N_Cook wrote:
As far as heaters are concerned. I went to a lecture by Tony Sale of
Colossus rebuild fame
http://www.codesandciphers.org.uk/lorenz/mk2.htm
About 2,500 valves in this computer.
They power up Colossus each day via a motorised variac.

How many valve failures a year, due to failed heaters in those 2,500, would
you expect ?
Please reply here

Answer tomorrow

In my field (audio amp repairs) actual valve heater failures are very
rare - Occasionally you see a case where the contact between valve pin
and internal riser is intermittent. I wouldnt like to guess at Colossus,
maybe one or two, possibly none. Of course valves ain't what they use
to be!

Ron