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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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    105J 630V Capacitor on the ST-120 but not on the ST-70?

    mrbegnt
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    mrbegnt


    Posts : 1
    Join date : 2015-08-19
    • Post n°1

    105J 630V Capacitor on the ST-120 but not on the ST-70? Empty 105J 630V Capacitor on the ST-120 but not on the ST-70?

    Post by mrbegnt Mon Jul 22, 2019 10:51 pm

    I have noticed that the VTA ST-120 has a capacitor (105J 630V) between the quad and the ground.
    What's the benefit of that cap and why is it missing on the ST-70 setup?


    105J 630V location:
    https://i.servimg.com/u/f94/20/09/46/82/105j_610.jpg
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    Bob Latino
    Bob Latino
    Bob Latino
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    Location : Massachusetts
    • Post n°2

    105J 630V Capacitor on the ST-120 but not on the ST-70? Empty Re: 105J 630V Capacitor on the ST-120 but not on the ST-70?

    Post by Bob Latino Tue Jul 23, 2019 7:33 am

    mrbegnt wrote:I have noticed that the VTA ST-120 has a capacitor (105J 630V) between the quad and the ground.
    What's the benefit of that cap and why is it missing on the ST-70 setup?


    105J 630V location:
    https://i.servimg.com/u/f94/20/09/46/82/105j_610.jpg

    That capacitor is a "bypass capacitor" for the entire high voltage system of the ST-120. It is not absolutely necessary for the running of the amp but it will help to break up any resonances in the high voltage system by setting up a low impedance path to ground. The VTA ST-120 and the VTA M-125's both use this 1 uF @ 630 volt capacitor because the high voltage system is larger and more complex than the VTA ST-70. The VTA ST-70 has a smaller power supply with with only one choke and no SCM (supplementary cap module) and does not really need this bypass capacitor.

    Bob
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