Can someone explain why ?............Bias content

Discussion in 'The Workbench' started by Kelia, Mar 29, 2020.

  1. Kelia

    Kelia Well-Known Member

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    Hi guys,..........a few days ago ,...I've picked up my amp back from the tech
    with an Output installed and new bias done with a probe . Had my tech install
    some 1 ohm resistors so I can do the bias myself when I want to change tubes .

    Today I've checked the bias and both tubes were around 26 and the amp sounds phenomenal !
    So again with some curiosity ,......I checked the plate voltage and it's at 475v so made the calculation with 65% dissipation and it gave me 34mv so I biased it at 34 and tried the amp,
    for some reason ,....now there was more compression but with less articulation and to my ears
    I prefered the amp the way my tech biased it with the perfect curve on his scope .

    So why is this ,.....am I running cold and it sounds good for some reason ?

    Any thoughts on Scope visit Percentage ?

    I also have another question ,.........I wanted to install some old RFT's while
    the amp was open and without touching the bias trim when it was set low at 26mv, one of the RFT was at 33mv and the other at 55mv so what does this mean?..............why is there so much difference between the Yogo's and the RFT's and why did one clime up at 55mv ?

    Tx
     
    Last edited: Mar 29, 2020
  2. junk notes

    junk notes Well-Known Member

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    yeah, After calculations and knowing the plate voltage, as long as it was done right, it pretty much up to the individual whether it sounds better to them hotter or colder, within a few numbers. It's about listening with your fingers. One of three choices.
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    You didn't say which amp you have.

    The lowest distortion is usually about 70%.
    That lowest distortion is measured (viewed) with a scope. Not a bias probe.
    But if you set it lower than 70% it will fizz when you drive the output harder.
    If you don't drive the output harder, then you might get away with cold bias. That is, you are playing the amp on "2."
    But if you crank the amp up, master volume on "7," (as god herself intended) then you will really start to hear the fizz of cold bias.
    Yes hot bias is always cleaner.

    Tubes are 33ma and 55 ma?
    all tubes are not the same, there can be large differences.
    This is why tubes are matched into sets.

    There may be 1 bad tube and 1 good tube,
    or you may have 2 mismatched good tubes. Don't know yet.

    There is also the screen grid resistors.
    If the screen grid resistor is bad, it can cause a mismatch in current between the 2 tubes.
    Take the tubes out and measure the screen resistors with the ohm meter.
     
    scotheath and Kelia like this.
  4. Kelia

    Kelia Well-Known Member

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    Thanks !,...........this is my 85 JCM800 .
    Don't think I know how to check screen grid resistors so I will ask my tech instead to be safe.
     
  5. Kelia

    Kelia Well-Known Member

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    Would it be a good thing after all these years to have the screen grid resistors replaced ?
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    You don't need to unless the resistors are bad.
     
  7. Kelia

    Kelia Well-Known Member

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    Thank you ams........asking because when I"ve put the old RFT's in
    and saw that one was at 55ma ,....heard a pop noise so I stopped and install
    the ones that were there before ,....also heard the pop noise when I flicked the standby switch on when the RFT's were in so I'm wondering what that pop noise
    was and a little squared to try those again and hope I didn't stess a component inside ?
     
    Last edited: Mar 30, 2020
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    You may have damaged something.
    My recommendation: always use new matched tubes.
    Don't install any old, used, "tested" or "NOS" tubes.
     
  9. Kelia

    Kelia Well-Known Member

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    Thanks ams ,...amps works and sound great now ,.....should I bring it to the tech for a components check?
     
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    A real technician with an Oscilloscope can tell if the amp is working correctly within 5 minutes.
    If the tech does not use an O scope - look for another tech.
    Real technicians use test equipment.
     
    Kelia likes this.
  11. Kelia

    Kelia Well-Known Member

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    Thank you !......appreciate "
     

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