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Thread: Variations in non-hifi valves

  1. #1
    Join Date: Aug 2008

    Location: Suffolk, UK

    Posts: 1,359
    I'm Paul.

    Default Variations in non-hifi valves

    Iím currently mucking around with some PL82 valves strapped as triodes. Both are NOS UK made Mullards but they are quite different in their performance. With exactly the same setup one draws around 20mA and the other draws only 16mA.

    Has anyone else seen such a variation with old TV or radio valves that were not made especially for high fidelity audio?

    The currents mentioned are quite low for this type of valves but I am not sure that would really matter.
    ~Paul~

  2. #2
    Join Date: Apr 2012

    Location: London

    Posts: 442
    I'm james.

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    Sounds completely normal to me, assuming you are not using a self bias circuit?

    Sent from my BLA-L09 using Tapatalk

  3. #3
    Join Date: Aug 2008

    Location: Suffolk, UK

    Posts: 1,359
    I'm Paul.

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    Quote Originally Posted by Jazid View Post
    Sounds completely normal to me, assuming you are not using a self bias circuit?

    Sent from my BLA-L09 using Tapatalk
    I am using a self bias circuit..
    ~Paul~

  4. #4
    Join Date: Apr 2012

    Location: London

    Posts: 442
    I'm james.

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    Might it be worth checking the heaters to make sure they are drawing similar current?

    Another thought is that old tubes can get gassy if unused for a while. If they haven't been recently tested might it be possible that this is responsible for the increased Ia in one?

    I also wonder if,, because you're using them rather off their 'spec', if it's worth trying their AC performance rather than their DC? After all they'll be used in earnest that way.

    Sent from my BLA-L09 using Tapatalk

  5. #5
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 4,526
    I'm Alan.

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    All valves can be subject to a plus and minus 25% variation.
    The published specification is only really an average.

  6. #6
    Join Date: Aug 2008

    Location: Suffolk, UK

    Posts: 1,359
    I'm Paul.

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    The project is on hold for now as a few things have gone sideways recently but I do have a few more, so can swap them out to find a pair that are better matched. The triode curve was estimated from the plate and g2 currents added together and plotted against the plate voltage as a point on a gride curve. This provided only limited characteristics that were a loose estimate anyway. Cant really say if the valves are working as expected but initial THD tests were pretty good at least.

    Thanks for the help chaps.
    ~Paul~

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