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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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    vta driver tube change & sound question

    Bob Latino
    Bob Latino
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    Post by Bob Latino Thu Aug 12, 2021 11:29 am

    Can you tell the the difference between dynakit stereo 70 and 120? The only obvious thing that stands out to me is the center transformer, which is a little thicker, but not the thickest. It measured about 2 inches thick at the widest part of it, not including the cap and the space at the bottom of it. I've seen many members in forum and online have almost exact transformer but keep referring as "st 70".

    First of all the tube amps sold by Tubes4hifi.com amps are a VTA ST-70 and a VTA ST-120 not a "dynakit" ST-70.

    The VTA ST-120 has ..

    1. A larger power transformer than the VTA ST-70
    2. Two larger output transformers than the VTA ST-70
    3. Two chokes instead of one as found on the VTA ST-70
    4. A larger power supply (more DC storage) than the VTA ST-70
    5. A slightly different resistor set on the driver board as compared to the VTA ST-70

    Bob
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    Cl75


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    Post by Cl75 Thu Aug 12, 2021 11:57 am

    I'm confused. Obviously I don't know the difference between "dynakit" or "vta", they look the same to me.I said "dynakit" because on the front panel that is what it written, as the same exact print "dynakit stereo 70" or "dynakit stereo 120" that found in the photos on your website. So can you reassess my amp? Is it st70 or st-120. I already purchased kt120 and cv4024s.
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    Cl75


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    Post by Cl75 Thu Aug 12, 2021 1:03 pm

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    Cl75


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    Post by Cl75 Thu Aug 12, 2021 1:05 pm

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    Bob Latino
    Bob Latino
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    Post by Bob Latino Thu Aug 12, 2021 2:12 pm

    You have an early VTA ST-120 that was built on a Dynakit chassis.

    Bob
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    Cl75


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    Post by Cl75 Thu Aug 12, 2021 2:23 pm

    But there is only one c 354 choke, not two. As you stated, vta st-120 has two.
    Bob Latino
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    Post by Bob Latino Thu Aug 12, 2021 2:48 pm

    Cl75 wrote:But there is only one c 354 choke, not two. As you stated, vta st-120 has two.

    You are correct .. That is an early VTA ST-70 not a VTA ST-120.

    Bob

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    Dave_in_Va
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    Post by Dave_in_Va Thu Aug 12, 2021 3:11 pm

    Since you have a VTA ST 70 you should try and cancel your order for the KT 120's and get a quad of EL 34's.
    (Unless the currently installed KT 88's are still good.)
    Good luck.

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    Cl75


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    Post by Cl75 Thu Aug 12, 2021 3:18 pm

    Bob Latino wrote:
    Cl75 wrote:But there is only one c 354 choke, not two. As you stated, vta st-120 has two.

    You are correct .. That is an early VTA ST-70 not a VTA ST-120.

    Bob
    So it is vta st 70, 35 watts per channel. Can I still use kt88 and cv4024s? Will bias setting differ?


    Last edited by Cl75 on Thu Aug 12, 2021 3:20 pm; edited 1 time in total
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    Cl75


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    Post by Cl75 Thu Aug 12, 2021 3:19 pm

    Dave_in_Va wrote:Since you have a VTA ST 70 you should try and cancel your order for the KT 120's and get a quad of EL 34's.
    (Unless the currently installed KT 88's are still good.)
    Good luck.
    The kt120 and cv4024s are in transit.
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    Cl75


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    Post by Cl75 Thu Aug 12, 2021 3:21 pm

    Will it be simple upgrade to st 120?

    If I can't use kt120d I would prefer kt88s if it is safe.
    Dave_in_Va
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    Post by Dave_in_Va Thu Aug 12, 2021 3:35 pm

    I tried KT 88's in VTA ST 70 and preferred the EL 34's it was designed for, but that's subjective.

    I'd explain the situation to whoever you ordered the KT 120's from and see if you can send them back for a refund or swap them for something you can use. I'm pretty sure you don't want to use a KT 120 in a VTA ST 70.

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    corndog71
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    Post by corndog71 Thu Aug 12, 2021 5:02 pm

    I too would recommend sticking with EL34’s as they are cheaper and sound just as good as the larger tubes. The power difference isn’t as noticeable as one might think.

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    Cl75


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    Post by Cl75 Thu Aug 12, 2021 5:36 pm

    Thanks guys for your inputs. Currently my st70 has set of kt88.I just want to know if it is safe to run them? I got the amp secondhand and don't have any prior knowledge or any electrical background to understand what I have or to deal with. That was why I reached out to the forum to seek for more information. Also Bob gave me a list of pre-amp tubes that are compatible with my driver board.I ordered a set of cv4024s. Have anyone use this type of pre-amp tube, please comment... Thank you very much.
    Dave_in_Va
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    Post by Dave_in_Va Thu Aug 12, 2021 5:50 pm

    4024 is just another number for a 12AT7. You're fine with those.

    There has been a difference of opinion on this forum about KT88's in a VTA ST70. When I got my VTA ST70 I was told it was safe (by Bob). Recently some posters here have suggested changing a resistor when using the KT88's.

    I'd really try to return those KT120's for a quad of EL34's and use those.

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    Cl75


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    Post by Cl75 Thu Aug 12, 2021 6:12 pm

    Dave_in_Va wrote:4024 is just another number for a 12AT7. You're fine with those.

    There has been a difference of opinion on this forum about KT88's in a VTA ST70. When I got my VTA ST70 I was told it was safe (by Bob). Recently some posters here have suggested changing a resistor when using the KT88's.

    I'd really try to return those KT120's for a quad of EL34's and use those.

    I will.I have a set of EL34 and a set of 6CA7. Is 6CA7 good too?

    Do you have the photos of your st70? What version? Is it same as mine, newer or older?
    Dave_in_Va
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    Post by Dave_in_Va Thu Aug 12, 2021 6:52 pm

    6CA7 (US number) is interchangeable with the EL34 (British number). Basically the same tube, different number.

    MT VTA ST 70 is the newer version that uses the lower gain 12AU7. I believe I bought it in 2013.

    When you set the bias for the EL34's set them at 40 mA.

    I'd have to move my amp around to remove the cage to take a pic of it. It looks like yours without the attenuator and mine has only two speaker connections per side.
    12AU7's look just like 12AT7's.

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    Cl75


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    Post by Cl75 Thu Aug 12, 2021 7:05 pm

    [quote="Dave_in_Va"]6CA7 (US number) is interchangeable with the EL34 (British number). Basically the same tube, different number.

    MT VTA ST 70 is the newer version that uses the lower gain 12AU7. I believe I bought it in 2013.

    When you set the bias for the EL34's set them at 40 mA.

    I'd have to move my amp around to remove the cage to take a pic of it. It looks like yours without the attenuator and mine has only two speaker connections per side.
    12AU7's look just like 12AT7's.[/quote
    Mine has the volume control. My transformers are about 2 inches thick at the thickest part, not including the caps.

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