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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


    upgrade vta st 70 after kit build

    Midwestside
    Midwestside

    Posts : 62
    Join date : 2019-03-20

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    Post by Midwestside on Sun Jun 28, 2020 12:29 am

    Outside of a stock build, i was wondering where i could potentially improve things.
    I already have the PIO coupling caps and descent tube set.
    I was wondering more along the lines of tube sockets, electrolytics and that sort of thing.
    PeterCapo
    PeterCapo

    Posts : 845
    Join date : 2008-12-05

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    Post by PeterCapo on Sun Jun 28, 2020 7:56 am

    Bias diode.  Try a Cree silicon carbide zero recovery schottky. Check out a suitable heat-sink if you decide to try one.
    Bob Latino
    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

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    Post by Bob Latino on Sun Jun 28, 2020 8:23 am

    Midwestside wrote:Outside of a stock build, i was wondering where i could potentially improve things.
    I already have the PIO coupling caps and descent tube set.
    I was wondering more along the lines of  tube sockets, electrolytics and that sort of thing.

    The main electrolytic on a VTA ST-70 is the German made "Authenticap" quad cap. The 4 electrolytic power caps on the driver board are Nichicon caps "Made in Japan". The bias supply power caps are also Nichicon caps.

    The easiest and cheapest way to upgrade the VTA ST-70 amp is to change out the center driver tube with a different tube. The center 12AU7 voltage amplifier tube has a lot of influence on the sound of the amp. Besides a standard 12AU7 tube, you can also use the military/industrial versions of a 12AU7 such as the 5963, 5814 and 6189. Many USA made NOS versions of these tubes (GE, RCA, Sylvania) may still be found on Ebay at fairly reasonable prices. Try a few different driver tubes in the center position.

    Bob
    Midwestside
    Midwestside

    Posts : 62
    Join date : 2019-03-20

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    Post by Midwestside on Sun Jun 28, 2020 1:45 pm

    Bob Latino wrote:
    Midwestside wrote:Outside of a stock build, i was wondering where i could potentially improve things.
    I already have the PIO coupling caps and descent tube set.
    I was wondering more along the lines of  tube sockets, electrolytics and that sort of thing.

    The main electrolytic on a VTA ST-70 is the German made "Authenticap" quad cap. The 4 electrolytic power caps on the driver board are Nichicon caps "Made in Japan". The bias supply power caps are also Nichicon caps.

    The easiest and cheapest way to upgrade the VTA ST-70 amp is to change out the center driver tube with a different tube. The center 12AU7 voltage amplifier tube has a lot of influence on the sound of the amp. Besides a standard 12AU7 tube, you can also use the military/industrial versions of a 12AU7 such as the 5963, 5814 and 6189. Many USA made NOS versions of these tubes (GE, RCA, Sylvania) may still be found on Ebay at fairly reasonable prices. Try a few different driver tubes in the center position.

    Bob

    Good to know, sounds like its spec'd pretty nicely already.
    DavidR
    DavidR

    Posts : 75
    Join date : 2017-08-10
    Location : MetroWest, MA

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    Post by DavidR on Sun Jun 28, 2020 8:28 pm

    Bob, is the ST70 capable of using a 12BH7 in the center tube like the ST-120 ?

    GE and RCA 5963 tubes from the 50's are quite nice. Sylvania made some nice 5814A tubes.
    Bob Latino
    Bob Latino
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    Post by Bob Latino on Sun Jun 28, 2020 8:51 pm

    DavidR wrote:Bob, is the ST70 capable of using a 12BH7 in the center tube like the ST-120 ?

    GE and RCA 5963 tubes from the 50's are quite nice. Sylvania made some nice 5814A tubes.

    Yes - although a 12BH7 is slightly different than a 12AU7, it is close enough so that it will work in the VTA ST-70. As an aside > Do not ever use a 12AT7, 12AX7, 12AY7 type tube or any other 9 pin tube that will fit in the socket. If you do, you can damage the two small LM334 current regulator IC's on the driver board.

    Bob
    DavidR
    DavidR

    Posts : 75
    Join date : 2017-08-10
    Location : MetroWest, MA

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    Post by DavidR on Sun Jun 28, 2020 9:10 pm

    Some of the RCA and Tung Sol (my fav) from the late 50's are really nice in that center position. I've never had the chance to try a black plate GE 12BH7 from the 50's. Maybe some day.
    Midwestside
    Midwestside

    Posts : 62
    Join date : 2019-03-20

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    Post by Midwestside on Sun Jun 28, 2020 10:02 pm

    DavidR wrote:Some of the RCA and Tung Sol (my fav) from the late 50's are really nice in that center position. I've never had the chance to try a black plate GE 12BH7 from the 50's. Maybe some day.

    I have a NOS mullard in the center and 2 raytheons on the outside. It definitely upped the sound quality for my ears. I am possibly getting 2 mullards to switch out with the raytheons.

    DavidR likes this post

    knotscott
    knotscott

    Posts : 33
    Join date : 2019-12-28
    Location : Rochester, NY

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    Post by knotscott on Mon Jun 29, 2020 9:30 am

    Experimenting with tubes is among the easiest ways to change (hopefully upgrade) the sound.  I've tried a few variations of the 12AU7s, and am currently running an RCA 12BH7 in the center position of my Dyna/VTA ST-70s with RCA clear top 12AU7s on the outside.  Love the sound I'm getting.

    I've also experimented with output tubes....have tried EL34s from Siemens, Shuguang, EH, and Nesstone.  Love the mids on the EH and the bass extension on the Nesstone.  I have not yet tried any 6550s, but they're on my radar.

    It's pretty easy to experiment with different RCA interconnects too.

    It's often more a matter of finding the perfect match for your system than it is one being clearly better than another.  As long as you're not doing something to damage the system or going broke doing it, half the fun is experimenting.

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