The U.S. Military often had 50 or 100 hour burn in on certain tubes to stabilize them. Any tube is going to have characteristics changes as it ages. Yes, correct, you will run in circles trying to match tubes and still never really succeed. There was quite a discussion on "Matching" tubes I read. Some sellers only match brands, some match date codes, and some match who knows what. I remember rebuilding an old Minschel organ as a Mothers Day surprise gift to my mother. My friend was an organ repairman and he saw my pile of new tubes for it. He told me don't put new tubes in, but put used ones in. If I had put new tubes in, he told me it was very likely he would have to retune it a couple of times, due to characteristics changing as they age. I installed used tubes, and the tuning stayed pretty accurate until she passed away, when I sold it.

Cheers for doing that video Andy, that subject has been on my mind a lot regarding “matched” pairs and how important it is. Your experiments / tests have proved it seems not? Looking forward to the sound tests, bung a bit of dub on! : ))

My all "matched" tubes after few hour of playing are now not matched:)