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  • Very dumb mistake

    I just got done debugging a grounding issue with an old Premier tube amp and forgot to disconnect the 10k resistor I hooked up to one of the filter caps to drain it to ground before turning it on. It's now blowing fuses (w/ or w/o tubes). What do you think I blew up? I had restuffed the old multi section can so it's going to be a joy to take it all apart again (they look ok visually). Could this have blown up the PT?

  • #2
    It doesn't sound good. I would expect that resistor to go open rather than short. But a short might cause a PT failure. Try disconnecting everything from the DC end of the rectifier (if it's tube rectified do intall a known good cap good for 600V from the rectifier output to ground). Does it still blow fuses? If yes then disconnect the rectifier (or unplug it if it'a a tube rectifier). Does it still blow fuses?
    "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

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

    Comment


    • #3
      That 10K resistor did not cause the problem.
      .04 amps was flowing through it.
      Troubleshoot the problem as Chuck suggests.

      Comment


      • #4
        No that resistor couldn't break the amp as a resistor. But as a jumper it could cause problems. I would test the resistor to see if it's open or shorted. I think it;s more likely therre is a short in the faux can cap.
        "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

        "If you're not interested in opinions and the experience of others, why even start a thread?
        You can't just expect consent." Helmholtz

        Comment


        • #5
          Thank you for your assistance.

          Sorry for the late reply. This is a mid 60s Premier Twin 8 w/ a solid state rectifier. Schematic here: http://www.prowessamplifiers.com/sch...ier_twin_8.pdf

          I disconnected all the secondaries and it did not blow the fuse. I measured 9 ohm between the primary leads and open between each of them and the chassis. I tested both rectifier diodes and got a 0.55v drop one way and open the other using the diode test on the multimeter. I connected them to the PT, and disconnected the DC side of the recto and measured 258 VDC there.

          I measured the positive leads of the filter caps inside the pseudo can and did not find any shorts to ground but I will take it apart tomorrow and make sure.

          Comment


          • #6
            Don't ignore the heater winding. It may just be coincidence with the bleed resistor being left on. You said you were dealing with grounding problems, did you do anything to the heater circuit?
            Like JPB said, the extra 40mA shouldn't damage a PT. However, if it was being run to it's capacity, or if it was already weak, I suppose the extra current could be the "straw that broke the camels back".
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


            Comment


            • #7
              Originally posted by g-one View Post
              Don't ignore the heater winding. It may just be coincidence with the bleed resistor being left on. You said you were dealing with grounding problems, did you do anything to the heater circuit?
              Like JPB said, the extra 40mA shouldn't damage a PT. However, if it was being run to it's capacity, or if it was already weak, I suppose the extra current could be the "straw that broke the camels back".
              I think you're right that that was a coincidence. The PT seems to be ok, I measured 275VAC from the center tap to either HT side and 6.6VAC on the filament winding.

              I got this amp with hum issues and bad speakers. Upon replacing all of the electrolytics and the speakers there was still quite a bit of hum left. I noticed that if I moved the 100 mf cathode cap on pin 3 of V2 the hum volume changed. I noticed that the ground lug on the tube socket was not properly grounded to the chassis. If I pressed the lip of the tube socket into the chassis with a pair of pliers the hum disappeared. I sprayed the socket and around it with deoxit and that improved the contact. That's when I forgot the bleeder resistor on and the fuses started popping!

              Comment


              • #8
                A good cleaning and retensioning of all grounds, tube sockets, jacks and switches is in order. Sometimes removing oxides can make matters worse because the oxidation is the only thing left making some contact.

                So now connect the next thing in line with the HV rail and disconnect the thing ahead of it. That would probably be one of the sections of your multi cap. If that works connect the next thing in line and disconnect the thing ahead of it. Continue until the amp pops a fuse again and you'll have found your problem.
                "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

                "If you're not interested in opinions and the experience of others, why even start a thread?
                You can't just expect consent." Helmholtz

                Comment


                • #9
                  Thanks Chuck!

                  Comment


                  • #10
                    Sometimes removing oxides can make matters worse because the oxidation is the only thing left making some contact.
                    This reminds me of the joke about the old car which disintegrated to pieces the day it was washed, caked mud and grime had been holding it together.
                    Juan Manuel Fahey

                    Comment


                    • #11
                      I once went to look at a car I was considering buying. Advertised with "fresh paint". Always a red flag. I was young. Anyway, I got there before the seller. The paint looked nice but for one area on a fender that had a couple of small bumps. So I poked at it and my finger went right through the fender leaving a jagged, rusty brown and flakey hole behind.

                      I left before the seller arrived.
                      "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

                      "If you're not interested in opinions and the experience of others, why even start a thread?
                      You can't just expect consent." Helmholtz

                      Comment


                      • #12
                        Originally posted by Chuck H View Post
                        I once went to look at a car I was considering buying. Advertised with "fresh paint". Always a red flag. I was young. Anyway, I got there before the seller. The paint looked nice but for one area on a fender that had a couple of small bumps. So I poked at it and my finger went right through the fender leaving a jagged, rusty brown and flakey hole behind.

                        I left before the seller arrived.
                        I got the other side of that. I went to look at an Austin Healey Sprite one time, the guy wanted $300 for. I spent an afternoon getting it running-took a lotta work and parts-and then went to settle up. The guy says "Oh. It's $500."

                        I said I'm not having it. I took all my parts off it and left, and I figure it's still sitting in the guy's back yard.

                        Comment


                        • #13
                          Originally posted by Prairie Dawg View Post
                          I took all my parts off it and left
                          Hell yeah. Even if It was worth $500 I would have done the same on principal.

                          I once had a customer try to stiff me on a paint job. For no more reason than I was young and scabing the work (read-side job). Knowing I had no recourse as a licenced professional he thought he'd just screw me and save the money. So I went to my truck and got my biggest scraper. He said "What are you doing?" I told him "Taking my paint back." He agreed to pay me but tried to talk me down from the agreed price. So I explained what "agreement" means to this jerk and assured him I would continue to make trouble until I was payed. It worked out.
                          "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

                          "If you're not interested in opinions and the experience of others, why even start a thread?
                          You can't just expect consent." Helmholtz

                          Comment


                          • #14
                            Agree.
                            Sometimes acting crazy is the only thing that works with some jerks.
                            They respect nothing else.
                            Juan Manuel Fahey

                            Comment


                            • #15
                              Originally posted by J M Fahey View Post
                              Agree.
                              Sometimes acting crazy is the only thing that works with some jerks.
                              They respect nothing else.
                              Right. People like that are looking for a victim. Not a fight. It's all in the posturing. But you can't go there without being prepared to commit or you might get your a$$ handed to you.
                              "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

                              "If you're not interested in opinions and the experience of others, why even start a thread?
                              You can't just expect consent." Helmholtz

                              Comment

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