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  • #16
    Originally posted by Enzo View Post
    Easy enough to check. But usually if those fail, it blows fuses.


    So did you ever remove Q7? That is my main go-to for this symptom.
    yes on Q7 -- no change

    ill try to test those diodes

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    • #17
      It's not likely the diodes, but easy enough to test, as Enzo says. FWIW, you can just unsolder one leg of each and run the amp without them as a test. They are just there for protection.

      Edit: Do you have a spare OT laying around that you could temporarily sub in?
      "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

      Comment


      • #18
        Originally posted by The Dude View Post
        It's not likely the diodes, but easy enough to test, as Enzo says. FWIW, you can just unsolder one leg of each and run the amp without them as a test. They are just there for protection.

        Edit: Do you have a spare OT laying around that you could temporarily sub in?
        diodes test good---

        i have a few amps here i could use.....ive got a jcm 900 (el34) apart already

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        • #19
          Post #6 there was question about the signal at the grids. Your response in post #8 was incorrect. Measure AC volts at pin5 with black probe to ground.
          Post result.
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


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          • #20
            Originally posted by g1 View Post
            Post #6 there was question about the signal at the grids. Your response in post #8 was incorrect. Measure AC volts at pin5 with black probe to ground.
            Post result.

            ok with or without 1k test tone into input...amp cranked up

            all pin 5 make the ac meter jump all over the place from 0vac up to 16vac

            Comment


            • #21
              That makes me suspect lack of drive to the power tubes, rather than the OT.
              Maybe try at the plates of the PI tube to see if you can get a more stable reading.
              Originally posted by Enzo
              I have a sign in my shop that says, "Never think up reasons not to check something."


              Comment


              • #22
                Originally posted by g1 View Post
                That makes me suspect lack of drive to the power tubes, rather than the OT.
                Maybe try at the plates of the PI tube to see if you can get a more stable reading.

                Ahhh------------------found something!!

                tube removed and PI tube socket has 450vdc at pin6........and 100mv dc at pin 1 .......thats in socket and also thru the ribbon cable at the main pcb . I should have a 200-400 vdc at the PI plates...yes ?

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                • #23
                  With no tube in the socket, you have no current, so plate voltage will go high.
                  "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                  • #24
                    FOUND IT !!!

                    Thanks to all you guys for your time........i think what was throwing me off was...i didnt consider where the fx loop was in the path......(before the PI )

                    It was an open 100k plate resistor on the PI tube !!

                    Amp is properly loud now and sounds great ...............cheers mates !!!!! Thanks G1 !!

                    Comment


                    • #25
                      Originally posted by Valvehead View Post
                      Thanks G1 !!
                      Just rehashing what got lost in the shuffle from post #6 by Jazz P Bass.
                      Originally posted by Enzo
                      I have a sign in my shop that says, "Never think up reasons not to check something."


                      Comment


                      • #26
                        "It was an open 100k plate resistor on the PI tube !!"

                        Pin #1 I'm guessing?
                        It's weird, because it WAS working fine.....

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