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Odd hum on powerup in late '70's UL 70W Super Reverb

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  • Odd hum on powerup in late '70's UL 70W Super Reverb

    OK...late '70's UL 70W Super reverb. I just did a cap job service on this one. This amp has a hum problem - when I power it up, it hums briefly, maybe 30 seconds, then the hum fades away. This isn't normal, to my ears and experience. Swapping out the power tubes didn't change anything. I adjusted the "hum balance" and "output tube matching" - adjusting the output tube matching seemed to reverse the overall hum noticeably, but this amp still has the weird 30-second initial hum. Turning the amp volume up and down doesn't seem to have much effect. I haven't tried subbing out preamp tubes, but that will be my next step. Thoughts?

  • #2
    The cap job included the bias circuit filter caps?
    If you remove the PI tube, do you still get the turn on hum?
    "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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    • #3
      Yes, the cap job included the bias caps. I haven't tried pulling the phase inverter tube.

      I had more time tonight to fiddle with the amp, and here's a more concise description of what's going on: The power-up hum actually only lasts about 10 seconds, and then gets quieter. The remaining hum can be adjusted mostly out of the audible range, but it is still there - the "Output Tubes Matching" pot has to be turned all the way to one direction, though. The "Hum Balance" control has no effect. When I use a new matched set of 6L6's, I can adjust it away even more, but it is still faintly there, and it seems if I had more "range" in the control I could possibly adjust it completely away. I checked the resistor on the pot - it has drifted from 33K to about 19K, but I tested it in-circuit - that may be invalid.
      Last edited by Fred G.; 05-04-2021, 03:47 AM.

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      • #4
        Check all of your screen resistors. If they have drifted out of spec or are open, "Output Tubes Matching" will be difficult if not impossible. A large conduction mismatch will cause hum.

        Also, check both tubes matching and hum balance pots. Make sure they are not open.
        "Yeah, well, you know, that's just, like, your opinion, man."

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        • #5
          Does the amp use a GZ34?

          Asking because the GZ34 should delay the B+ ramp-up by ~20s.

          If you're getting the initial hum with a GZ34, it might be damaged.
          Last edited by Helmholtz; 05-04-2021, 04:13 PM.
          - Own Opinions Only -

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          • #6
            Originally posted by Helmholtz View Post
            Does the amp use a GZ34?
            If it's like this schematic it has a solid state rectifier. If that's the case then the 10 seconds of power up hum could be due to the different warm up times of the output tubes (assuming it's switched on with the Standby switch in "Play" mode).

            Click image for larger version  Name:	image_36785.jpg Views:	0 Size:	1.76 MB ID:	930718
            Last edited by Dave H; 05-04-2021, 06:27 PM.

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            • #7
              Started up the amp without PI tube, made noo difference in hum.

              ok, took some measurements:

              V8

              Pin 5: -51.12
              Pin 4: 502.5
              Pin 3: 501.6
              Pin 1; -50.92

              Screen grid resistor; 501 ohms (drifted a bit, but still within tolerance)

              V8

              Pin 5: -58.7
              Pin 4: 502.0
              Pin 3: 505.3
              Pin 1; -58.92

              Screen grid resistor; 509 ohms (drifted a bit, but still within tolerance)

              Anything jump out, here, that may be an issue?

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              • #8
                Looks like one tube is conducting and the other isn't, well not very much. That alone will leave you with a hum that persists in spite of removal of the drive tube. The tube with what -58 on pin 5 is biased to cutoff.
                Enjoy. Every. Sandwich.

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                • #9
                  To find out how much each tube is conducting, it requires either 1R cathode resistors or a bias probe.

                  A 6L6 isn't necessarily in cutoff with a grid bias of -58V and a screen voltage of 500V.

                  Unbalanced bias voltages can be necessary for balanced idle currents with unmatched tubes.
                  - Own Opinions Only -

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                  • #10
                    An additional measurement - the output tubes matching pot. This is the tapped (?) pot with an additional lug on the opposite side of the pot.

                    The "tap" lug measures at 58.77; the center lug, connected to the bias resistor, opposite the tap lug measures at 52.02.

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                    • #11
                      Originally posted by Fred G. View Post
                      An additional measurement - the output tubes matching pot. This is the tapped (?) pot with an additional lug on the opposite side of the pot.

                      The "tap" lug measures at 58.77; the center lug, connected to the bias resistor, opposite the tap lug measures at 52.02.
                      Voltages look good.
                      The matching pot is meant to be adjusted for minimum hum.
                      - Own Opinions Only -

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                      • #12
                        OK, some TOTAL weirdness going on here:

                        1. I swapped the 6L6's to the opposite sockets. The filaments on V7 didn't even light up. Swapped them back, and everything lit up and went back to "normal (except the hum issue). While this was occurring, the Hum Balance pot began to show some life and affected the hum when it was rotated. This happened exactly ONCE and never again;

                        2. I got out my test sockets with the 1 ohm resistor, in order to test the current. These tubes are mismatched, the amp is biased pretty cold. V8 at 25 mA, V7 at 13.2 mA;

                        3. Swapped tube positions - V8 at14.5mA, V7 at 20.8 mA. And NOW, the "tube matching" pot setting NEEDED to be REVERSED in order to kill the hum! This was NOT happening before when I moved the 6L6's around. And now again, the hum balance pot isn't having much noticeable effect. I also think the biases for each respective tube should have been a little closer to their readings in the opposite sockets, too?

                        WTF is going ON here?

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                        • #13
                          Bad socket contacts?
                          - Own Opinions Only -

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                          • #14
                            Originally posted by Helmholtz View Post
                            Bad socket contacts?
                            I have been suspicious of that all along. I cleaned and re-tensioned both sockets, but maybe I just didn't do a good enough job. To my mind, the events in my bullet point #3 above make sense, but all the other stuff going on is just flat WEIRD! It does kinda point to some intermittent issue at the sockets. Maybe some crap had hung up in one of the sockets and has been cleaned out by all my swapping around...

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