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Marshall JCM900 Dual Rev 100w SLXq-06 Problems Bad OT ?

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  • #16
    Ok guys. I soldered the OT back in. Removed all output tubes. Powered on, in play mode, I get -48 VDC on pin 5 of V4 and V7 sockets.

    Screen resistors (470 ohm 5 watt) are all around 483 VDC, with output tubes in, and amp in play mode.

    Before this, I was checking resistor values, and noticed the 1.5 kohm resistor going from pin 4 to pin 5 on V4 socket was reading 1.8kohm. It was a carbon comp, so I changed it out for a metal film 1.5 kohm. Could that carbon comp resistor been causing the red plate problem? I know they're known to drift in value with age, and heat.

    I don't have a bias socket, but I did just order one from China. From my understanding, I can get my bias voltage at the intersection of R26 & R27. Is this correct? And, how would I determine a proper bias voltage setting?

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    • #17
      Could that carbon comp resistor been causing the red plate problem?
      No, to cause redplating resistance would need to be very high (several hundred k) or open.

      But you might have bad tube socket contacts. Or a leaking coupling cap C9/10.

      And, how would I determine a proper bias voltage setting?
      You need to measure tube idle currents.

      and adjusted bias to -49.0 mvdc.
      How did you measure this?
      Last edited by Helmholtz; 02-28-2020, 09:43 PM.
      - Own Opinions Only -

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      • #18
        I checked the capacitance of C9 & C10 (The two 50uf + 50uf @ 500v can caps) while I had the OT unwired. All were more than 20% off tolerance. Like 76uf , and 69uf. So, I should change those, right? As far as a bad tube socket contact, thats a possibility. Even though I've retensioned them already.

        I set my bias voltage at the intersection of R26 and R27

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        • #19
          No, those values are fine. Big caps like that are not close tolerance. besides, as filter caps, a little extra capacitance works even better.
          Education is what you're left with after you have forgotten what you have learned.

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          • #20
            At R26/R27 you are setting bias voltage, and I think it is -49V, not -49mV.
            You need to measure idle current, not bias voltage. You can do it by putting a current meter in place of the output valve fuse. It will give you a mA reading for a pair of tubes.
            "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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            • #21
              I noticed two resistors in parallel, connected between an output transformer secondary, and the purple wire. Which is connected to VL1. Is this factory? Or someones attempt at stabilizing the amp? I can't find these resistors on the schematic. Of course, the schematic doesn't show them on the transformer either.
              Attached Files
              Last edited by CoopDaKill; 04-12-2020, 06:40 PM.

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              • #22
                negative feedback iirc

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                • #23
                  Originally posted by CoopDaKill View Post
                  I noticed two resistors in parallel, connected between an output transformer secondary, and the purple wire. Which is connected to VL1. Is this factory? Or someones attempt at stabilizing the amp? I can't find these resistors on the schematic. Of course, the schematic doesn't show them on the transformer either.
                  Look at pg.3 of the schematic you linked in post #11. Lower right, there is a 56K resistor and 10n cap, that's them. The dotted lines indicate wires.
                  Like alexradium said, part of the NFB network.
                  "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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                  • #24
                    Thanks. Now I see. I pulled the output tubes, turned on power (left standby in warm up mode), I'm getting -93 VDC on pin 5. Shouldn't it be around -42 VDC?

                    Next, when I turn off Standby (play mode), I get -54 VDC on pin 5, with output tubes removed. I read on another forum that this difference means the coupling caps are bad. Please help !!!! lol

                    g1 ?
                    Enzo ?
                    Last edited by CoopDaKill; 04-12-2020, 10:29 PM.

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                    • #25
                      The bias circuit in these is different because of the cap coupled feed from the AC. Ignore what it says in standby.
                      "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

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