One of the problems with any kind test equipment is doing some real world calibration and I tried a number of different ways of doing this with my recently constructed somewhat modified Jack Price tube matcher.
Ultimately what I figured out was to take a trio of tubes-6V6, 6L6GC, and 6550. Warm them up until they're stable, say, fifteen or twenty minutes or until the current gets as stable as it's ever going to be, and then get out your leather gloves and switch them to each socket in turn without touching the control settings. I figured that anything under 0.5 ma spread was all good and so it was.
These are now carefully stashed reference tubes, and I'm sitting on my string of 1 ohm resistors from the same batch.
One thing that did occur to me was to figure out a way to incorporate a trimmer into each tube cathode circuit for really tight calibration.
Film at 11.