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  • #31
    Originally posted by Helmholtz View Post

    Measure ripple voltage at each end of the choke. There should be a significant difference before and after the choke.
    6.14 VAC before choke
    0.37 VAC after choke

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    • #32
      Bare with me, I am not used to using the scope. Perhaps I am not setting it correctly. At 5ms I have too many peaks. At 5us I was able to get it to two peaks.

      Here is 5ms image.

      Click image for larger version

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      • #33
        Originally posted by misterc57 View Post
        Bare with me, I am not used to using the scope. Perhaps I am not setting it correctly. At 5ms I have too many peaks. At 5us I was able to get it to two peaks.

        Here is 5ms image.

        Click image for larger version

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        Ok, so we have 120Hz ripple with superimposed HF oscillation. Is this with volume at zero?
        - Own Opinions Only -

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        • #34
          Originally posted by misterc57 View Post

          6.14 VAC before choke
          0.37 VAC after choke
          Choke is good.
          - Own Opinions Only -

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          • #35
            Originally posted by Helmholtz View Post

            Ok, so we have 120Hz ripple with superimposed HF oscillation. Is this with volume at zero?
            No, volume was up, not sure how high. Two new pics below with volume at zero and volume at 5.

            Click image for larger version

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            • #36
              Judging from the left picture, there should be no hum with vol. at zero?

              (Actually the first thing I'd do is rewire the amp for a GZ34.)
              - Own Opinions Only -

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              • #37
                Originally posted by Helmholtz View Post
                Judging from the left picture, there should be no hum with vol. at zero?

                (Actually the first thing I'd do is rewire the amp for a GZ34.)
                Yes no hum at zero volume. This is getting more bizarre. Hooked the speaker back up (was on a dummy load). I have a probe at the first filter cap watching DC. Guitar plugged in.

                Amp is producing bizarre noises (like blowing into a trumpet mouthpiece, I will call it high pitch squealing) that is jumping all around in frequency. I am also seeing the voltage at the first filter cap fluctuating up and down in sync with the weird noise, from 100 to 350 V. Layered behind this noise I can still hear the 120 cycle hum.

                Eventually the V at the first cap drops to 90 V and the squealing stops, while the hum is still there.

                Regardless of the V at cap 1, I can still strum and clearly hear the guitar.

                Yikes!

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                • #38
                  Well, you have an oscillation problem probably with the 12AX7. Have you tried a different tube?

                  What happens with V1 pulled and full volume?
                  - Own Opinions Only -

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                  • #39
                    Originally posted by Helmholtz View Post
                    Well, you have an oscillation problem probably with the 12AX7. Have you tried a different tube?

                    What happens with V1 pulled and full volume?
                    V1 pulled, no noise or hum heard at full volume. Swapped V1 and I and no longer hearing squealing sounds.

                    I am left with just a slight hum at loud volumes. Louder than I would ever play. Possible this is due to mismatch output tubes current?

                    In monitoring the voltage at the first cap (about 555 V) once in a while it drops to 97 V. I think the wiring connections between the rectifier, standby switch, and first cap needs to be touched up with solder. If I poke around at those joints the 555 V will come back. Also I detect a small squeal when poking the wire between the standby switch and first cap.

                    I also wonder if I should use a dropping resistor to knock that 555 V closer to 500 V?

                    I feel I am getting closer to resolution.

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                    • #40
                      Originally posted by misterc57 View Post

                      I also wonder if I should use a dropping resistor to knock that 555 V closer to 500 V?
                      Rewire to GZ34, will lower B+ as well as hum.

                      But check if you still have ultrasonic preamp oscillation.
                      Last edited by Helmholtz; 05-01-2021, 09:36 PM.
                      - Own Opinions Only -

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                      • #41
                        Originally posted by Helmholtz View Post

                        Rewire to GZ34, will lower B+ as well as hum.

                        But check if you still have ultrasonic preamp oscillation.
                        Looks like it is still there. Scope images below volume 0 and volume at 5.

                        Click image for larger version

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                        • #42
                          Go to 5Ás/div for better HF resolution.
                          - Own Opinions Only -

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                          • #43
                            Originally posted by Helmholtz View Post
                            Go to 5Ás/div for better HF resolution.
                            Click image for larger version

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                            • #44
                              Originally posted by misterc57 View Post

                              Click image for larger version

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                              So there's still some ultrasonic signal (around 42kHz).

                              Could be either an oscillation of V1 or a signal picked up from nearby equipment (switching power supply, phone etc.).

                              - Own Opinions Only -

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                              • #45
                                Originally posted by Helmholtz View Post

                                So there's still some ultrasonic signal (around 42kHz).

                                Could be either an oscillation of V1 or a signal picked up from nearby equipment (switching power supply, phone etc.).
                                Interesting. Beyond human hearing. Will see what I can figure out.

                                Thank you!

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