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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


4 posters

    VTA-ST120 Bias resistor settings,

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    edman44

    Posts : 24
    Join date : 2021-06-14
    Age : 59
    Location : PENNSYLVANIA

    VTA-ST120 Bias resistor settings, Empty VTA-ST120 Bias resistor settings,

    Post by edman44 Tue Sep 14, 2021 12:32 am

    Hello,
              Well After many hours of building my VTA-ST120 kit. I can say that it is complete. Sounds wonderful and has no hum. I just have one question about biasing the VR's? Believe it or not all four are set dead on 0.50 Ma. The one in question is the right rear Bias resistor. The other three Bias resistors are set in the same position right around 11 o'clock. When I first power the amp up after a few minutes of warm-up I check the Bias setting. And I find every time the left rear Bias is higher than the rest? After I adjust it the vr is at 8 o'clock. Does it matter that it does not line up with the other three Bias settings? I am using a headphone tube preamp until I can afford a real preamp/Thanks, Rolling Eyes
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    mijohn

    Posts : 97
    Join date : 2013-06-19

    VTA-ST120 Bias resistor settings, Empty Re: VTA-ST120 Bias resistor settings,

    Post by mijohn Tue Sep 14, 2021 2:20 am

    It doesn't matter at all. Usually I find that that each tube requires a different setting and it's just a coincidence that three of yours are closely matched. Congratulations on a successful build.
    Bob Latino
    Bob Latino
    Admin

    Posts : 3074
    Join date : 2008-11-26
    Location : Massachusetts

    VTA-ST120 Bias resistor settings, Empty Re: VTA-ST120 Bias resistor settings,

    Post by Bob Latino Tue Sep 14, 2021 8:58 am

    mijohn wrote:It doesn't matter at all. Usually I find that that each tube requires a different setting and it's just a coincidence that three of yours are closely matched. Congratulations on a successful build.

    Yes ... Even in a matched set of new output tubes one output tube may show some bias abnormality. Note also that NEW output tubes will also wear in slightly differently over the first few months. Check the bias once a month and make your adjustments after the amp has warmed up a half hour or so. If you set the bias when the amp is "cold", as the amp warms up the bias will rise slightly.

    Bob
    DavidR
    DavidR

    Posts : 146
    Join date : 2017-08-10
    Location : MetroWest, MA

    VTA-ST120 Bias resistor settings, Empty Re: VTA-ST120 Bias resistor settings,

    Post by DavidR Tue Sep 14, 2021 11:24 am

    I find I have the same issue. My right rear output tube bias always rises with use. The other 3 are always somewhat close to each other and to 0.5v set point but the RR is higher - .510 to .512v. Drives me crazy.
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    edman44

    Posts : 24
    Join date : 2021-06-14
    Age : 59
    Location : PENNSYLVANIA

    VTA-ST120 Bias resistor settings, Empty Re: VTA-ST120 Bias resistor settings,

    Post by edman44 Tue Sep 14, 2021 2:22 pm

    Thanks, guys for your thoughts, David my RR tube like yours is always on the high side also. And a big thanks to Bob for sending me the extra parts and his help with the amp. I have found the Dyanco customer support to be the finest you will ever find anywhere thanks to Bob. Other companies should treat their customers, as well as Bob, does. Now all I need to do is save money to buy a tube preamp instead of using an aiyima tube-b3 headphone preamp. I will be asking if there are any online preamps available in another post.

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    VTA-ST120 Bias resistor settings, Empty Re: VTA-ST120 Bias resistor settings,

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