Do small preamp tubes 12ax7 need to be matched?

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.