You can affect the tone of your amp by swapping out different brands of these tubes as well as different numbers in the 12a?7 family. The major difference between the 12ax7 and the other is the amount of gain or amplification that they create in the circuit.
Of all the Brands the Telefunken (Western Germany) is the most valuable and best sounding it was used in highend preamps and tuners back in the 50’s and 60’s most of the time when you get an old HH Scott, Fisher or Marantz (just to name a few) the preamps would use the Telefunken 12a?7 tubes. Other high quality tubes manufactures of these style tubes are Mullard (Holland), Amperex Bugle Boys (England) and Sylvania and TungSol (USA) and the lesser quality manufactures were RCA, GE, Sevetlana, Ruby, JJ and Tesla.
The following chart shows the different amount of gain for each of the tubes:
|12AX7||(gain of 100)|
|12AT7||(gain of 60)|
|12AY7||(gain of 45)|
|12AU7||(gain of 19)|
So what does this mean? – If your device calls for a 12ay7 and you replace it with a 12at7 you will get more distortion or your device will be louder, the reverse is true if you replace a 12ax7 (gain of 100) with a 12ay7 (gain of 45) your device will be about 55% quieter
Here are a few datasheets
Some Photos of some Telefunken 12a?7 tubes