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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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Hops
davidness
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    Older VTA ST-120 tubes

    davidness
    davidness
    davidness


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    • Post n°1

    Older VTA ST-120 tubes Empty Older VTA ST-120 tubes

    Post by davidness Tue Sep 22, 2020 7:27 pm

    It’s been many years since I fired my ST-120 up, but I’ve forgotten what driver tubes to use in the 3 positions. I thought it was a couple of different tubes, but now I only see references to “use 3 12AU7’s”. Maybe it depends on the board I’m using, so I have a picture I’ll try to include in this post. Please respond with what is most standard...

    Older VTA ST-120 tubes Image10
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    Hops
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    Hops


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    Location : South Detroit, Michigan
    • Post n°2

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    Post by Hops Tue Sep 22, 2020 7:50 pm

    Hello, Bob has some good suggestions in this post. https://dynacotubeaudio.forumotion.com/t4433-dirty-tube-sockets. I think every Amp Bob sold has a serial number somewhere. The Tube requirement changed a few years ago -- I think they went from a high gain to a low gain board. I am going to find the definitive post and repost a link
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    Hops
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    • Post n°3

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    Post by Hops Tue Sep 22, 2020 7:53 pm

    Hello, this is a useful post also: https://dynacotubeaudio.forumotion.com/t4212-best-rectifier-and-driver-tubes-for-latino-vta-st-70?highlight=driverl. I have saved many of the more useful posts as PDFs
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    davidness
    davidness
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    Post by davidness Tue Sep 22, 2020 8:35 pm

    I’m pretty certain that Bob built this amp before April 2012, but I bought the amp from someone else. How can I tell if this is an older ST-120? What do I need to look for?
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    Hops
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    • Post n°5

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    Post by Hops Tue Sep 22, 2020 9:06 pm

    Counting the pins on your driver sockets, it looks like you have a 9 pin ( older, high gain ) board. Let me look for some posts on that board.
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    Dave_in_Va
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    • Post n°6

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    Post by Dave_in_Va Tue Sep 22, 2020 9:09 pm

    Both the 12AT7 and the 12AU7 are nine pin tubes.

    I'm pretty sure the pre-2012 ST-70 was 12AT7 but I don't have any experience with the 120. Sorry.
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    peterh
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    • Post n°7

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    Post by peterh Wed Sep 23, 2020 1:59 am

    davidness wrote:I’m pretty certain that Bob built this amp before April 2012, but I bought the amp from someone else. How can I tell if this is an older ST-120? What do I need to look for?

    The later board uses 12au7 / ECC82 and has 2 small transistor-like devices. These are
    actually ConstantCurrentSources in seried with the outermost tubes cathodes.

    The earlier board has no such devices and uses 12AT7 / ECC81 tubes.
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    Bob Latino
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    • Post n°8

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    Post by Bob Latino Wed Sep 23, 2020 10:50 am

    Yes - If your VTA ST-120 amp was built before April of 2012, it would be using 12AT7 driver tubes.

    VTA ST-70 amps before April of 2012 > Use 12AT7 driver tubes. From April of 2012 to the present day use 12AU7 driver tubes
    VTA ST-120 amps before April of 2012 > Use 12AT7 driver tubes. From April of 2012 to the present day use 12AU7 driver tubes
    All VTA M-125 amps from any date > Use 12AU7 driver tubes.

    If you are not sure and do not remember the date of your VTA amp, Email me your name and I can look it up your amp on my computer. If you are not the original owner of your VTA amp, let me know who you purchased the amp from and I might still be able to find the original sale ? As a last resort, you could Email me a photo of the top and the inside wiring of the amp and I should be able to tell the age of your amp.

    The amp in the photo below is an older ST-120 and would be using 12AT7 driver tubes ..

    Bob


    Older VTA ST-120 tubes Image10

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    Blitzen
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    Post by Blitzen Thu Sep 24, 2020 9:56 pm

    Bob, there must be something specific you can point at to tell the difference..."this resistor right here would be this value with a high-gain (12AT7) amp", or something like that...
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    Bob Latino
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    • Post n°10

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    Post by Bob Latino Fri Sep 25, 2020 8:01 am

    Blitzen wrote:Bob, there must be something specific you can point at to tell the difference..."this resistor right here would be this value with a high-gain (12AT7) amp", or something like that...

    The amp in the photo in post #8 above does not have the two small, black LM334 current regulators. All VTA amps since April of 2012 have TWO LM334's on the top of the driver board and use 12AU7 type driver tubes. Photo below of a newer VTA ST-120 amp with the two LM334's (one for each channel) circled in red.

    Bob

    Older VTA ST-120 tubes LM334-ST120
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