Biasing practice and power tube burn in..

Discussion in 'The Workbench' started by Adrian R, May 20, 2020.

  1. Adrian R

    Adrian R Well-Known Member

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    Since I've had some more time off lately due to the pandemic.. I decided to install a couple sets of power tubes, bias them..play a bit..check, then leave them idling for several hours with routine checks to see what happens. I have installed two types of tubes, both EL-34s. One set was a Tungsol quad, the other a JJ E34L set.

    I noted that the JJs were far more unstable then the T/sols initially, but eventually settled down. I also noted that the next day after they had completely cooled the bias changed in both sets by as much as 5mV/A!

    To me it appears that the 'burn in' option that some tube vendors provide may indeed be a viable expense.

    What's your experience?
     
  2. What?

    What? Well-Known Member

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    Bias seems to vary a bit in any amps I have checked. I think it is normal, but I haven't seen much discussion about it.
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    How do you know that this is not caused by mains voltage fluctuation?

    You monitor the bias, but you don't monitor the mains voltage.

    Every matching lab burns the tubes in.
    So does the manufacturer.
     
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  4. marshalltsl

    marshalltsl Well-Known Member

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    I tryed the JJE34L on my TSL100 years ago. Was good but IMHO I think exist much better tubes in the current production. I sold it like a used tubes. Do you have tryed the TAD EL34 STR? It sounds different than JJ tubes. Burn in? Yeah! It's expensive but maybe it's better. Once I bought a winged C new without burn in. In three mounth one heater stoped working.
     
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  5. Adrian R

    Adrian R Well-Known Member

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    This is why I bias in the middle to compensate...5mV variance can have a profound effect on some amps!
     
  6. Adrian R

    Adrian R Well-Known Member

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    My wall voltage here btw doesn't vary too much...200amp service..pretty modern grid..couple volts variance max.
     
  7. What?

    What? Well-Known Member

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    What differences have you noticed and in which amps?
     
  8. Gutch220

    Gutch220 Well-Known Member

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    Is there something you can buy to put between the wall outlet & amp to smooth out voltage fluctuation?
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    The transformer is 1:4
    2 volts variation = 8 volts variation
     
  10. What?

    What? Well-Known Member

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    A voltage regulator unit. $ though. But is it really needed?
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    https://www.jj-electronic.com/en/technology

    "All vacuum tubes and capacitors go through an extended burn-in period to ensure superior stability. A rigorous final test for tubes and capacitors verifies every important parameter. In particular, the parameters for flawless tube functioning include insulation characteristics, vacuum level, cathode emission efficiency, filament voltage, plate currents, and microphonics. The quality control process includes several thousand hours of reliability testing on samples of tubes and capacitors."

    [​IMG]

    It is normal for the manufacturer to burn the tubes in before matching.
     
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  12. Phony iommi

    Phony iommi Well-Known Member

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    The hell you say!
    I recently purchased a new set of power tubes and paid an extra $2 each for burn in service.
    Did I get ripped off or are you speaking only in terms of preamp tubes?
     
  13. Im247frogs

    Im247frogs Well-Known Member

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    Maybe that's an inside joke at the factory, they burn you for an extra 2 bucks.
     
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  14. ampmadscientist

    ampmadscientist Well-Known Member

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    The tubes are burned and tested at the factory.
    then
    The tubes are burned and tested at the matching lab / vendor.
    They burn the tubes to spot defects before testing.
    And so 2 burn ins and 2 tests is pretty standard.
    They do this to all the tubes preamp and power and rectifier.

    Burn in is standard practice.
    And so $2 extra is pretty fair for a lab match...

    But don't tell me that leaving the filaments on wrecks the tubes (in standby).
    Because all the tubes are burned in that way. 24-48 hours usually.
     
  15. Phony iommi

    Phony iommi Well-Known Member

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    I ain’t gonna tell you anything about amps. I’m an amp idiot and you are the man.
    But in the future are you suggesting that one could save a few bucks by leaving the amp on standby for a day? That seems pretty easy to me. As long as it’s healthy for the amp.
     
  16. mickeydg5

    mickeydg5 Well-Known Member

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    Leaving tubes in STANDBY mode with your amplifier is not a "burning in" procedure. That is just wasting the life of your filaments/tubes.

    "Burn in" is basically running the tube initially to check for failure/infant mortality at the factory.
    Matching involves additional run time so that is more "burn in" but monitored for characteristics used to match tubes for sets usually by a distributor or seller.
     
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  17. matttornado

    matttornado Well-Known Member

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    sometimes my plate voltage measures 474 and other times 490. depends on time of day but i just bias to whatever i measure at the time.
     
  18. mickeydg5

    mickeydg5 Well-Known Member

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    It is best to measure to your highest utility VAC flucuation but it kind of averages out.
     
  19. South Park

    South Park Well-Known Member

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    The more heat cycles you have on tubes the less magnetic field will effect them .
     
  20. mickeydg5

    mickeydg5 Well-Known Member

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    Are you saying the sun has no magnetic fields?
     

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