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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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    Which Version VTA 70 Is This?

    knotscott
    knotscott


    Posts : 48
    Join date : 2019-12-28
    Location : Rochester, NY

    Which Version VTA 70 Is This?  Empty Which Version VTA 70 Is This?

    Post by knotscott Thu Oct 27, 2022 8:34 am

    An audio friend showed me some pics of a VTA 70 he picked up (without tubes), but his driver board looks a bit different than current versions.  Can someone tell me which version this is, and what driver tubes are compatible with it?

    Which Version VTA 70 Is This?  Edgars10

    Which Version VTA 70 Is This?  Edgarf11
    peterh
    peterh


    Posts : 1689
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Which Version VTA 70 Is This?  Empty Re: Which Version VTA 70 Is This?

    Post by peterh Thu Oct 27, 2022 8:41 am

    knotscott wrote:An audio friend showed me some pics of a VTA 70 he picked up (without tubes), but his driver board looks a bit different than current versions.  Can someone tell me which version this is, and what driver tubes are compatible with it?

    Which Version VTA 70 Is This?  Edgars10

    Which Version VTA 70 Is This?  Edgarf11
    There seems not to be any CCS ( transistor-looking devices), thus this is an older
    for ECC81 / 12AT7 type tubes

    knotscott likes this post

    knotscott
    knotscott


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    Join date : 2019-12-28
    Location : Rochester, NY

    Which Version VTA 70 Is This?  Empty Re: Which Version VTA 70 Is This?

    Post by knotscott Thu Oct 27, 2022 9:01 am

    Thanks Peter. Agreed...there are no LM334s or similar that I can see.
    Bob Latino
    Bob Latino
    Admin


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    Join date : 2008-11-26
    Location : Massachusetts

    Which Version VTA 70 Is This?  Empty Re: Which Version VTA 70 Is This?

    Post by Bob Latino Thu Oct 27, 2022 7:57 pm

    Yes - that amp has the older VTA ST-70 driver board from 2007 - 2011. In early 2012 we changed the VTA ST-70 driver board to the CCS version that uses the LM334 IC to give the driver circuit a constant current source. Nearly all of those amps without the LM334's use 12AT7 driver tubes. The newer VTA amps (2012 - to the present) use 12AU7 type driver tubes. The newer board using the 12AU7 driver tubes has a very slightly lower signal to noise ratio.

    Bob

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    1973shovel


    Posts : 42
    Join date : 2009-12-13

    Which Version VTA 70 Is This?  Empty Re: Which Version VTA 70 Is This?

    Post by 1973shovel Sun Oct 30, 2022 10:58 am

    While most pre-CCS boards may have used 12AT7s, that doesn't mean that they all did. If your friend wants to be sure which tubes his amp is configured for, have him check specific resistors which converted the 12AT7 version to use 12AU7s.

    Below is a link to an old VTA Forum thread, one where, in post number 6, Roy identifies the resistors in question, and specifies their values. Have your friend identify those resistor values, either by their color bands, or by lifting one leg of those resistors and taking a measurement. That's really the only way to know which tubes the older VTA board is configured for.

    https://dynacotubeaudio.forumotion.com/t1860-what-are-the-current-resistor-values-and-power-ratings-for-the-hi-gain-12at7-and-lo-gain-12au7-12bh7-vta-mkiii-iv-upgrade-boards#13292

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    gener8r
    gener8r


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    Which Version VTA 70 Is This?  Empty Re: Which Version VTA 70 Is This?

    Post by gener8r Mon Oct 31, 2022 4:26 am

    I wouldn't want to bias that board, flipping the amp over and over to get the two back tubes sync'd
    Bob Latino
    Bob Latino
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    Which Version VTA 70 Is This?  Empty Re: Which Version VTA 70 Is This?

    Post by Bob Latino Mon Oct 31, 2022 8:36 am

    gener8r wrote:I wouldn't want to bias that board, flipping the amp over and over to get the two back tubes sync'd

    Those two 10K pots on the bottom of the OLD board were NOT USED TO BIAS the amp's back tubes. Those two pots were used to set the the phase splitter section of the OLD driver board. What you did before you installed them was to measure the resistance from the center pin to each of the two outer pins and adjust the pot until the you got about 5000 ohms on each side. These pots were installed on the bottom so that once set, you could not adjust them without taking off the bottom cover and once set properly, these two pots never needed any adjustment.

    The newer VTA driver boards from 2012 to the present use two LM334 IC's to do the job. The LM334's are current regulators that do the job that the two 10K pots once did.

    Bob

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