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A couple of weeks ago I posted about my new ST-70- based amp from Chris Keller in NC and I asked how much it might benefit from 6NS7s instead of 12AU7s. I learned from inmate airtime in a prior post about the ability to replace just the center 12AU7 with an adapter. So I did that earlier this week.My impressions are consistent with the consensus among those of you who have done this before, or replaced the driver board, or simply heard a lot of both tube types in different applications. It's a better sounding tube for this application. The amp now sounds bigger, clearer and more dynamic. The bass is stronger. I'd say the changes are beyond subtle but not revolutionary. And I hear no downsides. I lost no high end extension and there is no his or hum or buzz or other artifacts that I can hear. It's just a better sounding amp.
It doesn't even look bad. It's kind of cool. Although it does look a bit like it's ether very happy to see me or flipping me off, what with the smaller tubes flanking it on each side. Har Dee Har Har.
All of that said, I don't hate the 12AU7s. The amp went from sounding really, really good to really, really good+ so I'm not ready to join the !2AU7s Are Garbage Society as far as their impact on SQ. The amp sounded great with them even if not quite as clear, bold and dynamic. And I've noticed that warm up is more vital for the 6NS7s. When the amp is cold, there is a thinness that goes away over half and hour or so, and I didn't notice that nearly as much with the 12AU7s.
As a follow up comment and question, my preamp is an all 12AU7 piece (Rogue Audio RP-5). I'd be interested in knowing how much that preamp might be holding back the improvement potential. I love the RP-5, including its built in phono stage. That preamp is flawless operationally, silent, and sounds really nice. But I cannot deny being intrigued by the SP14 preamp from VAT/Tubes4HiFi.
As a related question, in what applications does the 12AU7 most impact performance and least impact performance?
Thanks for this most enlightening and interesting (at least to me) discourse.
P.S. I chose an NOS Sylvania "Chrome Dome," which I bought from Brent Jessee. And both Chris, the builder, and Roy from VTA, the driver board supplier, were super helpful in thinking this through. Roy actually recommended the tube type and the adapter I ended up buying.
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"If you are the owner of a new stereophonic system, this record will play with even more brilliant true-to-life fidelity. In short, you can purchase this record with no fear of its becoming obsolete in the future."
Edits: 04/01/23 04/01/23Follow Ups:
That the 5963 is a very linear tube replacement for the 12AU7. Anyone here used one in place of the 12AU7?
Below is a link to a post by Airtime, where he specifically tried a 5963 in his VTA, back in 2016. His results weren't good, even compared to a GE 12AU7, as you'll see in his post.
But the 6SN7 worked wonders for him as the gain tube, as a point of reference.
in the middle spot as well with the 6SN7.
I did order and receive the VTA Octal pcb for my VTA-120 from Roy. I'm going to upgrade some parts on the Octal pcb. First thing is the all the coupling caps, then some of the resistors. I will be installing the Auto-Bias Pcb that I had purchased awhile back.
...in comparison to your noval board. There was some talk of the elimination of octal board sales when the new owner took over VTA. I hope they don't, but maybe I should order one, just in case.
Like you, I'd be changing most (if not all) of the supplied parts anyway, so if I do order one, I'll opt for a blank board.
now that I have finished my EDGE M8 power amp and its back in the system sounding awesome. I now can shift my attention to the VTA-120 octal PCB and Auto-Bias installation.
Even though the Crown sound good, it will go back to why I originally bought it driving my other project the JBL 4345 in biamp mode. This should be the last project I do since my DIY 2 way horn/ 15" speaker is almost complete.
My first stereo system was literally built around a Dynakit ST-70 in 1966. Over the years I've owned a few more used examples, the most recent one now under Rod's care and feeding. So from time to time I enjoy reading what others are doing with this classic.
Thanks Josh and others for posting your experiences here. As a current owner of the Don Sachs version of the VTA SP-14 I'm a fan of the 6SN7 as well.
"The only cats worth anything are the cats who take chances. Sometimes I play things I never heard myself." Thelonious Monk
I'm having preamp envy!
If I do upgrade my preamp, it will almost certainly be with either VTA's fully upgraded SP14 or Don Sash's version. Right now I have an all 12au7 Rogue Audio RP-5 which I assume is holding back the SQ of the ST-70 a bit now, and also my CJ Premier 11A. But I do like the sound of that Rogue with (pre-war) Russian Gold Lion 12au7s, and it's been so reliable too.
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"If you are the owner of a new stereophonic system, this record will play with even more brilliant true-to-life fidelity. In short, you can purchase this record with no fear of its becoming obsolete in the future."
You must have found better 12AU7s than I ever did. I tried several brands, along with 12BH7s, but found nothing pleasurable enough to listen to, until I went to the 6SN7. Airtime and I corresponded extensively via email, trying whatever improvements we could think of. In the end we reached the same conclusion, that the non-linear 12AU7 in the gain section had been the source of our displeasure the whole time.
Glad you're happy with the outcome. Thanks for your follow-up.
I have a stock ST-70 that's been re-capped with new RCAs, etc. I tried the adapter to the 6GH8A and had a couple of old 7199s that were weak. Eventually, a couple of NOS RCA 7199s turned up and are nice.
Still, why use 12AU7s and add another 6NS7? I do like the octals, but I'd have thought that there would be a replacement driver board with just 2 of them.
-Rod
Here is what VTA says on their website about their "classic" ST-70 driver board:"The upgraded VTA driver board uses three new 12AU7 or 12BH7 triodes in place of the two mediocre sounding 7199 pentode/triode tubes. Dynaco saved money using a two tube 7199 board design but made some serious compromises in sound quality." Chris who built my amp says much the same thing in his owner's manual.
Since then, VTA started offering a three tube octal driver board as an upgrade option.
And here is what inmate airtime said about the 12AU7 driver board when I first posted about buying the amp in February on the Amp Asylum:
"Because of its lack of linearity the 12AU7 should never be used as a gain tube. It sounds awful in that position and no amount of tube rolling is going to fix it. Designers are only using it because it quickly fixes a number of gain issues. The 6SN7 is the tube to use on that driver board."
Based on my actual listening, I don't agree that the 12AU& sounds awful in that position. Far from it. I do think the 6NS7 sounds better though.
As you can see on the linked page, VTA (tubes4hifi) still sell both as replacement parts and still seem to show the 12au7 board as being the standard approach, but Roy was clear when I emailed him that replacing the center tube with a 6NS7 and adapter should be an improvement.
But I have no idea how any of this would apply to the original two tube circuit.
___
"If you are the owner of a new stereophonic system, this record will play with even more brilliant true-to-life fidelity. In short, you can purchase this record with no fear of its becoming obsolete in the future."
Edits: 04/03/23 04/03/23
Could you also use a 6CG7/6FQ7 and do away with the adapter?
...but filament traces on the VTA board would have to be cut, and then rewired.
I considered doing that when Tre' suggested trying a 6CG7 in place of a 12AU7. But given the price of a NOS 6CG7, coupled with the trace cutting, I was hesitant, since I'd tried so many other things on my VTA board (various caps, resistors, and other tweaks) to get it to my liking, none of which ever worked. Sometimes there were improvements, but never enough to allow me to enjoy music with the amp.
After learning that a 6CG7 was essentially a noval 6SN7, and since I already had a few NOS 6SN7s, the adapter and 6SN7 was a less invasive (i.e. no trace cutting) way to try yet another experiment. Except that this time it worked. In retrospect, I could have bought a few 6CG7s from Jim McShane, and eliminated the adapter, but at the time I had become very skeptical that I'd ever find anything which would allow me to enjoy the amp.
If I knew then what I know now, I wouldn't hesitate to use a 6CG7, especially if I had some stashed away.
I don't know enough to answer the question myself, but I'm curious how easy it would be to have the board replaced at some point. I don't feel the need at this point because this approach is working so well.
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"If you are the owner of a new stereophonic system, this record will play with even more brilliant true-to-life fidelity. In short, you can purchase this record with no fear of its becoming obsolete in the future."
Edits: 04/03/23
Using three 6CG7s would only involve cutting a few traces on the circuit board, and soldering in six jumper wires. Installing the octal board involves removing and reinstalling more wires, as well as the hardware of the board itself. Not that that's a lot of work, but technically it is more work. But buying a new octal board would allow you to sell the used 12AU7 version, along with the adapter, so there's that.
But the real question, one that Airtime and I have speculated on, is how much sonic improvement (if any) would be gained by changing the phase inverters (the two outer tubes) to 6SN7s or 6CG7s.
He even posted the question to the Asylum, and possibly elsewhere, but I don't know if he was fully inspired by any of the answers he got. If I needed a new board, I wouldn't hesitate to buy the octal version. But since the VTA ST-70 isn't my main amp, I'm going to stick with the 12Au7s in the phase inverter positions, at least for now.
I don't know whether this will become my main amp, or at least my most used one. I bought it when a friend flagged it for me because I've wanted to own an EL34 amp for years and the price seemed so attractive at $1195 plus $35 shipping. And now another $74 for the single 6NS7 plus shipping.
My main amp was a CJ Premier 11A, but it might not go back into this system for awhile, at least not without an A-B session. I do love that amp, but the ST-70 is singing with my easy to drive Devore Super Nines. I'm sure it's not as great a match with many other speakers.
Thanks again for all your great advice.
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"If you are the owner of a new stereophonic system, this record will play with even more brilliant true-to-life fidelity. In short, you can purchase this record with no fear of its becoming obsolete in the future."
I don't mind using the adapter, but if your suggestion would have all the benefits without the need for an adapter, I'd be happy to try.
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"If you are the owner of a new stereophonic system, this record will play with even more brilliant true-to-life fidelity. In short, you can purchase this record with no fear of its becoming obsolete in the future."
enjoying the new 6SN7 in the middle spot. As you listen more and more you really appreciate this new tube in your VTA-70. Happy Listening!
.
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"If you are the owner of a new stereophonic system, this record will play with even more brilliant true-to-life fidelity. In short, you can purchase this record with no fear of its becoming obsolete in the future."
"in what applications does the 12AU7 most impact performance and least impact performance?"
Most Impact: When used as a standard gain stage in a circuit with no local or global feedback.
Least Impact: When used in a topology with inherent negative feedback, such as a cathode follower (best), cathodyne phase splitter or long tail pair.
It's a poor choice as a gain stage in a preamp with no feedback.
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