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The Dynaco Tube Audio Forum

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


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    Scratchy noise from one channel in new ST-70 build after 5 weeks

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    bootstrapjesse


    Posts : 11
    Join date : 2022-07-17
    Location : Fort Worth, TX

    Scratchy noise from one channel in new ST-70 build after 5 weeks Empty Scratchy noise from one channel in new ST-70 build after 5 weeks

    Post by bootstrapjesse Thu Oct 27, 2022 6:04 pm

    Greetings.  I have a question.

    I just finished up an ST 70 build. All new. Dynaco A470/PA060 new from Kevin at Dynakitparts. The only mods I’ve done is the Dynakit 80/40/30/20 uf can cap, Dan Joffe’s Stereo 70 Bias Power Supply Upgrade, the yellow sheet mod, and a Triode EF86 board. Running 6CA7 power tubes which are biased a bit on the cool side. I put the amp in service with two other ST-70s and ran it for the past five weeks. I believe it’s got about 60-70 hours on it so far. The amp worked flawlessly and sounded amazing until yesterday.

    I now have a crackling “scratchy” sound from the left channel. It sounds almost like a dirty potentiometer on a guitar amp being turned, or a scratchy record being played. It’s constant, but seems to wax and wane in intensity every 5 seconds or so (at idle with no inputs connected.) I have eliminated tubes, the transformer post connectors (Everything seems to be soldered on well to the back of the posts) speakers and cables.

    At idle, the sound can be heard from the left channel, when no inputs are connected to the amp, the only thing being connected are the speakers on the 8ohm taps. I’ve done a visual inspection on the output transformer wiring, and everything seems to be neat and tidy.
    I was wondering if someone can point me in the right direction. I’m pretty handy with the multimeter, but I don’t have an oscilloscope.
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    New2Tubez


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    Join date : 2018-03-20
    Location : NY

    Scratchy noise from one channel in new ST-70 build after 5 weeks Empty Re: Scratchy noise from one channel in new ST-70 build after 5 weeks

    Post by New2Tubez Thu Oct 27, 2022 7:04 pm

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    bootstrapjesse


    Posts : 11
    Join date : 2022-07-17
    Location : Fort Worth, TX

    Scratchy noise from one channel in new ST-70 build after 5 weeks Empty Re: Scratchy noise from one channel in new ST-70 build after 5 weeks

    Post by bootstrapjesse Thu Oct 27, 2022 7:24 pm

    tubes4hifi
    tubes4hifi
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    Scratchy noise from one channel in new ST-70 build after 5 weeks Empty Re: Scratchy noise from one channel in new ST-70 build after 5 weeks

    Post by tubes4hifi Sat Nov 12, 2022 7:24 pm

    most likely a cold solder joint somewhere on that left channel

    bootstrapjesse likes this post

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    bootstrapjesse


    Posts : 11
    Join date : 2022-07-17
    Location : Fort Worth, TX

    Scratchy noise from one channel in new ST-70 build after 5 weeks Empty Re: Scratchy noise from one channel in new ST-70 build after 5 weeks

    Post by bootstrapjesse Sun Nov 13, 2022 9:57 pm

    tubes4hifi wrote:most likely a cold solder joint somewhere on that left channel

    Fixed the problem by removing and reseating the driver tubes and the phase inverter tube. It's a Triode board which uses two EF86 and a 12AU7. I did also touch up a few of the eyelet joints, but that didnt solve the problem. Re-seating the tubes did. Weird. They are new production Ruskie genelex tubes and the sockets new ceramic ones that look like they are decent quality. The tubes fit very snug. ...I also cleaned the pins with some Deoxit for kicks and giggles.

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