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Discolored silk screening on EL84'sd

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  • Discolored silk screening on EL84'sd

    Good morning or afternoon...depending on where you are......I have this 15W Traynor YGL1 combo which has a set of EL84 output tubes....one tube has perfect silk screening...the other is black....the tube socket looked like it was overheated as well....these tubes are approx a year old according to the customer...replaced the tube socket, re-soldered all tube socket connections, cathode bias resistors are spot on with no visable signs of overheating damage......all voltages check fine.....could this be just an issue with this tube or might there be an underlying condition that I should further investigate....can't find any intermittment issues so far...Thoughts anybody????
    Cheers

  • #2
    My vote is for a bad tube.

    Comment


    • #3
      Check individual idle currents.
      - Own Opinions Only -

      Comment


      • #4
        Correct schematic? https://www.thetubestore.com/lib/the...-Schematic.pdf
        Assuming it is, are R64 & 65 good?
        Check C2 & 4 for DC leakage, eg lift the P1 MV side legs and see what VDC appears there.
        There’s no safety back up grid leak resistor, to keep things in line in case a P1 wiper goes open circuit. Check to see if the pot is iffy and that might have happened. Even if it’s fine I’d consider fitting a couple of 470k or 1M between the bias feed and grid stoppers.
        My band:- http://www.youtube.com/user/RedwingBand

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        • #5
          For whatever reason, that tube ran very hot for a period of time. It is likely bad or worn out. That is why the thermal paint has changed color.
          Education is what you're left with after you have forgotten what you have learned.

          Comment


          • #6
            Ok. Checked R64/65...dead on....cathode resistors are good as well.....the cathode resistors are in parallel......total resistance is 50 ohms...voltage drop across them is 3.19V...current works out to .063A...divide by 2 and current is .0315A per tube.......plate voltage measures 287V.....no dc voltage measured at the grids of the tubes.....however, the volume pot has a loose shaft...probably too loose especially when compared to the rest of the pots...and these are cheap junky pots....both tubes tested in the tube tester tested great....no intermitment shorts...great emission..maybe the pot is flaky....I will have to order one as I don't have any on hand.....that will take a couple of weeks to get....if any of these measurements seem out of wack, let me know....the amp sounded good with a guitar plugged in....

            Comment


            • #7
              If it was my amp I would separate the cathode resistors, each with its own bypass cap. That would allow to determine each EL84's idle current separately.

              Also the advice of pdf64 in post #4 (last sentence) is highly recommended.
              - Own Opinions Only -

              Comment


              • #8
                I will have to have a talk with the owner and see how he wants to proceed....thanks for the info guys...much appreciated.....
                Cheers

                Comment


                • #9
                  Originally posted by bsco View Post
                  .no dc voltage measured at the grids of the tubes.....
                  Should be around -7VDC. It's not there?
                  "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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                  • #10
                    Originally posted by g1 View Post
                    Should be around -7VDC. It's not there?
                    -9vdc there...my apoligies for that mistake....

                    Comment

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