yng i don't care who told you this. it is utterly false.

the internal geometries and chemistries are different between the au, at, ax, ay, etc. the gain is different as is the torque [12ax7 have the most gain or hosepower; 12au7 have the least horsepower but much more torque] high torque means low plate impedance. it depends on the space between cathode and grid, grid and plate, overall distance between cathode and plate.

they are not sorted from one production run. maybe the early chinese ones were, but thats another story.

i love the internet and all the experts who post rubbish they heard.