Voltage reading vs. tubes...

Discussion in 'The Workbench' started by Dominic Lampron, Apr 6, 2020.

  1. Dominic Lampron

    Dominic Lampron Member

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    Hi everyone, hope you're all OK with what's happening right now. My question is about reading voltage on tube socket with a meter. I'm pretty sure that i've already read this in the past on this website, but just to be sure as i did'nt found my answer: can i put out all tubes (3 x 12ax7 preamp tubes and 2 x power tubes), power on the amp, stand by switch at off and be able to read all infos about plate (pin 3), screen and control grid (pin 4 and 5), cathode current (pin 8) without all tubes into my amp? Is that can be done without damage anything on the amp? Is there any advantage doing this or it's better to do this with all tubes? Thanks and have a nice day!
     
  2. Kinkless Tetrode

    Kinkless Tetrode Well-Known Member

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    You can power up the amp without tubes in it. It won't hurt it. This can be done to check to see if somethings is wrong without damaging tubes. If it is blowing fuses you can do that to check if it still blows fuses with or without tubes. However, the voltage readings will likely be higher than normal, so not really representative of operational conditions. You can't measure cathode current if there is no tube for electrons to flow through.
     
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  3. ampmadscientist

    ampmadscientist Well-Known Member

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    No you can't.
    The tubes must be installed, and the speaker must be connected.
    The tubes should be biased properly first.
    Then you can take some good readings.

    The voltage will not be the same as written on a schematic, for a tube circuit. There will be big variations in tubes and nothing is wrong.
    For solid state circuits, the voltage written on schematic will be pretty accurate. There will only be small variations for solid state circuits.
     
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  4. Dominic Lampron

    Dominic Lampron Member

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    Thanks AMS. I’m still waiting for some resistors (47k for NFB, 10k 1w to replace ine out of specs and a 47K for bias too) to finish my work into my JMP. I’ll do some test with tubes and let you know all reading.
     
  5. Dominic Lampron

    Dominic Lampron Member

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    thanks!!!
     
  6. Dominic Lampron

    Dominic Lampron Member

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    Do you have recommandation for screen resistor? There are a lot of brand... 1k 5w 5%? 1%? there are Vishay, Xicon, Yageo... also, wirewound? through hole? confused a bit....
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    Screen resistor for what amp?
     
  8. Dominic Lampron

    Dominic Lampron Member

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    my jmp 76 with EL34
     
  9. Dominic Lampron

    Dominic Lampron Member

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    I will order at mouser....
     
  10. Dominic Lampron

    Dominic Lampron Member

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    50w amp...
     
  11. Dominic Lampron

    Dominic Lampron Member

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    50 watt jmp 1976. I have XF2 double OO getter... I will order from mouser...
     

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