Inspired by a couple of threads at the moment about new and NOS tubes. What specifically separates the great old tubes from the ones being made today? I don't mean the reasons for the old ones being made better; more demand, military use requiring higher quality etc. Etc. What is it about the actual tubes that was better? Were they made with higher quality metals? Did they use materials that are outlawed now? Were they able to achieve greater vacuums? And as a follow up; what makes it so difficult / costly to produce valves to a similar standard today?