Does Plate Voltage Change With Different Tubes?

Discussion in 'The Workbench' started by KraftyBob, Sep 4, 2018.

  1. KraftyBob

    KraftyBob Well-Known Member

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    I’m changing the tubes in my DSL40C for the first time. It came with TAD’s and with those the plate voltage is 457v/458v.

    I have a matched set of Ruby EL34B-STR on order and when I install those do I need to check the plate voltage again before I bias or will it always be 457/458 no matter what tubes I use?

    I’m fine doing all this, that’s not the issue. But the less I need to poke around 450v the better.
     
  2. Micky

    Micky Well-Known Member VIP Member

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    Some tubes draw more current than others, and therefore will lower the B+ just a bit. The opposite is also true.
     
  3. KraftyBob

    KraftyBob Well-Known Member

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    Thanks Micky. I'll check it with the new tubes then.
     
  4. FourT6and2

    FourT6and2 Active Member

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    Plate voltage will also change with bias setting. So you need to check Vp, bias, recheck Vp, rebias, rinse/repeat.
     
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  5. KraftyBob

    KraftyBob Well-Known Member

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    Good to know!

    Thanks everyone!
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    The plate voltage will never stay the same, with a different tube.
    There is wide variations in tubes.

    When you adjust the bias current higher (bias hotter), the voltage will always drop down.
    When you make the bias lower, (colder) the voltage will always go up.

    This is Ohm's Law. More load drops more voltage, it's a law that cannot be broken.

    The bias should always be tested adjusted when you change the tubes.

    --------------------------------------------------------------------------------------------

    There are still some who believe that all tubes should be the same...
    But that is an unrealistic expectation.
     
    Last edited: Sep 8, 2018
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  7. KraftyBob

    KraftyBob Well-Known Member

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    I was thinking about this today and what about the amps that have an external bias adjustment? How are they checking plate voltage?
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    They don't need to check plate voltage.
    All they need is an O scope. No bias probe either.
    Before there were bias probes, techs tested bias adjustment with a scope and that's all they needed.
    That's why the bias adjust is on the outside....
     
  9. FourT6and2

    FourT6and2 Active Member

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    Users don't typically check plate voltage on their amps because they just listen to the manufacture about what the average plate voltage in the amps are at a given mains voltage. If a company puts out 100 amps of the same model and during testing at 120v mains, plate voltage on all of them ranges from 450v – 458v, and they set the bias to 65% at the factory, then they'll just tell customers to bias the amps to 65% based on a plate voltage of 455v or something like that since 65% leaves enough cushion even if it's a bit hot due to a customer who has a higher mains voltage. Bias setting doesn't have to be down to the 1,000th decimal point. 60% to 70% is fine. Just comes down to how you like the sound. The amp will survive whatever setting just fine.

    So when you buy an amp with external test points, the user manual will tell you to bias the amp to some number—for example, 34mA. And you can trust that number since I doubt it will be anywhere close to 70% or higher. Should you measure the plates anyway? Yes. It's always a good idea to trust, but verify.
     
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    True the plate voltage isn't being measured.
    But if voltage is high or low, imbalanced, normal or goofy, that will show up in a scope test.
    If bias is hot, cold, normal or wrong, that will also show.

    The scope only shows the final result of voltage current into load, (not the separate ingredients) that result is interpreted from the wave form.
     

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