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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


    Pic for Discussion entitled, "St-70, Left Channel, 60 Hz Hum"

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    DynacoGuy


    Posts : 16
    Join date : 2020-10-06

    Pic for Discussion entitled, "St-70, Left Channel, 60 Hz Hum"  Empty Pic for Discussion entitled, "St-70, Left Channel, 60 Hz Hum"

    Post by DynacoGuy Thu Nov 26, 2020 3:29 pm

    [url=Image hosted by servimg.com]Image hosted by servimg.com[/url]
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    DynacoGuy


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    Pic for Discussion entitled, "St-70, Left Channel, 60 Hz Hum"  Empty Please delete this blank post.

    Post by DynacoGuy Fri Nov 27, 2020 5:43 pm





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    Last edited by DynacoGuy on Fri Nov 27, 2020 5:50 pm; edited 2 times in total
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    DynacoGuy


    Posts : 16
    Join date : 2020-10-06

    Pic for Discussion entitled, "St-70, Left Channel, 60 Hz Hum"  Empty Re: Pic for Discussion entitled, "St-70, Left Channel, 60 Hz Hum"

    Post by DynacoGuy Fri Nov 27, 2020 5:48 pm

    Pic for Discussion entitled, "St-70, Left Channel, 60 Hz Hum"  St70_c11


    I put callouts on the pic.

    A couple of things I could adjust,
    but not sure it would make a difference.

    The center taps, power xfmr and
    the Heat CT GNDs could be
    Removed form the ground buss,
    tied together and I'd add a wire to
    Locate them on Xfmr & 3rd prong GND lug.

    In another thread somewhere, can't recall,
    There should be continuity only on pin 2 to pin 2
    and only on pin 7 to pin 7 on each pair of power tubes.

    Is that correct?

    In this amp pin 2 and pin 7 have continuity throughout
    the amp on either side and in board.

    Any ideas?
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    DynacoGuy


    Posts : 16
    Join date : 2020-10-06

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    Post by DynacoGuy Fri Nov 27, 2020 6:01 pm

    peterh wrote:What a nightmare !
    A few observations : Shorting inputs should never increase any hum. This seems to indicate a
    groundloop somewhere

    Removing the 7199 tubes : if hum remains will localize the area to the EL34 / bias / B+
    ( unbalanced powertubes might cause hum )

    How/where is the SDS board connected to ground ? That could be your source, and a source that
    will disappear if you rebuild with a cancap ( restoring the original config)

    As for diagnosing, a scope should be able to find and measure the hum, just turn sensitivity up.
    Having a value of the hum will make it much easier then to resort to listen with the amp
    moved from the workbench.


    I followed the Van Alstine protocol,
    Open inputs.
    left right swap:
    power tubes, hum stayed in left channel.
    7199 swap, hum stayed in left channel.

    Full tube complement with shorting plugs installed:
    Increased hum and Hum is now in both channels.

    Pulled 7199s, no hum.

    I have not added any additional caps other
    than the 8 in the power supply.

    The original multisection was disconnected and
    attached loosely with one glob of solder on a twistlock.

    When I tried to find the hum with the scope I couldn't find it.
    Unless I had brain fart and didn't recognize it.

    I couldn't hear it on my bench speakers...nothing.

    I went back and placed it into the system, then
    with extension chord plugged it into two different
    home power nodes...no difference. Hum remains.


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    DynacoGuy


    Posts : 16
    Join date : 2020-10-06

    Pic for Discussion entitled, "St-70, Left Channel, 60 Hz Hum"  Empty Re: Pic for Discussion entitled, "St-70, Left Channel, 60 Hz Hum"

    Post by DynacoGuy Fri Dec 11, 2020 3:35 pm

    Look at the common output!

    There was no reference to ground on either,
    so I ran a ground wire from each common output
    to the ground buss in the amps center; the same
    location used by Dynaco.

    DG

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