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Troubling idle current draw from unloaded 60's Super Reverb

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  • Troubling idle current draw from unloaded 60's Super Reverb


    After pulling the chassis from a blackface Super Reverb, there were some obvious problems which needed attention (pulled ground leads from brass solder joints, etc.).
    So I jumped in to repair those and reinforce the existing ground connections, as well as fixed the some of the tail/feedback connections. Much of the standard service one could expect from an amp of this vintage was done pretty well by a prev tech.
    Anyways, when powering up via variac with all tubes installed, I noticed that the mains current draw indicated a problem. Usually, this is the result of bias setting leading to the output tubes drawing excessive current at idle. However, even with the bias set to the most negative voltage, the amp was drawing in excess over 1A at idle (120V). So, I pulled the output tubes and brought the amp up to standard power on voltage, and the amp still drew over a half amp in standby.
    Without going through each step of working my way backwards, ultimately, with all tubes pulled (as well as the lamp), when I get to about 70VAC the amp appears to show some loading up through the nominal 120VAC on the primary, where the amp idles just over 1/2A.
    This isn't an overly large mains transformer where you might expect to see the magnetizing current show something like this unloaded.
    Any thoughts? The one thing I didn't pull was the bias tap. But here is the schematic. (AB763)

    https://el34world.com/charts/Schemat..._schematic.pdf
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  • #2
    Pull ALL the tubes out for testing. Amp should draw 10-15 watts at idle. If you're drawing too much current, disconnect the PT secondaries until you find the culprit.
    --
    I build and repair guitar amps
    http://amps.monkeymatic.com

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    • #3
      I recently measured power consumption at idle of a Fender Vibroverb (smaller PT than SR).
      Found apparent power of 67VA and real power of 49W.

      With all tubes pulled both values should be lower by around 40W.

      As your mains current seemes too high with all tubes pulled, this leaves PT, rectifier and filter caps.

      To see if there's something wrong after the standby switch, hook your mA- meter across the open switch.

      What happens to mains current if you pull the GZ34?
      Last edited by Helmholtz; 11-15-2022, 11:11 PM.
      - Own Opinions Only -

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      • #4
        Originally posted by Helmholtz View Post
        I recently measured power consumption at idle of a Fender Vibroverb (smaller PT than SR).
        Found apparent power of 67VA and real power of 49W.

        With all tubes pulled both values should be lower by around 40W.

        As your mains current seemes too high with all tubes pulled, this leaves PT, rectifier and filter caps.

        To see if there's something wrong after the standby switch, hook your mA- meter across the open switch.

        What happens to mains current if you pull the GZ34?
        I've pulled all the tubes including the GZ34. This is where it really puzzled me. I first lifted the reservoir capacitor to make sure that wasn't partially shorted or failing, and then pulled the GZ34 (after swapping another in), but the amp still idles just over a half amp.

        edit: to address your other points, the connection to the reservoir cap was opened while in standby, with no apparent effect. This would basically eliminate a faulty filter cap in the HV supply.
        Last edited by SoulFetish; 11-16-2022, 01:09 AM.
        If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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        • #5
          With all the tubes pulled, as well as the lamp, this essentially opens the filaments as the center tap is grounded vs the 100Ω resistors.
          If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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          • #6
            Originally posted by SoulFetish View Post
            With all the tubes pulled, as well as the lamp, this essentially opens the filaments as the center tap is grounded vs the 100Ω resistors.
            AB 763 has a grounded heater CT, so shouldn't have 100R balancing resistors.

            What are PT secondary voltages?
            - Own Opinions Only -

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            • #7
              Originally posted by Helmholtz View Post

              AB 763 has a grounded heater CT, so shouldn't have 100R balancing resistors.

              What are PT secondary voltages?
              that's correct. I apologize if that wasn't clear
              If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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              • #8
                Originally posted by SoulFetish View Post

                that's correct.
                Means what?

                - Own Opinions Only -

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                • #9
                  Originally posted by Helmholtz View Post

                  Means what?
                  It has grounded heater CT, not resistors. When he said 'vs', he meant 'rather than having'.

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                  • #10
                    Originally posted by SoulFetish View Post
                    With all the tubes pulled, as well as the lamp, this essentially opens the filaments
                    Unless there is a wiring issue or arcing at any of the sockets. Like xtian mentioned, best to disconnect all secondaries at PT end, just to be sure.

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                    • #11
                      Has the death cap been removed?
                      "Yeah, well, you know, that's just, like, your opinion, man."

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                      • #12
                        I have to admit that I've been a little confused by the problem description. You've been talking about "idle current". Including idle current with no tubes in place. Including the rectifier tube.?. But the amp, it's self, doesn't idle. The TUBES idle. So what current are you measuring and where?
                        "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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                        • #13
                          Originally posted by The Dude View Post
                          Has the death cap been removed?
                          After I pulled the Recti, the first thing I thought of. Clipped it out. Wasn’t the culprit.
                          (Usually one of the 1st things I do is remove that)
                          If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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                          • #14
                            Okay, Im referring to a steady state condition which a…..
                            Chuck, don’t be F’in wIth me, bruh. It’s late and Im typing on my phone. Don’t make me type out a some kind of thoughtful retort. I’m typing with one finger here.

                            anyways, I’m being a little flexible with the definition, but it fits. Even an unloaded transformer will draw a steady predictable current through its primary coil
                            If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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                            • #15
                              Hey. There are transformers and ...transformers. I don't know a rule about in respect for what designed for but if you leave it one hour and do not burn you hand when touching is ok. Normally in 20 minutes should get in regime. There are transformers with excessive iddling current for sure -like soldering-gun transformers. I.m not sure about microwave owens trannies. Think is all about design in respect with its functions


                              Late: also just check all secondaries are ok and not shorted one somehow...just to be sure, turns direction in case it use windings connected in series/parallel...and so on...
                              Last edited by catalin gramada; 11-16-2022, 08:46 AM.
                              "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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