I'm convinced that a large chunk of the surviving round plate Tung Sol 6SN7s are B stock pieces that went onto a military surplus shelf to be used in times of major shortage. They were parts that tested low off the factory line or had some other issue. The A stock tubes that passed all tests went out into the field and powered equipment that was used during the war.
Of course some of these supplies will have remained after the fighting ended, but I think a lot of it was actually used up for its intended purpose. It's those rare caches of tubes you find over in Europe that are the really good scores for US made tubes, because they were A stock sent over there for the war effort that never actually got used. The stuff that stayed in the states was the backup supply.
If that's the case I'm lucky as most of my US tubes are of European stock.
The AVVT 300BSL-C37 Ennemoser design are also great tubes and the the Mesh plates even better.
The AVVT PX4M considered by some as the holly grail of PX4.
Alesha like Glenn is a genius, their designs often copied and used by others.
I see some traitors here going SS...........
