Thanks Cal. I was at VTS last week. Ellen kindly tested about 14 of my 7308 Amperex JAN and USN tubes on one of their FOUR calibrated Hickok 539b&c's. Spent more than 30 minutes. And no charge.
She gave mU details on each. Allowing me to pick the most consistent internal measurements and match. And had a chat with Peter, the tube amp master.
My recently modded and ugraded SF Line 2 would sometimes freak out during warm up and go into mute. Neither Chris at Parts Connection /SF or I could figure out why and thought "maybe" a weak or borderline tube - since mine are all pulls from 1964. What I can say is that after the testing and matching, and putting two almost identically tested tubes (with the same internal measurements) in the most important position, the preamp never sounded so wonderful. Even the wife noticed.