[galyons]

I have to laugh at some of the responses.  First off one matches a preamp tube from each half of the tube (assuming it has two sides) for output, then for mutual conductance.

Output is a factor of plate voltage. Transconductance, (mutual conductance),  is the ratio of  current change at output to voltage change at input.  Lest one forgets the genesis of this discussion was the statement "More to the point, the 12AX7 is a "Preamp" tube and plate voltage for the most part is a non sequitur."

LOL!  When you find yourself in a hole,  it is amazing how many folks won't consider to simply stop digging!

Cheers,
Geary

[JoshK]

No need to get smug.  Jim might not understand load lines but he is arguing a somewhat valid point that a good tube (linear as I'd say) is a good tube.   If you want to argue that circuit variations (including plate voltage) matter, which I wholly agree with, there are better ways to do so, else you sound like you came just to show off how much you know.

[galyons]

No need to get smug. 

Not trying to be smug.  Really just trying to get past the inaccuracy of the gross generalization. How a tube sounds, beyond macro characteristics, is dependent on its role in the circuit, the circuit itself and how the tube is driven.  Voltage is a critical factor to the "sound" of all amplifying devices, tubes, transistors and opamps.

I am hardly an expert and  "showing off" is not on the agenda. 

Cheers,
Geary

[JoshK]

Agree wholly with your last post, but some of the ones (not only from you) came across as crass. 

[Jon L]

A linear tube will be better in any circuit

And Siemens "triple mica" E83CC certainly is one of my favorites in any circuit 

[SteveFord]

Ach, you're tempting a weak man!
I'm with Josh.  I joined Audio Circle because it's a site where the members treat each other decently.
It's only grown men and women playing with their little radios (anybody else ever hear that putdown from their spouse?).

P.S.
What I know about plate voltage would fill a very small thimble.

[JakeJ]

Gentlemen,

Please remove the boxing gloves and return to a civil discourse in this thread.  It is supposed to be a discussion about the sound quality of various 12AX7 types (which I have experienced) and yes, it can be the circuit, or the tube, or both.  If you don't like generalizations then please feel free to educate us as to the finer points of what a tube will do when one or more circuit parameters are changed.  And do it with good manners, please.

MediumJim, I'm sorry but I must disagree.  Proper tube matching, whether output tube to output tube or a preamp tube (or tubes), is accomplished on a tube curve tracer and is done at several points on the curves generated to assure truly balanced and matched performance in any circuit it will be used in.  Simply checking the balance of the two halves and transconductance measurements are not enough to assure excellent performance.  This especially true in a phono stage.

See?  I stated that I disagreed and then I went on to say why.  No snippy remarks.

OK, now you guys try it.

[medium jim]

JakeJ:

Agreed, you can put a preamp tube on a maxi-matcher. But, not truly necessary.

I would think most audiophiles have separate preamps/amps as such the preamp tube isn't subjected to plate voltages.  The ma is what is important in a preamp tube...the closer to spec the better, for a 12ax7 it is 1.1ma.  Most current production 12ax7's are closer to .8ma which drastically alters the gain and tone.

There are some current production 12ax7's that are in spec and will sound better regardless of the circuit...

Jim

[medium jim]

Let me put it this way, a great tube will not be rendered inferior because of a ill designed circuit, may not sound as good as it could, but a lesser tube will only sound worse. 

I go back to the premise that preamp tubes will change the landscape more than output tubes and one needs to know what they're looking for to achieve the best results.

As you probably know, the standard 12AX7 tube gives 1.1 to 1.2mA of plate current for 250V plate voltage and -2V grid.  But in real-life circuits preamp tubes are auto-biased with a cathode resistor.   I have to think that the use of the cathode resistor is what those who disagreed with my thoughts forgot about.  Or maybe this is what they were referring to with respect to the different circuits.


Jim

[Ericus Rex]

Hmmm.  I think maybe there is some confusion as to what exactly "plate voltage" means here.  Jim, a 12AX7 will be operated at roughly the same plate voltage regardless of whether it's in an amp or preamp...with variations as determined by the circuit designer, of course, but within the recommended range given in tube manuals.  The fact that it may be cathode biased (nearly always the case) is irrelevant.  The plate voltage is an operating point and that point must fall within the tube manufacturer's acceptable range.  To say "a preamp tube isn't subjected to plate voltages" is a bit like saying 'my car doesn't have an engine.'  The tube must have plate voltage in order to operate.  Anyone else see this as the cause of confusion?

[cheap-Jack]

Hi.

A linear tube will be better in any circuit unless one actually likes distortion or euphonic coloration. 

BINGO!

Let me elaborate more:- triode is always MORE linear than a pentode, period.
That's why the best sounding amps, brandname/DIYed regardless, always
use triodes ONLY!!.

FYI, my 2 DIYed tube phonestages used only triodes (50-year-young vintage Telefunken ECC83s). My tube power amps driver stages used only triodes & the 2 pairs of EL-34 pendtode O/P tubes are triode-strapped.

My ears tell me trodes sound so so so much better than any pentodes as triodes are more linear than pentodes.

c-J

[medium jim]

Cheap Jack said what I was trying to much better.  Maybe I was too simplistic.  The point I was trying to make was that preamp circuits are a fixed bias and amp circuits for the most part aren't.  As such the quality of the preamp tube becomes of greater importance.

Jim

[Quiet Earth]

I agree with the guys who said you can not generalize that if a tube sounds really good in one piece of gear, it will always sound good in other gear. It is better to say which piece of gear you are using first, and then state which tubes sound best to worse in it. The o.p. did exactly that, so give him an A+. 

For example; I'll bet those "distant" sounding RCAs sound fantastic in a different (more vivid?) sounding preamp, integrated, or guitar amp. Understanding that is mostly all that matters when it comes to grading the sound of tubes that share the same number.

Quality of the gear first, quality of the tube second usually works for me.

[medium jim]

I will concede that since most current production 12AX7's spec around .8mA which would give it a gain factor closer to 75 than 100, that many amp/preamp designers factor this in to their circuits.

That said, NOS tubes probably will have too much gain.  With that, maybe a lower gain tube, say a 5751 that only has a gain factor of 70 might be the ticket, or even a 12AT7 with a gain factor of 60.

BTW, the late SRV used 5751's in his guitar amps in lieu of 7025/12AX7's and it fattened up his tone.

Jim

[SteveFord]

I tried most of these tubes in a bunch of stuff: Jolida headphone amp, Jolida JD-9, Pacific somethingorother DAC Lite and that mercifully unique Aural Spills phono stage.
The basic sonic signature of each tube was pretty consistent throught the parade of low buck crud which is what I expected.
I needed new tubes for the JD9 so I wanted to see how the reviews stacked up with what I heard.
The one I read said go for the JJ Tesla ECC803S and that was good advice for my application.

It's quite likely that the old RCAs (or GEs or whatever) would sound totally different in a guitar amp or differently designed audio components.

Be that as it may, the wife somehow managed to plug up the bathtub drain(?) so I'm off to the hardware store.  How the hell can you totally plug up something like that?  A plunger won't touch it, I might have to blast it out.

[JoshK]

Hi.
BINGO!

Let me elaborate more:- triode is always MORE linear than a pentode, period.
That's why the best sounding amps, brandname/DIYed regardless, always
use triodes ONLY!!.

FYI, my 2 DIYed tube phonestages used only triodes (50-year-young vintage Telefunken ECC83s). My tube power amps driver stages used only triodes & the 2 pairs of EL-34 pendtode O/P tubes are triode-strapped.

My ears tell me trodes sound so so so much better than any pentodes as triodes are more linear than pentodes.

c-J

I wouldn't go that far.  Again it has to do with load lines.   Many common pentodes and triodes would possible qualify that remark, but I can think of at least a half dozen counter-examples off hand.   A C3M (Deutsche Post) pentode is more linear than a 12AU7, in pentode mode no less.   Of course, I am picking a superb pentode and a sh***y triode in this case, but it still suffices to break the generalization. 

There are plenty of great pentodes.   Have a look here for an accumulation of good data on some cool small signal pentodes.

http://www.pmillett.com/pentodes.htm

Then some of the best triodes are triode strapped pentodes (Russian 4P1L for example which arguably bests the better 45s and 2A3s...but shhh...I didn't say that, I am still collecting while they are cheap!).

[Quiet Earth]

I didn't know the 12AU7 was considered a sh***y triode...... 

I wonder why they sound so damn good in my DAC?  Must be the forest and not just two of the trees.

[JoshK]

Its always controversial, cause they are used over and over and over again in commercial audiophile gear.  I should have kept my mouth shut, cause ignorance is bliss and calling it out is never met kindly.   

To paraphrase Morgan Jones, 12AU7s have a wide variance in construction and measurements and all of them are shite.  Objectively, based on measurements, its easily the most non-linear of all tubes ever used in audio, but they [were] are cheap and readily available since they were made in mass quantity.   Compare linearity of the 6SN7 for example, polar opposites.  Not only is it not linear, the distortion is not the benign lower order 2nd and 3rd but the highly audible and fatiguing higher [and odd, i.e. not natural] harmonics. 

12AU7s are oft considered to be the euphonic tube.  When someone says something is syrupy or euphonic, I think of 12AU7. 

But tastes are unique and every like what they like, and who knows when your implementation is in your dac, it could be doing something novel to cancel distortion (there are some really clever techniques for this).    But I'd personally rather start with a linear tube, then rely on tricks, if I was doing the design.

[Quiet Earth]

So what you are saying is that if you heard a good implementation of a 12AU7 circuit, you would still dismiss it because you know better?  Yes, I would rather be blissful. 

[JoshK]

Well I have heard many 12AU7 circuits and have yet to like a single one.   I know there are super clever designers like Morgan Jones, John Broskie and Anatoliy who could design something brilliant with them but typically choose not to for reasons I stated above.  If you list your particular piece of gear we can see if we can find the particular implementation so we are not talking in generalities.