 04-04-2021, 01:11 AM.

        Comment


        • #5
          Also I've swapped out V6 for a known good tube to no effect. The output tubes look fine and sound okay but do test a little weaker than the "good" range on my meter.

          I was playing with it some more and discovered that turning up the normal volume all the way cuts out some of the noise, and turning up the brilliant volume cuts it even more. I don't understand why.

          Comment


          • #6
            It's hard to tell 60Hz hum (typically related to heater buzz) from 120Hz ripple hum by ear, especially as you often only hear the harmonics.

            We had a couple of AC30 hum issues here lately, where a heater humdinger helped.

            http://www.valvewizard.co.uk/heater.html
            - Own Opinions Only -

            Comment


            • #7
              The guy in the video elevates the heaters by disconnecting the heater center tap and making an artificial center tap to the cathode of one of the output tubes. I've done everything else except that. However the hum is bad enough that I don't think lifting the heaters 10 volts or adding a humdinger is going to cut it. Since it's apparently a known issue with these amps for the power transformer to be the culprit, and since the transformer is physically humming, I'm leaning towards that, or possibly the output tubes or rectifier.

              Comment


              • #8
                Originally posted by The Jonald View Post
                The guy in the video elevates the heaters by disconnecting the heater center tap and making an artificial center tap to the cathode of one of the output tubes. I've done everything else except that. However the hum is bad enough that I don't think lifting the heaters 10 volts or adding a humdinger is going to cut it. Since it's apparently a known issue with these amps for the power transformer to be the culprit, and since the transformer is physically humming, I'm leaning towards that, or possibly the output tubes or rectifier.
                You may try heater elevation but the humdinger was more efficient in the other cases (just use the forum's search function).

                A physically humming PT doesn't typically cause speaker hum.

                As the hum stopped with the PI pulled, we can exclude the power tubes.

                Try the humdinger, it's cheap and easy.
                - Own Opinions Only -

                Comment


                • #9
                  Does the amplifier hum if you remove the phase inverter tube V6.
                  If the amplifier does not hum, output tubes, rectifier and mains transformer are correct.

                  1)
                  https://el34world.com/Forum/index.php?topic=20030.0
                  Where to place the humdinger pot

                  Last edited by vintagekiki; 04-03-2021, 10:13 PM. Reason: 1)
                  Who does not know and knows that he does not know - teach him Confucius)
                  Who knows and does not know that he knows - wake him Confucius)

                  Comment


                  • #10
                    Originally posted by Helmholtz View Post

                    A physically humming PT doesn't typically cause speaker hum.
                    Perhaps not, and it would be a first for me, but it's a known issue with this amp, the following 2 minute video briefly describes the problem: https://youtu.be/NXYJGNw_9-4

                    I may try a humdinger, but I've used them before and I think the hum is so loud that it must necessitate another solution.

                    Comment


                    • #11
                      Originally posted by vintagekiki View Post

                      Does the amplifier hum if you remove the phase inverter tube V6.
                      If the amplifier does not hum, output tubes, rectifier and mains transformer are correct.
                      Correct me if I'm wrong, but I believe V6 is a balanced driver for the output tubes, and V7 is the phase inverter. But yes, pulling V6 removes the noise. Pulling any other preamp tube does not.

                      Originally posted by vintagekiki View Post

                      If the amplifier does not hum, output tubes, rectifier and mains transformer are correct.
                      Can you or anyone explain why this must be the case?

                      Comment


                      • #12
                        Referring to schematic, V6 is the PI . V7 is a DC coupled cathode follower.
                        Last edited by Helmholtz; 04-03-2021, 10:31 PM.
                        - Own Opinions Only -

                        Comment


                        • #13
                          Originally posted by The Jonald View Post
                          Can you or anyone explain why this must be the case?
                          Removing V6 stops the hum, because broken connection the preamp (V7-V11) with the power amp (V2-V5).
                          Question
                          Remove tubes V9, V10, V11 (tremolo & vibrato) What happens with hum?
                          Is there a hum when all 3 volume pots set to min (position far left)
                          Who does not know and knows that he does not know - teach him Confucius)
                          Who knows and does not know that he knows - wake him Confucius)

                          Comment


                          • #14
                            Hum occurs with all controls to minimum. Hum is decreased a bit when normal volume is dimed, and decreased more when brilliant volume is dimed.

                            Pulling any or all preamp tubes has no effect, except V6. Pulling V6 removes the hum. You said that means the power transformer must be good, why is that?

                            Comment


                            • #15
                              Originally posted by Helmholtz View Post
                              Referring to schematic, V6 is the PI . V7 is a DC coupled cathode follower.
                              Oh yeah, it's a long tailed pair. D'oh.

                              Comment

                              bostanci escort
                              sisli escort mecidiyekoy escort
                              pendik escort
                              sex vidio
                              altyazili porno
                              antalya escort
                              beylikduzu eskort bayan eskort bayan escort antalya sirinevler bayan escort
                              gaziantep escort
                              atasehir escort
                              antalya escort bayan escort atakoy
                              izmit escort
                              ankara escort
                              porno
                              replica watches
                              Working...
                              X