"Stephen J. Rush" wrote in message
news

I've actually seen _tubes_ repaired. They were big metal-ceramic
transmitting tubes, like 3CX15000Z. A little company in Louisiana (there
must be a lot of them, scattered about) cuts a dead tube open on a lathe
and installs a fresh cathode assembly, which includes a copper tube to
connect to the vacuum pump. They also have glassblower's lathes for
handling the old tubes, but I don't think any of those are still in
service.

The radio station I worked in my high school years had their transmitting
tubes rebuilt as well. The repair wasn't possible when lightening hit the
tower however, the tubes in the final had quarter sized holes melted
through the glass!