Both the points you mentioned. It's also possible that some material is not available today. The revitalised WE 300B production made mention in their ads that they still had some thoriated tungsten cathode material left from their previous production, and this was important. When it's gone, what then? The skills required to make great valves are quite an art. But surely, it comes down to demand, cost of production and estimated return on investment.
If you have a look at on-line visual tours through tube plants, you will see a lot of old machines. Maybe some of these are not even made anymore.
Another point to consider - there are some seriously dangerous chemicals required in making valves and the overhead in protecting the environment makes the cost much higher these days than in times gone by, when the regulations weren't as stiff.
Regards,
If you have a look at on-line visual tours through tube plants, you will see a lot of old machines. Maybe some of these are not even made anymore.
Another point to consider - there are some seriously dangerous chemicals required in making valves and the overhead in protecting the environment makes the cost much higher these days than in times gone by, when the regulations weren't as stiff.
Regards,