Electrically almost all are the same.
Physically there are often "no way" differences.
As a rule you can go by the EIA number on the tag. This usually tells
you who really made the tube. If you have a EIA of 343 its Zenith,
if 1101 Rauland div of zenith, 312 is Sylvania , 274 RCA etc.
Jap sets use either Jap or US tubes, usually the Jap jugs have no EIA #.
http://users.eastlink.ca/~mitchlane/EIA_Codes.html
Same mfg & vintage usually swaps. Beyond that best to query here
for advice.
73 Zeno
