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    New Member, New ST120!


    Posts : 2030
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: New Member, New ST120!

    Post by deepee99 on Wed Oct 18, 2017 10:48 am

    Tubes4ever wrote:
    peterh wrote:
    jsl1234 wrote:Does it make sense to swap power tube positions first?

    that would involve rebiasing all of them.

    But using the triode/UL switches might be an idea as the problem started at the
    time they were used.

    I had a very similar problem and swapping the power tubes fixed the issue.  Probably was a socket connection issue, but the problem was resolved.  Rebiasing will not have an effect on whether or not the noise issue exists.

    Pin diameters in octal tubes vary widely, too, so what might be too loose in one socket could fit another just fine.


    Posts : 32
    Join date : 2017-08-28

    Re: New Member, New ST120!

    Post by jsl1234 on Wed Oct 18, 2017 12:54 pm

    Well I did swap the power tubes (and rebiased) but that didn’t solve the problem. Have since touched up as many solder joints on the input driver board right side as possible without taking the board out of the unit. I sent Bill an audio clip of the noise. He feels strongly that it is driver noise, not power tubes.

    One thing I will do (again) if the noise reoccurs to be sure is remove the driver tubes and leave the amp on to double check the source is not coming from the output section (according to Bill you can pull the driver tubes out and run the amp without harming the circuit — you should hear nothing but silence of everything is running fine).


    Posts : 32
    Join date : 2017-08-28

    Re: New Member, New ST120!

    Post by jsl1234 on Fri Oct 20, 2017 10:08 am

    maybe it really is fixed...ran it for two separate multihour sessions over the past two days without issue. Listening again this morning, also clean.

    Am a little frustrated that I didn’t determine exact root cause. Could be touching up the soldering on the input board and driver sockets, pulling out and reinserting all of the tubes multiple times during triage, retightening the right side RCA input jack: who knows what resolved the issue for now (or even if it will come back).

    Used to more clarity on these things—leaky transistor, blown cap, fried resistor, that sort of thing.  

    maybe I better adjust my expectations for this new paradigm — DYI tube amp Smile

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    Re: New Member, New ST120!

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