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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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scottlowe
corndog71
peterh
Elrick
deepee99
baddog1946
sKiZo
Dave_in_Va
Bob Latino
guentherj2
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    VTA ST120 Bias Adjustment

    sKiZo
    sKiZo


    Posts : 1530
    Join date : 2013-04-01
    Location : Michigan USA

    VTA ST120 Bias Adjustment - Page 3 Empty Re: VTA ST120 Bias Adjustment

    Post by sKiZo Thu May 01, 2014 8:43 pm

    Soft kitty, warm kitty
    Little ball of fur

    Lazy kitty, sleepy kitty
    Purr, purr, purr


    Nighty night!

    Hang in there ... good to hear you got a good backup ...

    PS ... make sure your builder lets you know what was wrong ... and make sure you post up with the results. Enquiring minds need to know!
    Roy Mottram
    Roy Mottram
    Admin


    Posts : 1840
    Join date : 2008-11-30

    VTA ST120 Bias Adjustment - Page 3 Empty Re: VTA ST120 Bias Adjustment

    Post by Roy Mottram Thu May 01, 2014 9:01 pm

    " . . . and probably install 10 ohm 3 watt resistors."


    personally I like to use 1 watt resistors for those cathode resistors.
    If the tubes are biased properly, there will only be .025w across that resistor (based on 50ma).
    So even a 1/4w resistor would be 10X as big as needed.
    I'd much rather burn out a 20c resistor than a $40 tube !!!!
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    Guest
    Guest


    VTA ST120 Bias Adjustment - Page 3 Empty Re: VTA ST120 Bias Adjustment

    Post by Guest Thu May 01, 2014 9:01 pm

    sKiZo wrote:Soft kitty, warm kitty
    Little ball of fur

    Lazy kitty, sleepy kitty
    Purr, purr, purr


    Nighty night!

    Hang in there ... good to hear you got a good backup ...

    PS ... make sure your builder lets you know what was wrong ... and make sure you post up with the results. Enquiring minds need to know!

    you're in my spot!!
    guentherj2
    guentherj2


    Posts : 50
    Join date : 2013-06-22
    Location : Bellingham, WA

    VTA ST120 Bias Adjustment - Page 3 Empty Re: VTA ST120 Bias Adjustment

    Post by guentherj2 Fri Jun 27, 2014 11:47 pm

    Thanks to all who have been following this thread and providing input.

    UPDATE: What the problem was I can't say. What I can say is that after installing screen grid protection resistors on the output tubes, upgrading from KT88 to to KT120 output tubes and installing a Weber SS rectifier, all is wonderful. I also discovered my SP12 preamp came with a faulty 24 stepped volume control pot. That I replaced with a Blue Velvet volume pot. I can't beleive how good everything sounds and bias measurements are stable. This is what I expected when I purchased this gear but it took several months to work out the bugs. Probably not what Bob and his crew want to hear but such is my case. I have absolutely no regrets with my VTA stereo. It performs as it should now and I am loving it. I don't know what my problem was but I do beleive dcgillespie is on to something and really appreciate his guidance.

    The ST-120 and SP-12, when working propery, rock!

    John
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    jvinhj240


    Posts : 2
    Join date : 2022-06-19

    VTA ST120 Bias Adjustment - Page 3 Empty Re: VTA ST120 Bias Adjustment

    Post by jvinhj240 Sun Jun 19, 2022 11:20 am

    on right side i can set to .500vdc but left side lowest I Can set is 1.3 and .700 what could be wrong?
    Bob Latino
    Bob Latino
    Admin


    Posts : 3260
    Join date : 2008-11-26
    Location : Massachusetts

    VTA ST120 Bias Adjustment - Page 3 Empty Re: VTA ST120 Bias Adjustment

    Post by Bob Latino Sun Jun 19, 2022 1:43 pm

    jvinhj240 wrote:on right side i can set to .500vdc but left side lowest I Can set is 1.3 and .700 what could be wrong?

    1. Check that both tubes on the left side are OK by swapping the two LEFT tubes with the two RIGHT tubes. See if the problem moves to the RIGHT side ? If it does, you need two new output tubes to replace the two tubes that will not bias.

    2. With the amp OFF > Check the two 10 ohm 2 watt resistors on the LEFT side that go from pins 1 and 8 to chassis ground. See if they measure about 10 ohms. If they measure ZERO ohms or some high value, you will have to replace those resistors.

    Bob

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    jvinhj240


    Posts : 2
    Join date : 2022-06-19

    VTA ST120 Bias Adjustment - Page 3 Empty Re: VTA ST120 Bias Adjustment

    Post by jvinhj240 Sun Jun 19, 2022 4:04 pm

    ok Bob, thanks.

    Nice sêeing you still active. I got this amp prolly 4-5 years barely used. I will post the result.

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