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    Bias question single tube drifting

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    jsl1234

    Posts : 28
    Join date : 2017-08-28

    Bias question single tube drifting

    Post by jsl1234 on Tue Feb 27, 2018 8:42 am

    Hi,

    Have STA-120 with standard compliment tube set (6550) from last year.

    Up until last month all tubes held bias at .500 +- .01 consistently.

    Starting last month one tube in particular starting drifting .060 (from .500 to .560) after a an hours or more of use. I have reset the tube back to spec, but it doesn’t seem to hold at .500  since it then either drifts down from the reset point or back up.  The other tubes seem to hold rock steady with only slight adjustments needed to realign to .500 as I tweak the tube that is unstable.

    Issue follows the tube not the socket re: I moved said tube to other side (right front to left front), rebaised the unt and the bias fluctuations follow the tube.

    Should I just let it go until it drifts even higher over time. Or is this an early indicator of a definite but slow fail that when happens will take other components with it?

    Thanks in advance Jay
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    Bob Latino
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    Posts : 2684
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Bias question single tube drifting

    Post by Bob Latino on Tue Feb 27, 2018 10:43 am

    jsl1234 wrote:Hi,

    Have STA-120 with standard compliment tube set (6550) from last year.

    Up until last month all tubes held bias at .500 +- .01 consistently.

    Starting last month one tube in particular starting drifting .060 (from .500 to .560) after a an hours or more of use. I have reset the tube back to spec, but it doesn’t seem to hold at .500  since it then either drifts down from the reset point or back up.  The other tubes seem to hold rock steady with only slight adjustments needed to realign to .500 as I tweak the tube that is unstable.

    Issue follows the tube not the socket re: I moved said tube to other side (right front to left front), rebaised the unt and the bias fluctuations follow the tube.

    Should I just let it go until it drifts even higher over time. Or is this an early indicator of a definite but slow fail that when happens will take other components with it?

    Thanks in advance Jay

    Hi Jay,

    If that tube won't hold bias and it does the same thing in another tube socket, it is possible that this tube may have some type of partial short that increases as the tube warms up. Yes - What you describe is many times a slow fail of the tube. I would just pick up another Sovtek 6550 and replace the tube. You could also pick up a matched PAIR of Sovtek 6550's and use them in the same channel and keep the GOOD tube of the pair as a spare.

    Bob

    jsl1234

    Posts : 28
    Join date : 2017-08-28

    Re: Bias question single tube drifting

    Post by jsl1234 on Tue Feb 27, 2018 11:18 am

    Thanks Bob,

    Maybe time for a little KT88 / KT120 tune rolling experiment Smile

    If I go this route, do most folks keep the same input tubes as a starting point?

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    Bob Latino
    Admin

    Posts : 2684
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Bias question single tube drifting

    Post by Bob Latino on Tue Feb 27, 2018 11:22 am

    jsl1234 wrote:Thanks Bob,

    Maybe time for a little KT88 / KT120 tune rolling experiment  Smile

    If I go this route, do most folks keep the same input tubes as a starting point?


    Yes - I would keep the same input tubes ... The KT120's are just a little more mellow sounding than the KT88's. Which type of output tube that you get would be a matter of personal choice on the type of sound that you like in your music system.

    Bob

    jsl1234

    Posts : 28
    Join date : 2017-08-28

    Re: Bias question single tube drifting

    Post by jsl1234 on Tue Feb 27, 2018 2:18 pm

    Thanks Bob. Given my already degraded upper frequency hearing range, I prefer a 'brighter'  presentation. So sounds like KT88's are the way to go. Can you recommend a source (do you sell them?)

    Jay

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