I recently went through testing of a lot of different 12at7. Here is what I found out.
If the tube is used in the circuit it’s designed for, there is very little sonic difference between various tubes in this family. The difference is possibly 1%. I tried NOS Telefunken and ShuGuang with practically the same results. I also tried them in multiple amps with the same results.
If you put 12at7 in voltage amplification stage of your amp, you will definitely here a difference, but the tube has not been designed to be used this way – thus, don’t do it; use 12ax7 or 12au7 or other members of the families (see here). My attempts to put it in the input stages of an amp resulted and significantly shrunken sound stage and very poor sound quality.
Since in a lot of cases 12at7 is used in current amplification circuits, it has very little to do with gain (mu), so gain here is not relevant and theoretically 12au7 & 12ax7 could be used in place of 12at7. However, 12at7 has a higher transconductance (gm) than 12au7 and thus better suited for current amplification circuits. Also, 12at7 has a higher plate current than 12ax7, so again better suited for current amplification circuits. 12AT7: gm = 0.0055A/V; 12AU7: gm = 0.0022A/V; 12AX7: gm = 0.0016A/V; 5751: gm = 0.0012A/V;
So in conclusion, if you use 12at7 for what it’s designed for, you have a huge choice of tubes that range from .99 cents for tubes like GE gray plates to $15 for telefunken (yes you can get it that cheap if you are patient on Ebay). The majority of tubes will be quite good and produce very little audible distortion and coloration. Try to choose the most transparent ones for current (e.g., driver, cathode follower) stages and control the sound with input stages.