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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


    Stock MK III upgrade to octal driver boards?

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    Adnick

    Posts : 7
    Join date : 2020-02-08

    Stock MK III upgrade to octal driver boards? Empty Stock MK III upgrade to octal driver boards?

    Post by Adnick on Sat Feb 08, 2020 11:52 pm

    Hello:

    Thought I’d join the site, big fan of tubes4hifi products, have an original ST 70 with t4h driver board, and other common mods...

    Just picked up some relatively stock MK IIIs, they have updated power supply management and CAPs, but stock other than that.

    Interested in the octal driver boards and was wondering, is there any extra current needed for two 6SN7's, are the stock 6.3 heater winding sufficient?

    Really appreciate the site....

    Regards,
    Andy

    Bob Latino
    Bob Latino
    Admin

    Posts : 2916
    Join date : 2008-11-26
    Location : Massachusetts

    Stock MK III upgrade to octal driver boards? Empty Re: Stock MK III upgrade to octal driver boards?

    Post by Bob Latino on Sun Feb 09, 2020 10:02 am

    Adnick wrote:Hello:

    Thought I’d join the site, big fan of tubes4hifi products, have an original ST 70 with t4h driver board, and other common mods...

    Just picked up some relatively stock  MK IIIs,  they have updated power supply management and CAPs, but stock other than that.

    Interested in the octal driver boards and was wondering, is there any extra current needed for two 6SN7's, are the stock 6.3 heater winding sufficient?

    Really appreciate the site....

    Regards,
    Andy


    The original Dynaco Mark III used a single 6AN8 driver tube with a current draw of .45 amps. A 6SN7 draws .60 amps but you have two of them so > two 6SN7's will draw 1.2 amps. I don't known what the current rating was of the original Dynaco power transformer's 6.3 volt AC filament line was BUT you will now be drawing and extra 3/4 of an amp (1.2 - .45 = .75). At the very least the amp will run noticeably warmer than a stock Dynaco Mark III. The Dynaco Mark III came out in 1957 and some of these amps are now 60 + years old. I don't know if I would want to stress out a 60+ year old amp's original power transformer ? Dynaco, as a cost cutting measure, built very little reserve into these amps.

    If you use the standard VTA Mark III noval board that uses 2 X 12AU7 type tubes (which use .30 amp filament current per tube or .60 amps for a pair), you have .60 - .45 (a single 6AN8) = just an extra .15 amp of current draw. This would be a much easier load increase for an older original Dynaco power transformer to deal with.

    Bob
    avatar
    Adnick

    Posts : 7
    Join date : 2020-02-08

    Stock MK III upgrade to octal driver boards? Empty Re: Stock MK III upgrade to octal driver boards?

    Post by Adnick on Sun Feb 09, 2020 11:07 am

    Thanks Bob:

    While trying to research this for myself, I had come across an old post by you talking about the fact that the T4H MK III driver boards had their own on board power supplies, and my original supposition was that this, would reduce the stress on the amps power supply and thereby might offset the extra load of the 6AN8s....

    Given your reply it appears my supposition was was a bit off (-;

    Looks like I’ll be using the dual 12AU7 driver boards....

    Really can’t express my appreciation for thoughtful, concise, and honest response.


    My Highest Regards,

    Andy

    solderblob
    solderblob

    Posts : 82
    Join date : 2018-05-20
    Age : 74
    Location : Placerville, California

    Stock MK III upgrade to octal driver boards? Empty Re: Stock MK III upgrade to octal driver boards?

    Post by solderblob on Fri Feb 14, 2020 12:32 pm

    Just to give you another point of reference, I have a pair of MkIIIs I modified with Roy's octal boards.  The power supply transformers run hot.  I can't hold my hand on them for more than a couple seconds.

    But they work fine -- although I have blown one 5AR4.

    If I had to do it over again, I'd go with the regular VTA boards.  I think they may also sound a bit better too, at least it does on my ST70.

    dave
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    Adnick

    Posts : 7
    Join date : 2020-02-08

    Stock MK III upgrade to octal driver boards? Empty Re: Stock MK III upgrade to octal driver boards?

    Post by Adnick on Wed Mar 04, 2020 8:09 am

    Have one amp done with the dual 6SN7 board, sounds great, but haven’t decided which output tubes to try in conjunction with the new board. Can’t say I can tell a difference in the temperature, the KT88s will heat up that one transformer over time, but building the other amp with a new stock configuration board as my own little experiment, and i’ll check the temperature of both when done.

    Stock MK III upgrade to octal driver boards? A05b7610
    Stock MK III upgrade to octal driver boards? Da6dde10
    Stock MK III upgrade to octal driver boards? Ee776510
    corndog71
    corndog71

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    Location : It can get windy here

    Stock MK III upgrade to octal driver boards? Empty Re: Stock MK III upgrade to octal driver boards?

    Post by corndog71 on Wed Mar 04, 2020 2:51 pm

    Those look great. Love that color!
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    Adnick

    Posts : 7
    Join date : 2020-02-08

    Stock MK III upgrade to octal driver boards? Empty Re: Stock MK III upgrade to octal driver boards?

    Post by Adnick on Wed Mar 04, 2020 5:57 pm

    Thanks much,
    Andy

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