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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


3 posters

    Clipping below rated output

    jeffjmr
    jeffjmr

    Posts : 29
    Join date : 2020-08-22

    Clipping below rated output Empty Clipping below rated output

    Post by jeffjmr Sun Oct 17, 2021 2:38 pm

    Two issues needing attention, perhaps pointing to same problem.

    My (TriodeUSA) ST70 is clipping on both channels at around 22W as carefully measured by oscilloscope. Only the phase splitter has been changed (to a nicely balanced 12AU7A) since buying the amp used. Came with new looking JJ EF806Ss and EL34s. I changed out the phase splitter because the negative half of the signal was distorting long before the positive. With the RCA clear top 12AU7A the waveform is symmetrical but still clipping around 22W.

    Lately, the left channel bias is moving day to day. I aim for .700 on each side. I check after about two hours or so of listening. One day the left will be at .675 (right moved up slightly as expected but not by as much) and the next day it will be at .715, right lower by a bit again as expected. Is this excessive movement day to day?

    Is it time for new power tubes, new EF806Ss, or both?

    Or should I be investigating other areas?

    Thanks,
    Jeff
    peterh
    peterh

    Posts : 1589
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Clipping below rated output Empty Re: Clipping below rated output

    Post by peterh Sun Oct 17, 2021 3:17 pm

    jeffjmr wrote:Two issues needing attention, perhaps pointing to same problem.

    My (TriodeUSA) ST70 is clipping on both channels at around 22W as carefully measured by oscilloscope. Only the phase splitter has been changed (to a nicely balanced 12AU7A) since buying the amp used. Came with new looking JJ EF806Ss and EL34s. I changed out the phase splitter because the negative half of the signal was distorting long before the positive. With the RCA clear top 12AU7A the waveform is symmetrical but still clipping around 22W.

    Lately, the left channel bias is moving day to day. I aim for .700 on each side. I check after about two hours or so of listening. One day the left will be at .675 (right moved up slightly as expected but not by as much) and the next day it will be at .715, right lower by a bit again as expected. Is this excessive movement day to day?

    Is it time for new power tubes, new EF806Ss, or both?

    Or should I be investigating other areas?

    Thanks,
    Jeff
    Try new matched power tubes.

    Dale Stevens likes this post

    tubes4hifi
    tubes4hifi
    Admin

    Posts : 1709
    Join date : 2008-11-30

    Clipping below rated output Empty Re: Clipping below rated output

    Post by tubes4hifi Yesterday at 11:58 am

    bias variation will depend on voltage variation on your AC wall outlet, your variation seems within normal range (maybe +/-2%)
    As for the clipping, from your description it would seem to be the driver board isn't really optimized for the tubes, and some resistors would probably have to be changed
    so that the phase splitter creates equal outputs, you'd probably need an oscope to see those waveforms and then adjust the circuit accordingly.
    Well, you do say the waveform is now symmetrical, so maybe it just doesn't have enough output swing. I'm not familar with that board, but might possibly need a higher voltage
    to the phase splitter, or the phase splitter circuit parameters need to be adjusted (plate & cathode resistors).

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    Clipping below rated output Empty Re: Clipping below rated output

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