Why aren't new vacuum tubes as good as old ones?

There is also a Japanese company - Takatsuki - that makes a few tubes. . . notable the 300b. Very expensive, but also very good.

Charles. Which Psvane WE Replicas have you tried?

What is the definition of "good"? Sonic quality? Durability? Given the longevitiy of some tubes (300b comes to mind) and unless a tube fails "early," it will be hard to prove that new tubes are predictably shorter-lived compared to older ones. Or am I wrong?