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  • #16
    Well you only acknowledged part of what I wrote. If pulling the PI didn't increase voltages, and it didn't really, then it's less likely that the OT leads need swapping. But I suppose it's easy enough to lift the feedback loop and see.

    As far as what to expect from incorrect OT lead orientation I can agree, the near ultrasonic thing hasn't happened to me either. But I have seen it manifest in many ways ranging from motor boating to intermittent frequency dependent crackling, instant whistling or a steady screech. So it wouldn't surprise me if it could also cause an ultrasonic oscillation.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    • #17
      Lifting the NFB did eliminate the squealing, which had been somewhat intermittent, but nevertheless it did seem to be improved. So I went ahead and swapped the OT primaries. Again, seems to be an improvement. No squealing, and the tap-test sensitivity seems diminished, though not entirely eliminated.

      The low B+ voltage problem remains. So the next step is to start replacing filter caps one by one to see if I can find a problem there.

      Comment


      • #18
        Originally posted by Fletcher Munson View Post
        Lifting the NFB did eliminate the squealing, which had been somewhat intermittent, but nevertheless it did seem to be improved. So I went ahead and swapped the OT primaries. Again, seems to be an improvement. No squealing, and the tap-test sensitivity seems diminished, though not entirely eliminated.

        The low B+ voltage problem remains. So the next step is to start replacing filter caps one by one to see if I can find a problem there.
        Well... One less thing Glad my hunch worked out. But I wouldn't start replacing filter caps. Assuming they're new that might just be an expensive and unfulfilling exercise. Instead, test voltage drop across the power rail series resistors. If a cap is shorted it'll show itself with a higher than normal current across one. You could do this with the choke too if you know it's DCR.
        "Take two placebos, works twice as well." Enzo

        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

        "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

        Comment


        • #19
          Also check your AC voltage at the OT center-tap with no signal applied (looking for excess ripple).
          The voltage is definitely dropping with idle current increase, so I'm more suspicious of open caps rather than leaky/shorted.

          What PT are you using by the way?
          "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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          • #20
            Instead, test voltage drop across the power rail series resistors. If a cap is shorted it'll show itself with a higher than normal current across one.
            I was mainly thinking of the first 2 caps (reservoir and screen node), which don't have a dropping resistor. If one of the preamp/PI filter cap was leaky enough to significantly drop B+, the preamp supply voltage would be much lower than the measured 241V.
            Also if a cap is leaky it will heat up and leakage will increase, so voltage drops may not be stable.
            - Own Opinions Only -

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            • #21
              A couple data points with my morning coffee:

              I measure 63V drop across the 10k resistor between the 2nd and 3rd filter caps. So 6.3 mA at this point. Unfortunately, I don't know the DCR on the choke.

              I measure 0.1VAC at the OT center-tap. The PT is a Mercury FTWB-P.

              A small mod that I failed to mention: 470 ohm / 3W screen resistors on the 6L6's.
              Last edited by Fletcher Munson; 04-16-2020, 05:09 PM.

              Comment


              • #22
                Originally posted by Fletcher Munson View Post
                I measure 63V drop across the 10k resistor between the 2nd and 3rd filter caps. So 6.3 mA at this point.
                That seems right. And after reading Helmholtz last post I should have expected this. He's right that the error is most likely earlier on in the PS.
                "Take two placebos, works twice as well." Enzo

                "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

                Comment


                • #23
                  I replaced the first three filter caps. No change in voltages. All three of the pulled caps measured within spec around 24-25uf.

                  Voltages:

                  Node 1 - 345
                  Node 2 - 340
                  Node 3 - 277

                  I'm going to post a copy of my actual layout drawing (which differs slightly from vintage) just to get fresh eyes on it. Have I miswired something?
                  Attached Files

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                  • #24
                    Originally posted by Fletcher Munson View Post
                    I replaced the first three filter caps. No change in voltages. All three of the pulled caps measured within spec around 24-25uf.
                    Ok.?. That was expected.

                    Originally posted by Fletcher Munson View Post
                    I'm going to post a copy of my actual layout drawing (which differs slightly from vintage) just to get fresh eyes on it. Have I miswired something?
                    Not being snarky, but that is the diagram you used for reference and not the actual amp. It's impossible to know if something is miswired by looking at this.
                    "Take two placebos, works twice as well." Enzo

                    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

                    Comment


                    • #25
                      Originally posted by Chuck H View Post
                      Not being snarky, but that is the diagram you used for reference and not the actual amp. It's impossible to know if something is miswired by looking at this.
                      OK, no worries. I'm getting a little frustrated with the build. Need to slow down and think more clearly.

                      Comment


                      • #26
                        Well, there are only 2 possible explanations for the low B+: Either something draws alot of current and power (and heats up after some time) - or the power supply is way too saggy.

                        So another idea: Are you sure that both rectifier diodes are ok and are wired correctly? If only one is operative this could explain the sagginess (does this term exist?).
                        Last edited by Helmholtz; 04-16-2020, 08:39 PM.
                        - Own Opinions Only -

                        Comment


                        • #27
                          Originally posted by Helmholtz View Post
                          Are you sure that both rectifier diodes are ok and are wired correctly? If only one is operative this could explain the sagginess (does this term exist?).
                          It does now

                          I attach "ing", "ness" or some other suffix or prefix to words "improperly" all the time. In fact I'm sure I've done it twice just today.
                          "Take two placebos, works twice as well." Enzo

                          "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                          "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

                          Comment


                          • #28
                            I replaced the first three filter caps.
                            What about the reservoir cap (first after standby switch)?
                            - Own Opinions Only -

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                            • #29
                              Also, do you have a way of verifying your compubias unit, like another amp to check?
                              If power tubes are running way hotter than it shows, that would explain your low B+ voltages.
                              "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

                              Comment


                              • #30
                                Originally posted by Helmholtz View Post
                                What about the reservoir cap (first after standby switch)?
                                Yes, this was replaced. No change. The pulled cap was tested and found good.

                                Sorry, I didn't state this clearly. I should've said I replaced the three 22uf caps (16uf on the vintage schematic).

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