Scott Dorsey wrote:
spamhog wrote:
On Jan 17, 12:48=A0pm, Chuck Harris
No, but don't let that stop you from trying it anyway.

-Chuck
:-) What keeps me is the fact that I have no idea how I could measure
the core temperature in a tube!

With an infrared thermometer or optical pyrometer. Auto parts stores
should have a model in the $20 range.

The hand held infrared thermometers will end up measuring the
temperature of the glass, not the elements inside.

An optical pyrometer would be the way to measure the filament's
temperature (quite crudely), but they are expensive.

Is heater current temperature-dependent enough as to provide a proxy?

No, because the plate temperature is very different than the cathode
temperature, and the plate temperature is what you worry about.

The heater temperature should be somewhat dependent on the plate temperature,
half of the plate's radiation goes into the center of the tube, which is
where the heater is. The heater temperature should vary slightly with
plate temperature. A sensitive bridge might be able to measure it, but
it would be down in the noise.

The plate of a tube doesn't wear out, so it's temperature isn't
by itself important. When the tube is evacuated, the plate is
induction heated to a nice red/yellow temperature. This is done
to remove any absorbed gases and other contaminants. As long as
the plate is never heated above this temperature, it won't release
more gas.

The big reason to worry about the plate getting too hot is the
heat it radiates will heat the glass envelope, and may cause
it to crack, or melt.

-Chuck