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The Dynaco Tube Audio Forum

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    Differences / worthy upgrades from older VTA boards?

    calinet6
    calinet6
    calinet6


    Posts : 19
    Join date : 2011-06-01
    Location : Massachusetts
    • Post n°1

    Differences / worthy upgrades from older VTA boards? Empty Differences / worthy upgrades from older VTA boards?

    Post by calinet6 Mon Feb 17, 2020 11:03 pm

    Hey all—I've got an original ST-70 I retrofitted with the VTA board around 10 years ago (2010-ish). It has the 12AT7 tubes in the driver section rather than the 12AU7s, and 33uF capacitors on the driver board. I have installed the pentode/triode switches.

    The amp has been a dream for the last ten years, truly helping me enjoy the music and it continues to amaze me every time I listen.

    I've built a couple tube amps from scratch since then and I've begun to understand the functionality better (the VTA mod was my first introduction!) and I'm just curious if it's worth comparing the VTA boards from ten years ago to the stock now and if any of the upgrades are worth doing to bring it up to snuff.

    My first thought is the power supply capacitors: is there anything to gain from upgrading them? It would certainly be fairly inexpensive and easy, but I'm not certain if it would be a benefit or not given the rest of the circuit. Is there a reason the larger ones are sold, and if so what's the reasoning there?

    The other question I had is on the new constant-current-source biasing for the driver tubes on the new version-- I just built a headphone amp with an LM317 CCS bias and it's stunningly consistent and stable, and it makes me wonder if it would benefit the driver tubes on the VTA board. I'm fairly confident I could design a modification if there is a reason to do so; I'm just not certain there is or not. Any idea on the difference that would make as well?

    Also just for the heck of it: why the change from 12AT7 to 12AU7 tubes? Just curious.

    Thanks! 10 years of enjoyment so far. Here's to many more.
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    Bob Latino
    Bob Latino
    Bob Latino
    Admin


    Posts : 3263
    Join date : 2008-11-26
    Location : Massachusetts
    • Post n°2

    Differences / worthy upgrades from older VTA boards? Empty Re: Differences / worthy upgrades from older VTA boards?

    Post by Bob Latino Tue Feb 18, 2020 9:57 am

    calinet6 wrote:Hey all—I've got an original ST-70 I retrofitted with the VTA board around 10 years ago (2010-ish). It has the 12AT7 tubes in the driver section rather than the 12AU7s, and 33uF capacitors on the driver board. I have installed the pentode/triode switches.

    The amp has been a dream for the last ten years, truly helping me enjoy the music and it continues to amaze me every time I listen.

    I've built a couple tube amps from scratch since then and I've begun to understand the functionality better (the VTA mod was my first introduction!) and I'm just curious if it's worth comparing the VTA boards from ten years ago to the stock now and if any of the upgrades are worth doing to bring it up to snuff.

    My first thought is the power supply capacitors: is there anything to gain from upgrading them? It would certainly be fairly inexpensive and easy, but I'm not certain if it would be a benefit or not given the rest of the circuit. Is there a reason the larger ones are sold, and if so what's the reasoning there?

    The other question I had is on the new constant-current-source biasing for the driver tubes on the new version-- I just built a headphone amp with an LM317 CCS bias and it's stunningly consistent and stable, and it makes me wonder if it would benefit the driver tubes on the VTA board. I'm fairly confident I could design a modification if there is a reason to do so; I'm just not certain there is or not. Any idea on the difference that would make as well?

    Also just for the heck of it: why the change from 12AT7 to 12AU7 tubes? Just curious.

    Thanks! 10 years of enjoyment so far. Here's to many more.

    The newer 12AU7 CCS version of the VTA driver board came out in early 2012. The switch to the newer board was made for a number of reasons.

    1. The newer board uses 12AU7 driver tubes have much lower gain that the original 12AT7 driver tubes on the older board. Lower gain also means a lower residual noise level. You are not amplifying the audio signal quite as much. The newer board is somewhat "quieter" than the old board.

    2. The older board had the bias pots turn counterclockwise to INCREASE the bias. This was counterintuitive to turning things clockwise to increase volume, levels etc. On the newer boards your turn the bias pots clockwise to increase bias.

    3. The new board has CCS (constant current source) > A constant current source is a power source which provides a constant current to a load, despite changes and variance in any load resistance. The older boards had a 10K AC bias pot to set the current balance manually between the two output tubes of the pair. The newer board has a small LM334 integrated circuit which keeps the amount of current balanced between each output tube of the pair, even if the two tubes present a slightly different load resistance.

    Bob
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    calinet6
    calinet6
    calinet6


    Posts : 19
    Join date : 2011-06-01
    Location : Massachusetts
    • Post n°3

    Differences / worthy upgrades from older VTA boards? Empty Re: Differences / worthy upgrades from older VTA boards?

    Post by calinet6 Tue Feb 18, 2020 11:29 am

    That's all great. Thank you.

    A few specific follow-up questions:

    1) I have the four 33uF supply electrolytics on the driver board. Are these worth upgrading? Would it be any benefit to increase the capacitance there to ~400uF (100uF each) or so? What would the difference be?

    2) Would it be worth retrofitting the board with the LM334 in place of the bias pot?

    3) Another question on that I guess -- I've never touched that bias pot (it's on the underside of the board as I can remember). Is there a right way to bias the driver board tubes such that they're in the best balance? Though it's almost the same amount of work to just replace with the LM334....
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    Bob Latino
    Bob Latino
    Bob Latino
    Admin


    Posts : 3263
    Join date : 2008-11-26
    Location : Massachusetts
    • Post n°4

    Differences / worthy upgrades from older VTA boards? Empty Re: Differences / worthy upgrades from older VTA boards?

    Post by Bob Latino Tue Feb 18, 2020 12:56 pm

    calinet6 wrote:That's all great. Thank you.

    A few specific follow-up questions:

    1) I have the four 33uF supply electrolytics on the driver board. Are these worth upgrading? Would it be any benefit to increase the capacitance there to ~400uF (100uF each) or so? What would the difference be?

    2) Would it be worth retrofitting the board with the LM334 in place of the bias pot?

    3) Another question on that I guess -- I've never touched that bias pot (it's on the underside of the board as I can remember). Is there a right way to bias the driver board tubes such that they're in the best balance? Though it's almost the same amount of work to just replace with the LM334....

    1. The 100 uF caps give the driver board more of a "reserve" for DC storage. This is important only when you play the amp at higher volume levels.

    2. The circuit is slightly different from one board to the other. You would have to replace your board completely with the newer board..

    3. You are not supposed to touch the 10K AC balance pot. When you build the older board, you set that 10K bias pot at its center position which assumes that the two output tubes are matched properly. The 10K bias pot was put on the bottom of the driver board so that people wouldn't alter the setting after the amp was built.

    Bob
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    • Post n°5

    Differences / worthy upgrades from older VTA boards? Empty Re: Differences / worthy upgrades from older VTA boards?

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