[Amps] 3-500Z plate durability

[ka1xo at juno.com]

In light of the observation of internal parts becoming so hot they glow a dull red in glass envelope tubes, and the ensuing call for comments thereon, please allow a reflection from the last thirty five years of broadcast engineering (and the like):

Before AM BC transmitters began using ceramic PA's there were mostly glass envelope tubes in use, and most BC transmitter bays had a neat window facing the finals so the operator could see the lit filaments.

I've worked at stations in the 1,000, 5,000, 15,000, 50,000 and 1.2MW  classes, and have seen glass envelope tube service in all but the 50KW and 1.2MW venues. The stations have used everything from 250TH's on through 4-1000A's. Not all of these stations were well maintained, meaning the output transmission lines and tower tuning sections were deteriorated from environmental exposure, lightning and static discharge and from internal heat. By and far, the stations with tuning and transmission line problems seemed to have tubes where the anodes glowed red, so much more so than well-maintained properties that even now I wonder if there's a connection. 

Transmitters whose tube anodes were red hot needed replacement more often. Additionally, replacing output finals in a BC transmitter means dipping the plate and that changes the output tuning. Very often, my supervisors would forego the extended tuning procedure because I felt they knew how fragile the system was, and they often waited until Proof-Of-Performance to line things up correctly.

So to supply one possible answer, which doesn't really answer anything, tubes whose internal parts glow red hot work just as well as those who don't. The factor to be reckoned with is Mean Time Between Failure.

Respectfully,

Hal Mandel
KA1XO/2
Secaucus, NJ