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.
cleaneduphippy
Here's an interesting article on how tubes work:

http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm

I use the Electro-Harmonix 12AX7 and 12AT7 tubes and really like them.
I use the EH 6922 gold pins in my cj premier 16lsII pre-amp and like them very much
I don't have any experience with 12xxx tubes, but I own both the EH and Sovtek 6SN7s, and I can tell you that they are definitely not the same tube, either in construction or sound. In my Wright WLA12A the EHs are very superior to the Sovteks, and close enough to NOS tubes I've tried that I don't worry about whether the supply of NOS gets too expensive. The Sovteks sound pretty bad in comparison, grainy, constricted, no bass. If only Sovtek existed and not EH, I'd be hoarding NOS 6SN7s whenever I could.

David
Attached is a link to an e-bay seller who I have bought eh tubes from on several occasions. He is very reputable and trustworthy. I believe I have seen him on audiogon as well.

http://cgi.ebay.com/Electro-Harmonix-6922-pre-amp-tubes-Gold-Pin_W0QQitemZ150074152358QQihZ005QQcategoryZ14974QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Chuck