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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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    ST-120 and summer time voltage swings

    DavidR
    DavidR


    Posts : 148
    Join date : 2017-08-10
    Location : MetroWest, MA

    ST-120 and summer time voltage swings Empty ST-120 and summer time voltage swings

    Post by DavidR Mon Aug 02, 2021 2:05 pm

    I'd like clarification as to what I experience in summer months can harm my amp or the tubes. My wall voltage is typically 122 to 123 VAC. I use a variac and set it for 117VAC both to run and when I adjust the bias. In the cooler months voltage swings are minimal and swings are not large BUT summer time with AC units off and on causes issues. My variac voltage dropped to 115VAC and, obviously, the bias set point dropped and was below the recommend .490 VDC. My variac was maxed out on the dial.

    Is there an issue running the amp with the bias below .490VDC?
    Bob Latino
    Bob Latino
    Admin


    Posts : 3101
    Join date : 2008-11-26
    Location : Massachusetts

    ST-120 and summer time voltage swings Empty Re: ST-120 and summer time voltage swings

    Post by Bob Latino Mon Aug 02, 2021 5:40 pm

    DavidR wrote:I'd like clarification as to what I experience in summer months can harm my amp or the tubes. My wall voltage is typically 122 to 123 VAC. I use a variac and set it for 117VAC both to run and when I adjust the bias. In the cooler months voltage swings are minimal and swings are not large BUT summer time with AC units off and on causes issues. My variac voltage dropped to 115VAC and, obviously, the bias set point dropped and was below the recommend .490 VDC. My variac was maxed out on the dial.

    Is there an issue running the amp with the bias below .490VDC?

    Hi David,

    No > The variation of line voltage is normal when AC is used by many people to keep cool in the summer. You could run the bias anywhere from .450 VDC per each output tube up to the recommended .500 VDC. Let the bias go up and down when the line voltage swings. It won't harm the amp unless you get into the 525 - 550 VDC B+ range. Above 550 VDC, you will exceed the voltage rating of the quad cap and could possibly damage the quad cap. The SCM (supplementary cap module) is 700 volt rated and the ESL bypass capacitor is rated at 630 volts.

    Bob
    DavidR
    DavidR


    Posts : 148
    Join date : 2017-08-10
    Location : MetroWest, MA

    ST-120 and summer time voltage swings Empty Re: ST-120 and summer time voltage swings

    Post by DavidR Mon Aug 02, 2021 7:03 pm

    Thanks Bob. I don't have to worry about being above 550.

    IIRC the bias was about .470 - .473

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    ST-120 and summer time voltage swings Empty Re: ST-120 and summer time voltage swings

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