Alan Ibbetson wrote:
The guy sent the open-heater 8877 back. Yes, the heater is open and yes, it
is the tube I sent him (date code and serial numbers match). So maybe the
valve really was broken in transit.
Electrons are produced when the filament wire is heated above 2200 ^o C.
Adding small
amounts of thorium to the tungsten in the filament wire *reduces this
temperature
substantially*, to about 1700 ^o C. This increases the efficiency of
electron production and
increases the life of the filament wire.
Unfortunately after many hours of operation the thorium moves around
in the tungsten,
eventually the molecules align in lines, gradually the tungsten becomes
very brittle.
This has for years caused it to be inadvisable to move a tungsten light
"bulb" that shows a
blackening (even ever so slight) between lamps. I believe the increase
in electron production
results from the fact that thorium is mildly radioactive.
Since thorium-232, has a half-life of about 14,050,000,000 years the
ability to increase the
electron production will not fade over the lifetime of vacuum tubes.
This has for many years made it imperative to handle "emergency spare"
broadcast
transmitting tubes extremely genitally, without any bumping, when it is
time to test and
re-degassing.
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