a 12ax7 tube should work in a position where a 12ax7 came out of
given the circuit was working before

I must have gotten a bad tube, but I was able to return it. The one I bought was a 12AX7R but the one that is in it is a 12AX7A. The guy at the music store was convinced that the last letter in the name references the country of production. Is he right?

possibly
...
err but not the way I know it

some brands may use the extra letters for that

12 volts with heater warm up control and a ceramic base ... 7 elements
http://www.duncanamps.com/technical/tubenumber.html
http://thetubestore.com/tubeinfo.html
http://thetubestore.com/12ax70ectyp.html

I thought the extra letters were to indicate special features like
higher gain ... lower noise ... things like that
or could be just for design revisions

could be country
could be manufacturer as in Electro-Harmonix - 12AX7-EH
12AX7A does seem to come from China

but it can all get confusing
from one of the above sites
" For instance, what is a 12AX7 in the U.S. is an ECC83 in Europe is also a 7025 to Fender owners because for a while Fender sold tubes labelled 7025 that were a low noise 12AX7. These tubes are all the same thing. "

not something that is easily generalised
a 12AX7A seems to be made in a few countries by a few manufacturers ... if you can believe what you find on the net

the one from the tube MP was 12AX7A and it has china stamped on the side. I think they sold me a bad tube.

The only good thing I ever got from China was the new monitor pic I posted over [[url=http://[/URL]="http://recording.or…"]here[/]="http://recording.or…"]here[/].

(Sarcasm not included).

Did you check to see that the pins were free of corrosion and clean? A "bad" out of the box tube often just has some corrosion on it. Take a wire brush and go to town.

Also, when you removed the tube from the socket, did you have to pull hard? Is it possible you broke one of the legs on the tube socket?

FWIW, the GT line of tubes has letter designators at the end of their AX and AT series tubes which indicate the manufacturer and/or origin. For example, their GT Gold 12AX7M is a Mullard (M for Mullard) where the 12AX7R or 12AX7S is a Sovtek.

Most GT tubes are tubes which are from other manufacturers that have been test and then rebranded. They do (IIRC) make some of their own as well, but I'm not sure which ones (again, IIRC, mostly power amp tubes).

I've personally found that, regardless of country of origin or brand, you can find a good and a bad tube from just about anyone.

Cheers-
J