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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

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    paysonmark


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    Post by paysonmark Mon Aug 27, 2018 9:10 pm

    hello all,
    regarding changing driver tubes for the vta board, do I change all 3 or just the outer 2
    thank you

    edit--- or just the center one


    Last edited by paysonmark on Mon Aug 27, 2018 9:24 pm; edited 1 time in total
    Bob Latino
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    Post by Bob Latino Mon Aug 27, 2018 9:16 pm

    paysonmark wrote:hello all,
    regarding changing driver tubes for the vta board, do I change all 3 or just the outer 2
    thank you

    Just change out the CENTER driver tube .. The center tube affects both channels. The two side driver tubes have very little influence on the sound of the amp.

    Bob
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    paysonmark


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    Post by paysonmark Mon Aug 27, 2018 9:25 pm

    thank you bob
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    Cl75


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    Post by Cl75 Sat Aug 07, 2021 12:59 pm

    Hi Bob, I just acquired vta st 70 with ecc81s, and kt88. What are the tube types for preamp and power tubes, and why? vta driver tube change & sound question Screen11
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    Bob Latino
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    Post by Bob Latino Sat Aug 07, 2021 1:06 pm

    That is an older VTA ST-120 from about 2008-2009. It uses 12AT7 driver tubes. You can use either 6550, KT88 or KT120 output tubes. For the rectifier you can use a 5AR4/GZ34 tube or a Weber WZ68 or Weber WS-1 solid state rectifier.

    Bob

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    Cl75


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    Post by Cl75 Sat Aug 07, 2021 3:10 pm

    Thank you for quick response.It says dynakit st 70 on the front panel.I have a pair of 12ax7 and a pair of 6sn7, are there direct replacement with the 12at7?if not, what need to be done in order to use them. Are there any additional power output or quality if I use the kt120? Thank you very much.
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    Post by Bob Latino Sat Aug 07, 2021 4:01 pm

    Cl75 wrote:Thank you for quick response.It says dynakit st 70 on the front panel.I have a pair of 12ax7 and a pair of 6sn7, are there direct replacement with the 12at7?if not, what need to be done in order to use them. Are there any additional power output or quality if I use the kt120? Thank you very much.

    It says Dynakit ST-70 on the front because at the time I was buying the chassis from Dynakitparts.com. Yes - You will get about an 8% increase in power by using the Tung-Sol KT120 output tubes instead of 6550 or KT88 output tubes.

    Bob
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    Cl75


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    Post by Cl75 Sat Aug 07, 2021 4:10 pm

    Thanks for the clarification. What about the use of 12ax7 and 6sn7gtb in place for the 12at7? Any suggestions?
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    Post by Bob Latino Sat Aug 07, 2021 4:31 pm

    Cl75 wrote:Thanks for the clarification. What about the use of 12ax7 and 6sn7gtb in place for the 12at7? Any suggestions?

    Don't use a 12AX7 in place of a 12AT7. A 12AX7 has more gain than you really need in that circuit. A 6SN7 is really an octal version of a 12AU7 with similar gain characteristics. BUT > The 6SN7 draws double the current of a 12AU7 (.3 amps per tube for a 12AU7 vs. .6 amps per tube for a 6SN7). If you have three 6SN7's in place of three 12AU7's then the driver circuit will draw almost and extra amp and the amp will run noticeably warmer.

    Bob
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    Cl75


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    Post by Cl75 Sat Aug 07, 2021 4:38 pm

    Then, can I use 2 of the 6sn7 and 1 of 12at7 in the center?

    Anything wrong with running it warmer?
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    Post by Bob Latino Sat Aug 07, 2021 7:29 pm

    Cl75 wrote:Then, can I use 2 of the 6sn7 and 1 of 12at7 in the center?

    No - You can't mix 12AT7 and 6SN7 tubes on any of the VTA amps. Note - A 12AT7 is a noval tube and fits into a 9 pin miniature socket. A 6SN7 is an octal 8 pin socket. If you had the newer VTA 12AU7 CCS board, you could use a 6SN7 driver tube in the center position if you had noval to octal socket adapter.

    Bob


    Last edited by Bob Latino on Sat Aug 07, 2021 7:35 pm; edited 2 times in total
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    Post by Bob Latino Sat Aug 07, 2021 7:32 pm

    Cl75 wrote:Anything wrong with running it warmer?

    Running the amp warmer than it should will put more stress on the power transformer. Whether this fact will have any long term negative effects, I can't say for sure ?

    Bob
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    Post by Cl75 Sat Aug 07, 2021 8:04 pm

    Thank you Bob for answering all of my questions. One last question I have is is it safe two mix power tubes like 1 kt88 and 1 kt120 for each side?
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    Post by Bob Latino Sat Aug 07, 2021 8:42 pm

    Cl75 wrote:Thank you Bob for answering all of my questions. One last question I have is is it safe two mix power tubes like 1 kt88 and 1 kt120 for each side?

    This is not a good idea .. I guess you could do this in an emergency situation to keep the music flowing but it is not a good idea to mix two different tubes on one channel. An analogy would be having a team of two horses pulling a wagon and the two horses are of different physical characteristics. This would not be a good or efficient way to pull the wagon.

    Bob
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    Post by Cl75 Sat Aug 07, 2021 10:51 pm

    Hi Bob, I forgot to ask you about the wattage output rating for kt88 and kt120 for my machine. Much appreciated.
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    Post by Bob Latino Sun Aug 08, 2021 7:18 am

    Cl75 wrote:Hi Bob, I forgot to ask you about the wattage output rating for kt88 and kt120 for my machine. Much appreciated.

    The VTA ST-120 will put out 60 watts per channel with KT88 output tubes and an extra 4 to 5 watts per channel if you use KT120 output tubes. Note - you can't really detect this slight power increase while playing the amp.

    Bob
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    Cl75


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    Post by Cl75 Wed Aug 11, 2021 11:31 am

    Can I run a pair of 6829 or 7062 for the sides and 12at7/12ax7 in the middle, or all 3 of 6829/7062?
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    Post by Bob Latino Wed Aug 11, 2021 11:41 am

    Cl75 wrote:Can I run a pair of 6829 or 7062 for the sides and 12at7/12ax7 in the middle, or all 3 of 6829/7062?

    Since your amp is an older VTA ST-120 that uses 12AT7 driver tubes, you can use 12AY7, 5751, ECC81 or a Mullard CV4024 as a substitute for a 12AT7. Do NOT USE a 12AX7 in place of a 12AT7. A 12AX7 has too much gain for any 12AT7 designed circuit.

    Bob
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    Cl75


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    Post by Cl75 Wed Aug 11, 2021 12:18 pm

    What about the 7062s and 6829?

    What is the lowest, highest and ideal setting for biasing kt88 and kt120?

    Is it normal to have the bias location at around the10 and 11 o'clock, next to each other?
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    Post by Cl75 Wed Aug 11, 2021 12:50 pm

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    Post by corndog71 Wed Aug 11, 2021 2:53 pm

    It doesn’t matter which pinhole is used. It is good to know which one goes to which tube.
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    Post by Bob Latino Wed Aug 11, 2021 3:03 pm

    Cl75 wrote:What about the 7062s and 6829?

    There are a few military/industrial versions of a 12AT7 that are also probably acceptable. I don't know much about these two tubes that you mentioned so I can't really comment on them.

    Cl75 wrote:What is the lowest, highest and ideal setting for biasing kt88 and kt120?

    Both KT88, KT120 and also 6550 output tubes should be biased at about .500 VDC in a VTA ST-120. This is the ideal setting. If you bias higher than this, then tube life will suffer. If you bias lower than this, you will probably get slightly longer tube life. You can probably bias these down as low as .450 VDC but at some point lower than this, the tube will not be operating in an area that will give best sound.

    Cl75 wrote:Is it normal to have the bias location at around the10 and 11 o'clock, next to each other?

    Usually two OPPOSITE pins on each tube socket are used as bias measuring points. Usually the pin next to the "BIASET 1.56V" marking on the chassis is used as the bias measuring point for the FRONT tube of the pair. For the REAR tube of the pair, usually the pin opposite this pin is used. Apparently in this case, the builder chose to use the pin just above the "BIASET 1.56" to bias the rear output tube of that pair.

    Bob
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    Post by Cl75 Wed Aug 11, 2021 4:02 pm

    Thank you very much Bob.
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    Post by Cl75 Thu Aug 12, 2021 12:51 pm

    Bob Latino wrote:That is an older VTA ST-120 from about 2008-2009. It uses 12AT7 driver tubes. You can use either 6550, KT88 or KT120 output tubes. For the rectifier you can use a 5AR4/GZ34 tube or a Weber WZ68 or Weber WS-1 solid state rectifier.

    Bob
    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".
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    Post by Cl75 Thu Aug 12, 2021 12:55 pm

    Bob Latino wrote:That is an older VTA ST-120 from about 2008-2009. It uses 12AT7 driver tubes. You can use either 6550, KT88 or KT120 output tubes. For the rectifier you can use a 5AR4/GZ34 tube or a Weber WZ68 or Weber WS-1 solid state rectifier.

    Bob
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