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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    Proper MkIII KT88 bias

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    sdcarroll


    Posts : 2
    Join date : 2018-02-18
    Age : 68
    Location : Denver, CO

    Proper MkIII KT88 bias Empty Proper MkIII KT88 bias

    Post by sdcarroll 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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    Guest
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    Proper MkIII KT88 bias Empty Re: Proper MkIII KT88 bias

    Post by Guest Mon Mar 26, 2018 10:23 pm

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    Last edited by PeterCapo on Mon Nov 30, 2020 6:06 pm; edited 1 time in total
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    sdcarroll


    Posts : 2
    Join date : 2018-02-18
    Age : 68
    Location : Denver, CO

    Proper MkIII KT88 bias Empty Re: Proper MkIII KT88 bias

    Post by sdcarroll 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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    Proper MkIII KT88 bias Empty Re: Proper MkIII KT88 bias

    Post by Guest Tue Mar 27, 2018 6:05 pm

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    Last edited by PeterCapo on Mon Nov 30, 2020 6:06 pm; edited 1 time in total
    WLT
    WLT


    Posts : 186
    Join date : 2013-07-13
    Location : Rochester NY

    Proper MkIII KT88 bias Empty tube life

    Post by WLT 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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    Proper MkIII KT88 bias Empty Re: Proper MkIII KT88 bias

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