Your best bet for all the answers is a good reference book about tubes. I recommend "Tube Lore" by Sibley Ludwell.
tube tester question
I'm new to tubes..How can a tube tester be calibrated? (can a good audio repair shop do this?) Does a tube tester use tubes itself? Are tubes still current production items? Why are old tubes (NOS) better than new (why can't high quality new tubes be made?) Also what does matched tubes mean? Can one tube of a matched pair age differently?