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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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    ST70 Voltages High

    moodyj2000
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    moodyj2000


    Posts : 18
    Join date : 2023-01-18
    • Post n°1

    ST70 Voltages High Empty ST70 Voltages High

    Post by moodyj2000 Tue Sep 26, 2023 12:46 pm

    I have a ST-70 I’m working on. New original spec can capacitor from Dynakit and new PC-3A board as well. Variac voltage is 117V. Input to 5AR4 is 365VAC and 363VAC. Output is 495VDC on both.

    I’ve tried two separate 5AR4’s with same results. Voltages at the can cap are higher as well.

    Any thoughts?
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    moodyj2000
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    moodyj2000


    Posts : 18
    Join date : 2023-01-18
    • Post n°2

    ST70 Voltages High Empty Re: ST70 Voltages High

    Post by moodyj2000 Tue Sep 26, 2023 12:47 pm

    Also, selenium rectifier has been replaced with Dynakit’s bias kit. Everything is installed correctly.
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    peterh
    peterh
    peterh


    Posts : 1861
    Join date : 2012-12-25
    Location : gothenburg, sweden
    • Post n°3

    ST70 Voltages High Empty Re: ST70 Voltages High

    Post by peterh Tue Sep 26, 2023 2:33 pm

    moodyj2000 wrote:I have a ST-70 I’m working on. New original spec can capacitor from Dynakit and new PC-3A board as well. Variac voltage is 117V. Input to 5AR4 is 365VAC and 363VAC. Output is 495VDC on both.

    I’ve tried two separate 5AR4’s with same results. Voltages at the can cap are higher as well.

    Any thoughts?

    Assuming all tubes in place, what is the voltage on the bias test taps ?
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    Bob Latino
    Bob Latino
    Bob Latino
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    Posts : 3271
    Join date : 2008-11-26
    Location : Massachusetts
    • Post n°4

    ST70 Voltages High Empty Re: ST70 Voltages High

    Post by Bob Latino Tue Sep 26, 2023 4:27 pm

    moodyj2000 wrote:I have a ST-70 I’m working on. New original spec can capacitor from Dynakit and new PC-3A board as well. Variac voltage is 117V. Input to 5AR4 is 365VAC and 363VAC. Output is 495VDC on both.

    I’ve tried two separate 5AR4’s with same results. Voltages at the can cap are higher as well.

    Any thoughts?

    The easiest thing to do to get the DC high voltage down would be to turn your variac down 113 - 115 VAC. For a Dynaco ST-70 you should have 440 - 460 VDC to chassis ground at the quad cap terminal that connects to pin #8 of the rectifier tube socket.

    Bob
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