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    Hmmmmmmmmmmm

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    tubenutr

    Posts : 76
    Join date : 2015-01-25
    Age : 60
    Location : Northern Ontario

    Hmmmmmmmmmmm

    Post by tubenutr on Mon May 28, 2018 1:46 pm

    Hey folks,

    I realize that this has been beaten to death, but here goes.

    My VTA ST70 has developed a hum in both channels and as well in the power transformer.
    I've changed out the middle 12au7 as well as the GZ34. No change.
    Not much has changed as in I haven't tube rolled or changed anything upstream.
    I should also note that when the preamp is off, the power amp hums very loudly.

    Any help would be greatly appreciated.

    Thanks,
    Mike

    wildiowa

    Posts : 187
    Join date : 2012-03-19

    Re: Hmmmmmmmmmmm

    Post by wildiowa on Mon May 28, 2018 2:00 pm

    Ground loop mania. Let the search begin. Some are easier than others. A detailed roadmap for diagnosis and, hopefully, a cure - will be forthcoming from the experts soon. Be patient, it is a holiday...
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    peterh

    Posts : 959
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Hmmmmmmmmmmm

    Post by peterh on Mon May 28, 2018 2:14 pm

    tubenutr wrote:Hey folks,

    I realize that this has been beaten to death, but here goes.

    My VTA ST70 has developed a hum in both channels and as well in the power transformer.
    I've changed out the middle 12au7 as well as the GZ34. No change.
    Not much has changed as in I haven't tube rolled or changed anything upstream.
    I should also note that when the preamp is off, the power amp hums very loudly.

    Any help would be greatly appreciated.

    Thanks,
    Mike
    Connect a large-area cable between pre and vta connected to the chassies. Crimp-eyelets and
    "toothed washer" will make sure of good connection.
    Then, disconnecting inputs one by one until humm vanishes.
    Failed input will be dealth with with isolation transformers or other ways.

    It goes without saying that 3-prong powercables will be beneficial when all is sorted out.
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    Peter W.

    Posts : 750
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Hmmmmmmmmmmm

    Post by Peter W. on Tue May 29, 2018 8:15 am

    *GLEEP*.

    Far too little information to diagnose accurately, or even suggest where to start. So:

    a) 60 or 120 Hz hum?
    b) Gradual or sudden onset?
    c) "... in the transformer." Have you tightened the bolts holding the transformer together and to the chassis?
    d) Can you feel the hum?
    e) Is there any heat associated with the hum? (New heat, that is.)
    f) Do you have a fine-pitch ammeter?

    Hum in both channels suggests the power-supply.
    60 Hz hum suggests the rectifier.
    120 Hz hum suggests capacitors.
    A humming transformer suggests loose laminations and/or excessive load.

    Any further suggestions are dangerous before some answers. BUT, I will *state* that the amp should be taken out of service except for diagnostic purposes until the source of hum is identified and corrected.
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    bluemeanies

    Posts : 235
    Join date : 2015-02-09
    Age : 67
    Location : Folsom Pa.

    Re: Hmmmmmmmmmmm

    Post by bluemeanies on Tue May 29, 2018 8:29 am

    Possible dreaded ground loop as mentioned!
    Have you added anything new to your system? If so I would start there. 
    If the answer is no I would literally disconnect every piece of equipment and one by one re-enter each piece.
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    tubes4hifi
    Admin

    Posts : 1463
    Join date : 2008-11-30

    Re: Hmmmmmmmmmmm

    Post by tubes4hifi on Sat Jun 02, 2018 9:34 pm

    I agree with Peter W on all points !
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    tubes4hifi
    Admin

    Posts : 1463
    Join date : 2008-11-30

    Re: Hmmmmmmmmmmm

    Post by tubes4hifi on Thu Jun 07, 2018 9:29 pm

    turned out to be a bad rectifier tube !!
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    sKiZo

    Posts : 1466
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Hmmmmmmmmmmm

    Post by sKiZo on Thu Jun 07, 2018 10:42 pm

    I'd try replacing the rectifier tube ...

    (oh ... wait ... never mind!) clown

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