Inherent Difference - 12AU7 and a 12AT7

EAT DOES make the 12AX7 tube, but it is referred to by the European moniker: ECC803S, which was the best of the best, for that tube family(http://www.gcaudio.com/cgi-bin/store/showProduct.cgi?id=501). The 12AX7, and the 6DJ8/6922(ECC88), are the two most popular 12 volt, 9 pin miniatures(most commonly used), in the industry right now. I'm certain EAT took that into consideration, when deciding what to manufacture.

Sorry,I meant, why don't they make a 12AU7 or a 12AT7...

Same pinout, but the gain is different. A 12AU7 only has a gain of 17 or so, a 12AT7 has a gain of 60. The 12AT7 was originally designed as a VHF oscillator or mixer tube, but it can be used in audio circuits.

The 12AT7 has more gain and will sound louder. Great if you want to beef up output from a DAC or preamp that normally uses 12AU7 tubes.

Because the current draw of a 12AT7 is similar to the 12AU7, in most applications a 12AT7 it can be substituted for a 12AU7 and the primary considerations are more gain (louder sound) and the perceived quality of the sound.

I used a 12AT7 in my Audio Note CD player in place of a 12AU7 and got wonderful sound from a particular Mullard that exceeded other 12AU7s that I had tried.

However, you cannot substitute a 12AX7 for either because of a huge difference in current.