Nice work, brianh!
The biggest, most expensive tube I've ever seen was in the amplifier of a vibration test system at Lewis Research Center in Cleveland (now the Glenn Research Center). You basically had an amplifier and gigantic speaker with a metal cone--you could bolt test articles on it and shake the dickens out of them--that tube was probably the length of my forearm. BIG, BIG, tube. They had to have the tubes custom made by hand when they died, to the tune of about $15K ea.
These days it's hubby involved with tubes. The last couple weeks he's been drooling over Fender tube amps on the 'bay.