Our British 'V' force were full of thermionic valves that saw active service over the Third Reich - not to mention the good old recently retired Shackleton - and there was no way that most of their kit would or even could ever be upgraded to solid state. There are certain advantages to using thermionic valves, especially in military applications [as well as top-of-the-range domestic audio systems!], and immunity to electronic pulses generated by nuclear detonations is only one of them. Plenty of the remaining 'Second World' military aircraft date back to the sixties, so I'm sure that a lot of thermionic avionic equipment remains in active service.
When I was an apprentice we learned lots of complicated thermionic theory that no-one bothers with very much today, right up to the heptode - try reproducing that fabled contraption in a solid state circuit! At the same time we were taught 'new' transistor circuitry that would be considered very primitive by today's electronic wizards. It impressed us though and we were promised greater reliability- a promise that was certainly delivered in no uncertain terms. The disappearance of the cosy glow from the valve heaters made trouble-shooting a lot more difficult though...
Many of our tutors had their doubts about the new technology and I recall old 'Wally' Parkhouse refusing to have anything to do with the large power transistor that replaced a triode switching valve in his beloved "Wide Speed Range" generating system. Wally would call another instructor into the class at any time his meanderings around the circuit encountered the dreaded 'transistor thingummajig'.
Last edited by Blacksheep; 6th April 2004 at 02:20.