The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all products and all products



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    Join date : 2008-11-30


    Post by tubes4hifi on Wed Sep 16, 2015 2:24 pm

    I get this question alot so I figured I may as well post this info on the board as a stickey . . .

    If you have a 30-50 year old Dynaco ST70, I'd highly suggest a new power transformer.

    Almost all of them have been running hot for so many years, that they aren't going to be happy

    with the extra current demands of the octal driver board, which uses 1200ma of filament current

    compared to 900ma for the original driver board that used two 7199 tubes.

    On the other hand, the ST70 was designed with the capability of powering an extra preamp thru the front octal sockets,

    and the specs for the amp state that EACH front octal socket can deliver 6vac at 1 amp.

    So realistically using the old power transformer to power the octal driver board should be no problem at all.

    I like to tell customers that if your power transformer doesn't run HOT, then it's OK to use the octal board.

    Almost all ST70 transformers run pretty warm, which is normal, but some of the older ones run really HOT.

    If you have a newer power transformer (they've been sold for around 20 years now as an upgrade)

    it is the same height as the output transformers, rather than almost an inch shorter.

    These all have even more current capability and run cooler than the original, and definitely have no problems at all

    with the extra 300ma that the octal driver board uses.

      Current date/time is Fri Feb 22, 2019 9:33 am