Do small preamp tubes 12ax7 need to be matched?

I think for best performance, I would use a matched pair.

I think "matched pair" for small signal tubes is a misnomer. What, exactly is being matched? Transconductance and current figures are a poor indicator at best as to how any given small signal tube will perform in situ. Anyone with extensive tube testing and utilization experience would agree that sometimes even tubes that read NOS on a calibrated tester can be absolute duds sonically be it noise, microphony or other sonic issues. My strategy is usually to find a half-dozen tubes that all test good, and then listen to all of them to find a good sonic match for a useable pair. Generally, I find that of the half-dozen, at least two will be unacceptable for the given application (which doesn't preclude their use elsewhere in a circuit). Hence, the old saying of 'if it sounds good it is good', applies quite well to small signal tubes. Power tubes, of course, are an entirely different story.

-Richard

I have always been told that matching only matters for the power (output) tubes but I think it would depend on how the circuit is done. I think the Aesthetix are balanced right? Maybe it can make a difference in that case. See if you hear a difference!

I have been told that the small signal tubes do not need to be matched but that the power tubes do unless you can adjust the bias on your amp.

Chuck

Palasr is correct on all points.

I would only add that perhaps the bigger issue is whether to bother with (i.e., pay for) tubes that are "low-noise" in the context of preamps. While it obviously depends upon the preamp, my experience is that low noise is not a necessity for line stages, but that for phono stages, it is very important.

The only small signal tubes I closely match are ones used as long-tailed phase invertors. Most amps don't have any provision to adjust the AC balance of the driver stage. For that I use a VTV small signal tube tester. It also lets you listen to the tube so you can check for noise and microphonics. Very helpful if you need to screen alot of tubes.

I always use matched tubes. Many amps allow output tubes to be biased individually but I have never seen that for smaller tubes.

Note: Tubes like the 12AU7 are stereo tubes, i.e. they each have a pair of plates, grids and cathodes. I am unfamiliar with the circuitry of the Aesthetix but many of the preamps and phono amps I have built and modified I found some tubes where half of the tube is used for one channel, the other half of the tube for the other channel. So if you are really concerned about matching tubes (like I am), make sure that the tube is matched with itself when a single tube is used as amplification stage for both channels.

I buy from a reputable dealer who always sends me matched tubes at no extra cost.

Regards
Paul

How can you tell if the tubes ate matched?  If you have the same brand etc.. are they likely matched?

I had a Telefunken 12ax7 tube go out on me in the V4 (R) position of my Aesthetix Rhea, so I replaced the pair with stock Sovtek 12ax7WBs. Do I need to replace the Teles with a matched pair, or can I just pick up a single and throw it in.

You can also use 12AT7 (ECC801s) instead of 12AX7 , military Telefunken tubes (if they are not fake) are much better than Russian Sovtek or any other new tubes.

Ideally yes but you don’t need to be too OCD - with small triodes, often you’re trying to match 4 elements (2 triodes per tube) so it can be frustrating to try to find 4 within 10% of each other on the major parameters (mu, transconductance) - especially vintage tubes. And even up to a 20% difference sounds like a lot, but it can be just fine for many applications. If your elements are within 15% (max) of each other you’re generally doing quite well. I believe Upscale matches their "Platinum" grade to within 10%.

If you have an egregious mismatch, well over 25%, then that can start to cause very audible issues. I had a pair of Cifte 6189 silver plates where one tube was much stronger than the other, and it caused about a 1 dB L/R channel imbalance when used in the mu follower slots of my phono stage.

Once I also bought a trio (pair and a spare) of Brimar CV4004 (12ax7) from Parts Connexion, unaware that they were going to be COMPLETELY unmatched from an extremely variable stock. They were a complete mess, egregiously mismatched on triodes and tubes (variances or 40% and more), and sounded like an absolute mess as a phase splitter my tube amps. Fortunately the vast majority of dual-triode tubes that have been sold to me as "matched pairs", over the years, have been as advertised and sound symmetric when in an amp. Parts Connexion is the only shop/dealer I'll never buy tubes from again. 

Some of the old tubes were abused by gear, more on one side than the other. I’ve seen a number of Tung-Sol BG RP 6SN7 (the holy grail tube) where one triode measures like new and the other is practically dead (measuring 50% less than the strong triode). Those are no good, so watch for that :(

Just recently I purchased two early 1960’s E88CC Tubes and two Genalex Gold Lion E88CC Tubes, both sets were advertised as matched pairs.

I thought I was buying a safeguard tube with the Genalex to tie me over whilst I was awaiting the usable vintage valves to be discovered.
On testing both sets of valves to Vintage measured as good as new.
One Genalex would not produce a reading.

I personally like to have a matched measured valves to use.

Matched tubes will always give better sound.

You can "use" a 12AT7 to replace a 12AX7, in that the pin connections of the two tube types are identical, but that would be far from ideal.  The 12AT7 has less gain, a lower plate resistance, and a higher transconductance compared to a 12AX7.  In a phono stage, the difference in gain would be quite noticeable, even with high output cartridges, if one were to replace a 12AX7 at the phono input with a 12AT7.  But also the 12AT7 would function suboptimally in a circuit designed for a 12AX7, so there might also be noticeable distortion.  (I am qualifying my statements, because I know someone is going to claim that he used a 12AT7 in this fashion and it sounded wonderful.)

Mulveling mentions encountering 6SN7s where one triode tests like new and the other triode tests like its near death.  (The 6SN7 is a dual triode, with two completely independent triodes in a single glass envelope.) This can happen with a tube that has been pulled from a piece of equipment where only one of the two triodes (the dead one) was actually attached to the circuit.   Often equipment was built this way so that one could swap L and R channel tubes after the tubes seemed to need replacing, and thereby gain use of the unused section in the opposite channel.  

I am not a big believer in paying a premium for matched tubes, because tubes do not age in synchrony.  So after a few months of use, one of those two tubes may test quite differently from the other.

Lewm brings up a very good point when he states that tubes age at different rates. I have always wondered how tube matching makes a lot of sense when a) the tubes will age at different rates b) what are the parameters that one can say a tube is matched to another? c) your circuit is either driving a tube hard or it is not-- which results in varying the life and the point in (a) and lastly d) noise and distortion vary greatly by tube design and placement in the circuit.
Anyone here truly know what a tube tester is accurately testing for?

You can "use" a 12AT7 to replace a 12AX7, in that the pin connections of the two tube types are identical, but that would be far from ideal. The 12AT7 has less gain, a lower plate resistance, and a higher transconductance compared to a 12AX7. In a phono stage, the difference in gain would be quite noticeable, even with high output cartridges, if one were to replace a 12AX7 at the phono input with a 12AT7. But also the 12AT7 would function suboptimally in a circuit designed for a 12AX7, so there might also be noticeable distortion. (I am qualifying my statements, because I know someone is going to claim that he used a 12AT7 in this fashion and it sounded wonderful.)

I know it works perfectly and a person who replaced 12AX7 with 12AT7 in his new Mac preamp noticed huge upgrade in sound and (no problem with gain). They are in use ever since for more than one year for everyday listening sessions as far as I know. German military NOS telefunken are superior to any Russian tubes. My advice to OP is to stay with NOS Telefunken tubes (...ax7 or ...at7), matched pairs!  

"I know it works perfectly."  It just might in this ONE case you site.   The two tubes are not close to each other at all.  12at7 gain 60 12ax7  gain 100
Say they are run at 250 volts B+   12ax7  plate current  1.2 ma  12at7  10ma 
Plate resistance 12at7  10900 ohms 12ax7  62500 ohms.  Grid voltage 12at7 -12  volts 12ax7  -2  volts.  Etc...  Lewm is on the money these two tubes, short of the heater current and can be run on the same b+ , are not even close.

It would be advisable  to have a schematic and know the size of the transformers and how much current the power supply can handle to know if there could be a problem  with the parts.  There is some math to do to set the tubes where load lines are ideal.   

That being said I prefer the sound of 12at7 over  12ax7 in phono stages.  But would design it accordingly.   If the guy who has the Mac preamp actually  figured out where to run the 12 at7  properly  it could even sound  better.

Enjoy the ride
Tom

PS.     In regards to matched tubes ,over rated in my book.  Plus short  of having a load  line tester , tube testers are ballpark at best.  Also the op could use 2 different  types he has in on hand one on each side see how much difference he hears even with  two different  brands.

It might be that in the case Chakster cites, the tube is used as a cathode follower. Or a phase splitter.

Some circuit slots are going to be more tolerant of ax7 / at7 / au7 / ay7 type swaps than others. Just depends on the circuit. Cathode followers and mu followers tend to be pretty tolerant, though 12ax7 probably aren’t the best choice for driving a line of cables & SS amps.

The Herron phono supports at7 / ax7 swaps in 2 slots to change gain +/- 5dB or so. The Rogue phono is fine with at7 or au7 in the mu follower slots (similar gain difference). If you change the type in the RIAA / MM ax7 slots then you’re going to get something that sounds very much not as intended, and not accurate, though 5751 are typically a "close enough" sub for ax7.

I once accidentally put 5814 / au7 in the V1 / ax7 / phase inverter slots of tube monoblocks (Rogue Apollo) and it actually sounded pretty good, but certainly very different and colored before I noticed the reason. It was a pleasing type of coloration, actually.

If a type sub changes the sound a lot, that’s usually a coloration because the circuit isn’t designed for that tube type. One exception to this would be using a 12bh7 in place of the stock 12au7, which usually sounds a lot better (amp must be able to provide the extra heater current), because 12au7 sucks and the 12bh7 does what it does, just better.

12AU7s are to be avoided.  In my experience, any other equivalent tube (12BH7, 6CG7, 6FQ7, even 6SN7 if you can change the socket) will sound superior to a 12AU7, in a gain topology.  This does not apply to cathode follower or phase splitter use (in an amplifier; preamplifiers don't generally incorporate a phase splitter).  In those latter cases, the 12AU7 is "OK".
I am aware that the Herron allows optional use of 12AX7/12AT7, so that the gain can be modulated thereby.  I have wondered how Herron gets away with that without a noticeable difference in SQ, at least according to its many admirers. Are there switches that must be re-positioned, for one tube choice vs the other? 12AT7s can be operated on relatively low current, so that parameter can be chosen to be compatible with a 12AX7.  But I have always felt they sound better with a tad more current than can be tolerated by a 12AX7.  Just one man's opinion based on some experimentation.
Mulveling, By "phase inverter", do you refer to what is also called a phase splitter?  In that case, I can easily believe that the 12AU7 could sub for the 12AX7.  Although the operation of the 12AU7 might be improved by increasing plate current.

I care mostly about noise and microphonics. Channel variations are what a balance control is for. Phono stages and preamps in particular deserve the quietest tubes you can find. I would use only the tube type specified by the manufacturer. Euphonic distortion is just that, distortion.