Detlof: I am pretty sure that special test amps are needed in order to measure noise. Maybe there is a way (with additional equipment) to test them while running in your amp, but this is just a guess? Most testers just measure for GM and MU (think that's it, haven't used one for over 25 years) and may have a feature that will allow you to drop the current down approx. 10% while they are still in test mode in order to see how much the first reading drops. Generally the greater the drop, the less life left in the tube, but there may be exceptions to this. I am far from an expert and just followed the test instrument's instruction manual when I used to test tubes for my guitar amps. I purchased my Hi-fi tubes new (as needed) at this time and never bothered with it, but then again I had never heard of NOS tubes 25 years ago. LOL. Speaking of 300B's (I am a fan of the late production Svetlana's from Conus Audio in Canada) there is talk of a new TJ Meshplate at AA that looks very interesting.
How to test those NOS tubes
After many decades of audiophilia nervosa und some years of tube rolling, I've finally bought myself a nice WE Tube Checker to try to measure and match those critters myself. Since I'm an absolute greenhorn in this game, what are the main parameters I must look out for? How do I test for noise for example ? Is there a site, which helps newbees like me? Are there books I should be on the look out for? Could anybody tell me the basics? Any help is greatly appreciated. Thanks in advance!