OLD VTA driver board vs NEW driver board

by 1bigpig Thu Apr 29, 2021 2:01 pm

1bigpig: Posts : 14
Join date : 2015-03-12

Post n°1

OLD VTA driver board vs NEW driver board

by 1bigpig Thu Apr 29, 2021 2:01 pm

I purchased my Bob Latino ST-120 used about 7 years ago and have been loving it ! It came with the older style 12AT7 VTA driver board. About 3 years ago, I purchased a newer VTA driver board which uses the 12AU7 tubes with the CCS power. The newer VTA board came with Audyn Q4 audio bypass caps while my original has the Russian K40 capacitors. Also, my older board has some beautiful sounding Mullard 12AT7 tubes that sound really sweet to my ears. The newer VTA board also came with clear top RCA 12AU7.

So I have 3 questions:

1st. My old VTA board currently is working fine, but is the newer driver board with the CCS power worth upgrading to?

2nd. If yes to #1, then is it worth swapping out the Audyn Q4 caps with the older Russian K40 bypass caps?

3rd. If yes to #2, since I love the Mullard 12AT7 sound AND would already be swapping out caps, can the newer VTA board be modified to accommodate a Mullard 12AT7 in the "pre-amp" (center tube position) but leaving the phase splitter tubes as RCA 12AU7s? I think I have read that changing from 12AT7 to 12AU7 involved just swapping out a few resistors but I am not sure and seem to be unable to find that post now. Since I would only be changing one tube, I am wondering WHAT resistors to change and what the new values would be.

Thanks,
Bruce

by **Bob Latino** Thu Apr 29, 2021 6:01 pm

Bob Latino

Bob Latino: Admin; Posts : 3277
Join date : 2008-11-26
Location : Massachusetts

Post n°2

Re: OLD VTA driver board vs NEW driver board

by Bob Latino Thu Apr 29, 2021 6:01 pm

1bigpig wrote:I purchased my Bob Latino ST-120 used about 7 years ago and have been loving it ! It came with the older style 12AT7 VTA driver board. About 3 years ago, I purchased a newer VTA driver board which uses the 12AU7 tubes with the CCS power. The newer VTA board came with Audyn Q4 audio bypass caps while my original has the Russian K40 capacitors. Also, my older board has some beautiful sounding Mullard 12AT7 tubes that sound really sweet to my ears. The newer VTA board also came with clear top RCA 12AU7.

So I have 3 questions:

1st. My old VTA board currently is working fine, but is the newer driver board with the CCS power worth upgrading to?

2nd. If yes to #1, then is it worth swapping out the Audyn Q4 caps with the older Russian K40 bypass caps?

3rd. If yes to #2, since I love the Mullard 12AT7 sound AND would already be swapping out caps, can the newer VTA board be modified to accommodate a Mullard 12AT7 in the "pre-amp" (center tube position) but leaving the phase splitter tubes as RCA 12AU7s? I think I have read that changing from 12AT7 to 12AU7 involved just swapping out a few resistors but I am not sure and seem to be unable to find that post now. Since I would only be changing one tube, I am wondering WHAT resistors to change and what the new values would be.

Thanks,
Bruce

Hi Bruce,

1. There won't be that much difference in the "sound quality" between the older (pre 2012) 12AT7 driver board and the newer VTA ST-70 CCS 12AU7. The older board does have more gain but the newer 12AU7 driver board has a somewhat lower residual noise level. The newer board has two LM334 current regulators and does balance the amount of current on the two output tubes of the pair so that the current is always equal on both tubes at all drive levels even if the two tubes of the pair are slightly mismatched.

2. Those Audyn Q4 caps are just regular polypropylene caps and are inexpensive. I have never heard them so I cannot comment on their sound. The VTA amp Russian K-42 caps are paper in oil (PIO) caps and are made to military standards and have a real spacious holographic sound to them. To me, they sound better than any film cap I have ever tried in my own VTA ST-120 and VTA M-125 monoblock amps. For the price they are excellent caps. I test every one that goes out the door and they are shipped in sets of four whose values are closely matched.

3. You cannot use a 12AT7 driver tube in the newer VTA CCS driver board so if you switch to the newer board you will have to get three new driver tubes. The nice thing about using 12AU7 driver tubes is that you can also use the military/industrial versions of the 12AU7 > the 5963, 5814 and 6189. Millions of these three 12AU7 variant were made for the military. Go on Ebay and you can sometimes pick up a 5963, 5814 or 6189 tube made by GE, RCA or Sylvania in NOS or near NOS condition. My favorite tubes are the Mullard CV4003 (a 12AU7) or a RCA cleartop or a Sylvania 6189 or a GE five star 5814.

Bob

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by Tom Pickett Thu Apr 29, 2021 7:42 pm

Tom Pickett

Tom Pickett: Posts : 203
Join date : 2020-11-22
Age : 59
Location : Texas

Post n°3

Re: OLD VTA driver board vs NEW driver board

by Tom Pickett Thu Apr 29, 2021 7:42 pm

"I test every one that goes out the door and they are shipped in sets of four whose values are closely matched."

Bob this is the reason your kits are so nice. You make sure the quality of your kits meets your personal standards before they go out the door to your customers.

Some wouldn't even give it a thought or care but you do.

by 1bigpig Fri Apr 30, 2021 12:16 am

1bigpig

1bigpig: Posts : 14
Join date : 2015-03-12

Post n°4

Re: OLD VTA driver board vs NEW driver board

by 1bigpig Fri Apr 30, 2021 12:16 am

Thanks Bob!

I appreciate you taking the time to answer my questions. I think I will keep my current 12AT7 driver board until pre-amp/buffer tube(s) die and then swap out the whole driver board.

I too am unfamiliar with Audyn Q4 caps, which is why I asked. I think a quick internet search brought them up as speaker x-over caps.

Your recommendation for 12AU7 tubes is interesting. I saw Mullard 12AU7s/CV4003 seem to be going for $100+ a tube, but the RCA/Sylvania are much less (~$20 a tube). Are the Mullards that much better or just supply/nostalgia?

In your opinion, how does the Mullard 12AU7/CV4003 compare to the Mullard 12AT7/CV4024?

Thanks again!
Bruce

by **Bob Latino** Fri Apr 30, 2021 8:31 am

Bob Latino

Bob Latino: Admin; Posts : 3277
Join date : 2008-11-26
Location : Massachusetts

Post n°5

Re: OLD VTA driver board vs NEW driver board

by Bob Latino Fri Apr 30, 2021 8:31 am

Bruce,

The Mullard (Valve Electronic) CV4024 and the CV4003 are similar in their sound. Their price is high because they are getting rare.

The GE, RCA and Sylvania 5963, 5814 and 6189 are all good tubes and the supply is still pretty good hence the pricing is usually reasonable. You can't really go wrong with any of them .. 5814 and 6189's tend to sell for more money than a 5963 but my opinion is that the 5963 is just as good a tube.
Check them all out at the links below ...

5963 tubes on Ebay

5814 tubes on Ebay

6189 tubes on Ebay

Bob

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OLD VTA driver board vs NEW driver board