lifespan transmitter driver amplifiers
 

Not sure how to put this but, Back when whole tv stations were tubed how frequently did the finals feeding the antennas need to be changed? I mean weren't many of them on the air 24/7 before the solid state age where some were even still used?*

Just stands to reason that by the nature of a thermionic device at those power levels it would always need swapping out.






* I'm starting a group about t.v. sign-offs etc and would appreciate any feedback on the subject...

Tube outputs are still in use.
Maybe someone with direct experience can comment on reliability.
Early stations did not stay on 24 hours a day.
Some stations install dual transmitters, so one can be off air if necessary, with a temporary 3 dB reduction in power.
High power solid state transmitters can be designed as many lower power modules, so failure of one or two is hardly noticeable.

By the way, reliability of electronic devices is often described by a "bathtub curve" having a component of infant mortality (decreasing with time), a component of constant random failure rate, and a component of wear-out (increasing with time).
Someone once explained to me that vacuum tubes start their life in the wear-out mode (due to constant depletion of the cathode).

http://en.wikipedia.org/wiki/Bathtub_curve

I recall stopping by the transmitter site of our local channel 7 outlet. At the time, they were running 2 very ancient RCA rigs...big black boxes labeled "Abel" & "Baker". "Baker" was semi-defunct & the engineer was deep inside it trying to get it running so they could take "Abel" off the air for work. He was carrying a bushel basket full of tubes around while he worked....IIRC LOTS of 6146s in the basket. I should've asked what the final was...probably a couple of 4CX10000s or something.