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Peavey VTM Conversion to EL34 & 1 ohm resistors

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  • Peavey VTM Conversion to EL34 & 1 ohm resistors

    I've cut the land between pins 1 & 3 and tied pin 1 to pin 8. I now need to insert the 1 Ω resistiors to ground. But since the PC board has pin 8's tied to each other as well as ground, Do i need to make another cut to isolate them and have the 1 Ω resistors be the only path to ground? This seems correct. I guess I just need some confidence here.
    I've also made a bias adjust ckt with a 3.3k resistor in series with a 50k pot (R8 & R9 respectively on the Peavey PSU board).
    I believe this is all there is to it although, when I try to trace the bias ckt on the VTM schematic, I get lost. I'm comfortable with Marshall's way of doing it but the Peavey schematic loses me.
    Thanks!

  • #2
    Yes you would have to make another cut to isolate the 2 pin 8's. But please explain what you mean about pin3, that is the plate?
    "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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    • #3
      Originally posted by g1 View Post
      Yes you would have to make another cut to isolate the 2 pin 8's. But please explain what you mean about pin3, that is the plate?
      If this is the correct schematic and PCB layout it looks like a really messy mod to do. All the pin 8s are connected together and pin 3 is connected through pin 1

      Peavy_vtm_120.pdf

      Edit:
      I see we have been here before - Peavey VTM - from 6L6 to EL34
      Last edited by Dave H; 05-26-2019, 12:38 PM.

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      • #4
        Pin 3 is the plate. The way I understand it, I must isolate pin 8 from all ground references (3 cuts to the pcb), one jumper will tie pin 1 to pin 8 and a 2nd jumper will tie the cut grounds (for the rest of the ckt) back together. I will then insert a 1 Ω resistor between pins 1&8 and ground.
        Click image for larger version

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        • #5
          Yikes this looks a bit sloppy, remember we are dealing with potentially deadly high voltage here!
          (outside the bias circuit)

          You could just use a bias probe to set the bias and not have to add the 1 ohm resistors onto the board. As per the other thread the JSX has a switchable voltage divider to allow both types of tubes to be used, and Eurotubes advocates making hybrid versions that run 2x EL34 and 2x 6L6s with separate bias circuits for each.

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          • #6
            Well, I cut the lands and soldered in the orange jumpers and resistors. The rest of the wiring is original. I don't have a bias probe so the 1 Ω resistors will have to stay. I did also cut the land between pin 3 and pin 8. This necessitated another jumper betwee pin 3 only and B+. And I changed the screen resistors to 1k 5w.
            Click image for larger version

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            Update- the operation was a success. I can't say there's a HUGE difference in sound at low volume, but I can only go be memory. Thanks for your help.
            Last edited by Just Mike; 05-28-2019, 12:40 PM.

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