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    very low level hum and reduced volume from one channel

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    rypriest

    Posts : 2
    Join date : 2014-01-13

    very low level hum and reduced volume from one channel

    Post by rypriest on Tue Dec 15, 2015 12:55 pm

    I built a ST-120 about a year ago and it has been performing very well. I recently moved it to drive more efficient speakers and have noticed a very very low level hum and slightly reduced output from the right channel. The reduced output has been evident for some time and is easily compensated with adjusting preamp balance, however, the hum was not apparent. I am just hoping for a little guidance as trouble shoot this. Thank you.

    Bob Latino
    Admin

    Posts : 2414
    Join date : 2008-11-26
    Location : Massachusetts

    Re: very low level hum and reduced volume from one channel

    Post by Bob Latino on Tue Dec 15, 2015 1:38 pm

    Try replacing the CENTER driver tube. This tube has two triode sections - one for the left channel and one for the right channel. The section for the right channel on this tube could be starting to fail ..

    Bob

    nmchiefsfan

    Posts : 33
    Join date : 2012-03-21

    Re: very low level hum and reduced volume from one channel

    Post by nmchiefsfan on Tue Dec 15, 2015 1:40 pm

    The hum may have always been there but not noticeable due to the speaker efficiency. When I had a similar issue the problem turned out to be the center driver tube on the VTA board. Have you tried a different tube in that position? If you don't a spare you could move the left or right tube to the center and see if it helps.

    rypriest

    Posts : 2
    Join date : 2014-01-13

    Re: very low level hum and reduced volume from one channel

    Post by rypriest on Tue Dec 15, 2015 2:37 pm

    I have rolled through various different driver tubes and rotated output tubes without effect. A little more info, the hum does not change with adjustment of the stepped attenuator and is constant 60 hz.

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: very low level hum and reduced volume from one channel

    Post by arledgsc on Wed Dec 16, 2015 10:53 am

    To aid debug you can try an old guitar amp repair trick of desperation by pulling the center tube, turning on the amp, and listening.  Does the amount of hum change?  Same with the phase inverters.  Helps isolate which stage is contributing to the issue.  I don't think there would be any harm in trying briefly other than perhaps tweaking bias to compensate for load changes on the B+ supply.

    But speaker efficiency may be letting you hear everything more clearly.

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