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Pinging airtime and others regarding the 12au7 and its favored/disfavored applications

73.143.183.29
Posted on March 15, 2023 at 08:15:58
JoshT
Audiophile

Posts: 6622
Location: Eastern Massachusetts
Joined: July 4, 2000
 So airtime, you peeked my interest recently regarding how terrible the 12au7 tube is, at least for use in the ST-70 driver board. I know you've expressed your negative views on that tube type before, and you're certainly not the only one by a long shot who I've heard express similar views here and elsewhere on the internet.

From what I've read, it is nonlinear, has high distortion, and sounds "small" compared to other options such as the 6SN7. Among others, the folks at Vaccuum Tube Audio (Tubes4Hifi) and Dan Sachs sing the praise of the 6SN7 compared to the 12au7, but I gather it's not a plug and play replacement.

In my case, I have 12AU7s exclusively in may ST-70 and my Rogue Audio RP-5 preamp. I also have three of them in my Cronus Magnum II integrated (together with a pair of 12ax7s). Here's the thing - I like the sound of all three components a lot. I really have no complaints at all, and I generally live my the maxim "if it sounds good it is good," but on the other hand . . .

On the other hand, I also have first hand experience with upgrades that revealed to me how much I was missing before the upgrade. And strictly from a sonics standpoint, my favorite preamp was a Conrad Johnson Premier 17LS with four 6922 tubes. But that was years ago and the Rogue sure is close to my recollection and has more features and is better mannered.

So I'd love to hear from the experts in some detail what they don't like about the 12au7 compared to other options, and whether the tube's defects are only audible in some applications and not all. Also, when can one replace the other with an adapter but without otherwise changing the circuit? The 12au7 sure is plentiful, both NOS and new production, and affordable.

TIA!
___
"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."

 

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the controversial 12au7, posted on March 15, 2023 at 08:50:02
Story
Audiophile

Posts: 10451
Location: NJ
Joined: December 11, 2000
 the waveform produced can run 'distorted' if implemented with the non-linear portions of the curve. It doesn't sound distorted per se, just 'non-linear' when run into the extremes of both ends. How to avoid this depends on the circuit requirements.

I suspect most designers know this but use the tube because of costs - it's a lot less expensive than a 12sn7. It has a sound that is 'compressed' in comparison and some might like that, others won't. If you are a dynamics fan with classical and jazz, you would prefer the 12sn7 for fidelity. Strictly rock music might prefer the 12au7.



 

RE: Pinging airtime and others regarding the 12au7 and its favored/disfavored applications, posted on March 15, 2023 at 09:22:54
Palustris
Audiophile

Posts: 2408
Location: Cape Cod
Joined: September 12, 2008
 Both Morgan Jones in his book "Valve Amplifiers" and Eric Barbour in "Vacuum Tube Valley" found that the 12AU7 has significantly more 2nd harmonic distortion than many of the other low mu triodes such as the 6SN7, 5687, 76 and 12SN7. Personally, I use the 1950s TungSol 5687, although others have their favorites. The 76 family are all low distortion and popular with builders of audio equipment. Read "Valve Amplifiers" for an education in vacuum tubes. The link to the VTV article is below.

 

I have used the 12AU7 with Good results., posted on March 15, 2023 at 09:26:45
Cougar
Audiophile

Posts: 4583
Location: SoCal
Joined: June 25, 2001
 I use them in my VTA-120 amp (outer tubes) but use a 6SN7 in the middle splitter spot. This has given me the best sound from my VTA-120. I also use them in my Transcendent Sound Masterpiece 300b preamp. Not all 12AU7's are created equal in all gear. Some sound awful and some sound really nice from what I have experience.

 

Oh wow! So I could do that too with my ST-70?, posted on March 15, 2023 at 09:39:37
JoshT
Audiophile

Posts: 6622
Location: Eastern Massachusetts
Joined: July 4, 2000
 Thanks! It sounds like you are doing what airtime recommended a couple of weeks ago as a comment to one of my prior posts, and I was partly following up to make sure I understood.

So a single 6SN7 could do the trick with an adapter and no other necessary modifications to the board, or is the ST120 circuit different from the ST-70? I can certainly ask the builder who's been super helpful

Do you have the VTA driver board in yours? I was worried that there wouldn't be enough space on either side where the 12au7s sit but I assume the board dimensions are about the same?
___
"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."

 

Thanks and will do. I'm still a bit green on tubes. After all, I'm only 57 LOL. nt, posted on March 15, 2023 at 09:44:38
JoshT
Audiophile

Posts: 6622
Location: Eastern Massachusetts
Joined: July 4, 2000
 .
___
"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."

 

Interesting. Thanks! nt, posted on March 15, 2023 at 09:45:46
JoshT
Audiophile

Posts: 6622
Location: Eastern Massachusetts
Joined: July 4, 2000
 .
___
"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."

 

RE: the controversial 12au7, posted on March 15, 2023 at 10:27:08
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002



"the waveform produced can run 'distorted' if implemented with the non-linear portions of the curve."

I have looked at the plate curves for the 12au7 and I can not find a linear portion. [that is not true, there is a limited area where the tube could be used and give reasonably linear results as long as you have a horizontal (high impedance) load line] The grid line spacing it wide to the left and narrow to the right for the entire width and that will lead to high harmonic distortion.

The 12au7 works great when there is a lot of feedback applied, like in a cathode follower circuit.

As a gain stage, when there won't be much feedback applied, they should be avoided.

For comparison take a look at the 6sn7 (or 6cg7) plate curves. Do you see how the grid line spacing is so much more even and how they don't scrunch together as you move to the right and down? With this tube you can run at many different operating points and get a linear (low harmonic distortion) result. You don't even need a horizontal load line to get good results, although a horizontal load is always better.




Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

know all about it as discussed before, posted on March 15, 2023 at 10:33:53
Story
Audiophile

Posts: 10451
Location: NJ
Joined: December 11, 2000
 and people use it for the reasons I said, popular with guitar amps

I don't have anything 12au7, but I do have 12sn7 and 12sl7's in a few pieces



 

RE: Oh wow! So I could do that too with my ST-70?, posted on March 15, 2023 at 12:40:22
Cougar
Audiophile

Posts: 4583
Location: SoCal
Joined: June 25, 2001
 Airtime is a member over on AK where we all who built the VTA kit/s went to help or get help. There was a thread over there a while back may around 2015 and Skizo was the one who mentioned about putting a 6SN7 in the middle splitter spot I tried iy and I think Airtime tried after I did but I could be wrong. That is where the biggest difference in sound for me happened. I was using Northern Electric 6SN7's in that spot but they kept humming. Then I was on the Upscale Audio site and saw Kevin found a bunch of Russian 6H8C (Equivalent to 6SN7) and were cheap. I bought some and that was the ticket! Had the same sound as the NE 6SN7 but with better mids and bass. I have been running it ever since.

Yes, you will need a 6SN7 to 12AU7 adapter to make this work. My PCB came with my kit but if I'm correct, Roy offers a 6SN7 Octal pcb for the VTA-120 and 70. Some will say you can't run that tube in the 12AU7 spots but I only used it in the middle splitter spot and has worked fine. I would check out the threads over there to see what we all did. My Moniker there is Drummerboy2.

Skizo, Airtime, and another person were all helpful and offered a lot of info and help to others. Just goto the Tube section and search for the VTA-70/120 kit builds threads around 2015-2017. Mine was ("Just Pulled the trigger on Bob Latino's VTA-120 tube power amp"). I think that's were most of the info is at for the VTA amp builds between those years. Also, these amps are nothing like the Dynaco amps other than they look like them. Totally different circuits and better heftier transformers.

btw, I got the Auto-Bias PCB for my VTA-120 a few months back, I still need to install it. I will do that after cleaning and tightening all the socket pins

Happy Reading! :)

 

RE: Oh wow! So I could do that too with my ST-70?, posted on March 15, 2023 at 15:33:28
Mick Wolfe
Audiophile

Posts: 3365
Location: AZ
Joined: October 10, 1999
Contributor
  Since: September 4, 2000
 I've had a VTA (or Bob Latino at the time) ST70 for over a decade. This amp came stock with the 12AT7 board. Fortunately at least in my case, I've been quite happy with the 12AT7. Another friend who has the ST120 recently had the 6SN7 board installed. This replaced his 12AU7 board. Although he wasn't unhappy with the 12AU7 board, he says the 6SN7 board and companion 6SN7's of course have been a noticeable improvement. He had bought a 6SN7 adapter, but opted instead to do the entire board. So using a 6SN7 adapter in the middle position of the 12AU board seems to be a good inexpensive option to try for either the ST70 or ST120. Some may later opt for the entire board or be perfectly happy with just the adapter. Regardless, I think the VTA amps are some of the best value tube amps out there. I never tire of listening to mine even though I've got another P-P tube amp that lists for 2.5X
the cost of the ST70.

 

Thanks! nt, posted on March 15, 2023 at 15:43:39
JoshT
Audiophile

Posts: 6622
Location: Eastern Massachusetts
Joined: July 4, 2000
 .
___
"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."

 

RE: " he says the 6SN7 board have been a noticeable improvement", posted on March 15, 2023 at 16:08:39
1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007



According to Roy Mottram, on the VTA forum, about 90% of his customers who tried both the 12AU7 and 6SN7 board, prefer the 6SN7 version.

I only wish I'd tried the 12AU7 socket to 6SN7 tube adapter in the gain section years sooner than I did. The improvement took my ST-70 with the VTA board from an amp that almost never left storage, to one I can now enjoy listening to.

Oh, and for the record, according to Bob Latino himself on the VTA forum, the amps aren't, and have never been "Bob Latino" amps, as shown above.

 

Why not just use a 6SN7?, posted on March 16, 2023 at 04:50:24
andy evans
Audiophile

Posts: 4382
Joined: October 20, 2000
 A 6SN7 sounds much better and has a lot less distortion.

Just a slightly bigger hole in the chassis and you're done.

 

I'm going to . . ., posted on March 16, 2023 at 07:25:27
JoshT
Audiophile

Posts: 6622
Location: Eastern Massachusetts
Joined: July 4, 2000
 The point of my post was to learn more about whether it was a workable swap with a socket adapter an no other modifications and what kind of improvements I might hear.

Now I know. Before my post I didn't.
___
"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."

 

Thanks, and one clarification question, posted on March 16, 2023 at 07:29:25
JoshT
Audiophile

Posts: 6622
Location: Eastern Massachusetts
Joined: July 4, 2000
 Are you continuing to use single 12AU7 socket to 6SN7 tube adapter or was that just a stepping stone for swapping in a new board?

I've emailed Roy and he confirmed the single tube approach works, but I get the sense many people do that as an interim step to confirm its an improvement before replacing the entire board. I'd really rather not replace the entire board because it's a new amp and I am already enjoying it as is.

Thanks!
___
"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."

 

Update: I ordered a VTA Octal pcb., posted on March 16, 2023 at 07:39:27
Cougar
Audiophile

Posts: 4583
Location: SoCal
Joined: June 25, 2001
 After posting yesterday I contacted Roy and asked some question about the Octal PCB and the Auto-Bias PCB. I decided to pick up an Octal PCB and do some upgrades like I did with the original pcb and some that I didn't do that came with the kit. This should be a nice improvement in sound. I will post some pics of what I do over on Tube/Diy and on AK when I'm finished.

 

Excellent, posted on March 16, 2023 at 08:04:50
JoshT
Audiophile

Posts: 6622
Location: Eastern Massachusetts
Joined: July 4, 2000
 I look forward to the updates and pictures.
___
"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 continue to use the adapter, posted on March 16, 2023 at 08:21:37
1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007
 ...but that's only because this ST-70 with the VTA driver board is one of my back-up amps, and only sees occasional use. If it were my main, best sounding amp, I'd opt for the octal board over the noval.

Regarding tube adapters in general, Jim McShane told me that they can sometimes cause somewhat diminished highs, depending on the particular tube and circuit. I didn't notice that in my system. but eliminating them with the proper octal socket(s) for the 6SN7(s) would really be the best way to go.









 

I'm looking forward to reading about your results, posted on March 16, 2023 at 08:33:23
1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007
 Airtime and I have long been curious how much improvement would be gained by 6SN7s used as the phase inverters, over the two 12AU7s.

I've thought about converting my phase inverters to 12AT7s (VTA's original configuration) since the 12AT7 is a slightly more linear tube than the 12AU7, but I suspect that your all octal upgrade is really the way to go.



 

I ordered an adapter, posted on March 17, 2023 at 06:48:56
JoshT
Audiophile

Posts: 6622
Location: Eastern Massachusetts
Joined: July 4, 2000
 I bought one based on a link from Roy and received an email from the seller - screen shot below.

Is this just a generic warning about putting a 6 volt tube in a 12 volt tube socket? I assume it would be a concern if I were replacing all three tubes but isn't where I'm just replacing the center tube?

Thanks for your input on this.




___
"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."

 

RE: I ordered an adapter, posted on March 17, 2023 at 07:33:19
1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007
 All tube filaments in a Dynaco ST-70 (including the VTA version) use 6.3 VAC, other than the 5AR4 rectifier. So the 6.3 V adapter is the one you want, even if you were adapting all three driver tubes.





 

Continuity meter, posted on March 17, 2023 at 08:07:54
1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007



I'm going to assume that you have a continuity function on the meter you use to check tube bias. If so, above is a crosscheck I came up with from the data sheets of the two tubes, to be certain that the adapter is wired correctly before you use it in your amp. I wasn't willing to leave the adapter to "It's probably OK", and it only took a minute to test.

The column on the left are the 12AU7 pins, and on the right are the corresponding 6SN7 sockets.

Good luck with it, and I strongly suspect you'll soon be a member of the 90% Club (who prefer the 6SN7 over the 12AU7 in the VTA). Please keep us posted.



 

Thanks again!, posted on March 17, 2023 at 10:57:10
JoshT
Audiophile

Posts: 6622
Location: Eastern Massachusetts
Joined: July 4, 2000
 Now to order a tube, most likely from Jim, Brent or the Tube Depot. Or an Inmate, lol. I've had good luck in the past with all four sources. :-)
___
"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."

 

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