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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


    Dynaco ST70 VTA board issues

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    Kulchinski1993

    Posts : 1
    Join date : 2020-09-24

    Dynaco ST70 VTA board issues  Empty Dynaco ST70 VTA board issues

    Post by Kulchinski1993 on Thu Sep 24, 2020 4:19 pm

    Hello, first time poster here. I am in need of some help troubleshooting some issues with my Dynaco ST70 (upgraded with VTA board and new transformers, running NOS RCA 6L6s, 12AT7s and Mullard 5AR4). The amp has started to produce a very loud hum, and vibrations can be felt throughout the amp. When I checked the bias for the tubes (which I normally have set to 40mA per tube), I was getting super low readings of 12mA on each tube. This was not happening before the humming started. My first guess was the rectifier tube, but I replaced that and the humming and low readings persisted . I have also tried changing the power tubes and it didn’t make a difference. Hoping someone can steer me in the right direction here!
    MechEngVic
    MechEngVic

    Posts : 81
    Join date : 2019-01-16

    Dynaco ST70 VTA board issues  Empty Re: Dynaco ST70 VTA board issues

    Post by MechEngVic on Thu Sep 24, 2020 8:26 pm

    Kulchinski1993 wrote:Hello, first time poster here. I am in need of some help troubleshooting some issues with my Dynaco ST70 (upgraded with VTA board and new transformers, running NOS RCA 6L6s, 12AT7s and Mullard 5AR4). The amp has started to produce a very loud hum, and vibrations can be felt throughout the amp. When I checked the bias for the tubes (which I normally have set to 40mA per tube), I was getting super low readings of 12mA on each tube. This was not happening before the humming started. My first guess was the rectifier tube, but I replaced that and the humming and low readings persisted . I have also tried changing the power tubes and it didn’t make a difference. Hoping someone can steer me in the right direction here!

    A loud hum you can feel is a transformer. Check the voltages at the transformer wire outs. If you find a bad voltage, unsolder that wire then measure it again. If it normalizes, it's the circuit, it it stays the same, it's the transformer.

      Current date/time is Thu Oct 22, 2020 12:01 am