Going to see if I can run EL34s in my 6550 JMP50

Discussion in 'The Workbench' started by The Grim Peeper, Jan 15, 2020.

  1. The Grim Peeper

    The Grim Peeper Active Member

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    I got my volt meter and bias probe ready. I just need a little refreshing. I’m about to test plate voltage. I have the EL34s in place. So I just let it warm up under speaker load for 5 minutes and test the pin with the white square cap across it with black lead on chassis for grounding right?
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    It won't work until you change the bias range resistor, the current will be too low.

    jmp50.png
     
  3. The Grim Peeper

    The Grim Peeper Active Member

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    But I’ve heard that sometimes there is enough range for both tube types without modifying. Just rebiasing
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    The worst that will happen is the tube will be too cold.
     
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  5. The Grim Peeper

    The Grim Peeper Active Member

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    Ok I’ll try it then. I’ll let you know what the PV is and what the bias probe says
     
  6. The Grim Peeper

    The Grim Peeper Active Member

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    Tubes are glowing and it is reading zero.
     
  7. The Grim Peeper

    The Grim Peeper Active Member

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    I put the 6550s back in and still 0 DC
     
  8. The Grim Peeper

    The Grim Peeper Active Member

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    Only one 6550 is glowing. There’s a slight glow in the dead one near the base.
     
  9. scott-colorado2016

    scott-colorado2016 Well-Known Member

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  10. The Grim Peeper

    The Grim Peeper Active Member

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    Ok Scott givit to me strait.
     
  11. The Grim Peeper

    The Grim Peeper Active Member

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    Something is not feeding the amp voltage.
     
  12. The Grim Peeper

    The Grim Peeper Active Member

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    But the tubes are glowing.
     
  13. junk notes

    junk notes Well-Known Member

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    Bias selector
    upload_2020-1-15_22-29-8.jpeg
     
  14. The Grim Peeper

    The Grim Peeper Active Member

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  15. The Grim Peeper

    The Grim Peeper Active Member

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    That’s the 120k I see close to the trim pot.
     
  16. The Grim Peeper

    The Grim Peeper Active Member

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    The amp turns on and the tubes light up.
     
  17. The Grim Peeper

    The Grim Peeper Active Member

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    But there’s got be another reason I’m not getting any voltage. I had the best tech in the world but ge retired. I think I’ll leave it alone till I find a new one.
     
  18. scott-colorado2016

    scott-colorado2016 Well-Known Member

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    Sorry, I'm not the one to ask, I'm a troubleshooter, but I've never seen a JMP50 in person.
    The amp was working before you did something, then you did something, and the amp stopped working, that's a Scooby "rut row" to me... and, one I've done before.
    Patience, there will be someone to come around to help. Ampmadscientist already offered some advise earlier.

    What is the bias selector for? Hard to tell from the picture. Did you have the bias probe in correctly? Maybe blew the HT fuse. Do you have a meter to check the fuse?
     
  19. The Grim Peeper

    The Grim Peeper Active Member

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    Arg I’m too damn stupid. I changed the fuses. I had a 6550 blow. Now it won’t work. I got zero on plate voltage trying old 6550s and EL34s I’m still reading 0 on plate volts. I can’t get it working. It’s going on the shelf till I figure it out. My tech retired so I’m Out of luck.
     
    Last edited: Jan 16, 2020
  20. The Grim Peeper

    The Grim Peeper Active Member

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    I always have my traynor YBA1. It has never let me down. I have a show tomorrow
     
    Last edited: Jan 16, 2020

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