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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


    VTA Driver Board

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    rudes

    Posts : 50
    Join date : 2012-03-07

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    Post by rudes on Sun Dec 27, 2020 11:28 am

    Hey Guys, Is anyone familiar with this VTA driver board? I have never seen one with six trim pots. Also, the PS cap is not configured as it should be as far as I know based on their other boards. The jumper and resistor are in the wrong place. The resistor is a 6.8K instead of 2.2K. Also additional capacitance has be added to the CE 80/40/30/20 cap. When I turn one bias pot, it affects both tubes and I can not get them both up to bias at .4v. Would appreciate any input.VTA Driver Board Vta12
    VTA Driver Board Ps_cap12
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    rudes

    Posts : 50
    Join date : 2012-03-07

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    Post by rudes on Sun Dec 27, 2020 1:14 pm

    Have 372 B+ at the board


    Last edited by rudes on Sun Dec 27, 2020 1:55 pm; edited 1 time in total
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    stewdan

    Posts : 213
    Join date : 2010-03-07
    Age : 82
    Location : Houston Texas

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    Post by stewdan on Sun Dec 27, 2020 1:40 pm

    Hi ---- It looks like a VTA Board. I wonder if it was a prototype board??  Have you had it awhile or did you just get it and from where?

    You might try contacting Roy Mottram directly and ask him  ==>>>>   roymottram@yahoo.com
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    nmchiefsfan

    Posts : 63
    Join date : 2012-03-21

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    Post by nmchiefsfan on Sun Dec 27, 2020 6:29 pm

    Is there a chance that the AC balance pots were put on the top side of the board instead of the bottom side?
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    rudes

    Posts : 50
    Join date : 2012-03-07

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    Post by rudes on Sun Dec 27, 2020 9:20 pm

    Yes, After looking at another amp I figured this person did install pots on top. Bias problem turned out to be a bad quad cap.

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    Bob Latino
    Bob Latino
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    Posts : 2976
    Join date : 2008-11-26
    Location : Massachusetts

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    Post by Bob Latino on Mon Dec 28, 2020 11:55 am

    That amp is an original Dynaco ST-70 which was upgraded with a VTA driver board from before 2012. Yes - This is one of the older VTA driver boards with an AC balance control. That AC balance control should have been placed on the BOTTOM of the driver board because once set, this pot should not be readjusted. If that pot is on the underside of the board, the owner is less likely to tinker with the pot once the board is installed into the amp. What you are supposed to do before you install this 10K pot is to measure from the center pin to the two outer pins and adjust the pot until both sides read equally. Ideally these should read 5.00K from center pin on each side but many times you get something like 4.92K per side when you adjust the pot so that the two measurements are equal.

    The newer VTA boards (2012 to present) have a small LM334 integrated circuit that replaces this 10K pot on the older VTA boards. The LM334 will keep the current the same on both sides of the phase splitter part of the circuit automatically.

    Bob

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