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    Proper MkIII KT88 bias

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    sdcarroll

    Posts : 2
    Join date : 2018-02-18
    Age : 62
    Location : Denver, CO

    Proper MkIII KT88 bias

    Post by sdcarroll on Mon Mar 26, 2018 9:24 pm

    I own a couple MKIIIs and am rather confused as to the proper bias for the KT88s. The original Dynaco manual specifies 1.56VDC with total current of 140ma. However, I've seen posts that recommend less than ~.9VDC with current of 80ma, i.e. .089V=.08a x 11.2ohms. Will someone explain why the later bias value is recommended?
    Thanks
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    PeterCapo

    Posts : 491
    Join date : 2008-12-05

    Re: Proper MkIII KT88 bias

    Post by PeterCapo on Mon Mar 26, 2018 10:23 pm

    Lowering the bias extends the life of the tubes. Whether or not it is worth the accompanying increase in distortion is up for discussion.

    sdcarroll

    Posts : 2
    Join date : 2018-02-18
    Age : 62
    Location : Denver, CO

    Re: Proper MkIII KT88 bias

    Post by sdcarroll on Tue Mar 27, 2018 2:07 pm

    Thanks Peter. I actually found a good write-up from tube-town.de that provides specific tube dissipation in Watts with plate voltage variables. It's definitely worth a squint. https://www.tube-town.net/cms/userfiles/media/bias/bias-setup-e.pdf According to its table, the Dynaco manual is recommending a very hot bias.
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    PeterCapo

    Posts : 491
    Join date : 2008-12-05

    Re: Proper MkIII KT88 bias

    Post by PeterCapo on Tue Mar 27, 2018 6:05 pm

    You might also be interested in seeing "The Power Output Stage - Pt. 1" on page 5, here: http://www.audioregenesis.com/documents/ST-70%20Base%20Line%20Testing.pdf
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    WLT

    Posts : 7
    Join date : 2013-07-13
    Location : Rochester NY

    tube life

    Post by WLT on Tue Mar 27, 2018 8:34 pm

    I built my Dynaco MK III kits in 1974 and have logged many hundreds (thousands?) of hours on them. Lowering the output current thru them does help extend tube life. In the 1960/70s the KT88s were not expensive. Today the Genalex KT88s can cost about half a pair of MK IIIs. Lower current puts less stress on the power transformer as well. Regardless of distortion the money should help us decide to turn them down some. I have not noticed higher distortion but others may have more experience with it. I doubt it would be significant. If you want really low distortion then maybe MK IIIs are not the way to go anyway.

    I am on my third set of tubes and components (resistors and capacitors). I think they sound great, simple to maintain and are a wonderful value. Enjoy. Bill

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