Electro-Harmonix and Sovtex tubes

I'm a tube newbee so please forgive me if my questions are somewhat elementary.

First of all in a pre-amp what exactly do the 12at7 and the 12ax7 tubes do? I know that each type of tube has a particular function, but not exactly sure what the function of each is.

Second question, I understand that both E-H and Sovtex are both owned by the New Sensor Corp but what wondering from a sonic perspective what are differences between the E-H and Sovtex tubes (12at7 and 12ax7). At present I'm running NOS Sylvanias or NOS RCA 12at7 and Sovtex 12ax7 in my amp (Jolida SJ-502A) but down the line I thinking of getting away from NOS and going to current production stock simply because of availability (E-H/Sovtex tubes seem very available at the local music shops where I live).

Bottom line, I trying to educate myself on tubes (and it can get a little confusing).

Anyway, appreciate any comments.