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    Rectifier damaged?

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    hwaitung

    Posts : 6
    Join date : 2018-05-05

    Rectifier damaged?

    Post by hwaitung on Mon Jul 16, 2018 8:19 am

    I was playing with the triode and pentode switch the other day on my VTA ST70. Basically I turn off the amp, switch the mode to triode and turn it on again, but I did it quite quickly within 1 or 2 min of turning off the amp. A couple cracking noises from the tubes was heard when I turn the amp back on but it plays fine. The sound seems to be from the rectifier. When I check the bias again, the bias of all 4 power tubes drops to something around 0.2V from 0.4V originally. If I replace the rectifier with another one, the bias goes back up. Sounds like the rectifier is partially damaged but not totally, any idea is appreciated?
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    peterh

    Posts : 985
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Rectifier damaged?

    Post by peterh on Mon Jul 16, 2018 11:02 am

    Switching off and on is not want the rectifier likes. At least wait until cathode has cooled down.
    Switching triod/pentode don't need to power down ( i think).
    Consider that GZ34 toast, don't use it again.

    New2Tubez

    Posts : 19
    Join date : 2018-03-20
    Location : NY

    Re: Rectifier damaged?

    Post by New2Tubez on Mon Jul 16, 2018 12:21 pm

    By any chance, did you do Roy's "yellow sheet diode mod"?
    It protects from current traveling backwards from the capacitor into the rectifier at power down.

    See post 5 here:  http://dynacotubeaudio.forumotion.com/t1518-soft-start-rectifier-for-st70

    hwaitung

    Posts : 6
    Join date : 2018-05-05

    Re: Rectifier damaged?

    Post by hwaitung on Mon Jul 16, 2018 7:18 pm

    peterh wrote:Switching off and on is not want the rectifier likes. At least wait until cathode has cooled down.
    Switching triod/pentode don't need to power down ( i think).
    Consider that GZ34 toast, don't use it again.

    Lessons learned. Luckily that is not an expensive NOS rectifier.

    hwaitung

    Posts : 6
    Join date : 2018-05-05

    Re: Rectifier damaged?

    Post by hwaitung on Mon Jul 16, 2018 7:22 pm

    New2Tubez wrote:By any chance, did you do Roy's "yellow sheet diode mod"?
    It protects from current traveling backwards from the capacitor into the rectifier at power down.

    See post 5 here:  http://dynacotubeaudio.forumotion.com/t1518-soft-start-rectifier-for-st70

    Unfortunately no. Should get that done earlier... Crying or Very sad
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    sKiZo

    Posts : 1490
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Rectifier damaged?

    Post by sKiZo on Mon Jul 16, 2018 9:36 pm

    And yes ... no problem hot switching the TP settings ... power down not required.

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