spamhog wrote:
On Jan 17, 12:48 pm, 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!


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

Yes, it is rather temperature dependent.

What I might do, is put two identical tubes with their filaments in series
into a bridge configuration with two identical resistors, and measure the
voltage between the junction of the two filaments, and at the junction of
the two resistors. Then paint the one tube, and leave the other unpainted.
If the paint causes any change in the tube's internal heat, it should cause
the bridge to move one way or the other. Tungsten's resistance rises as
its temperature rises, and lowers as its temperature lowers.


~ +12.6V(REG)
~ |
~ +---------+-----------+
~ | |
~ FIL-A R
~ | |
~ +--------DVM----------+
~ | |
~ FIL-B R
~ | |
~ +---------+-----------+
~ |
~ -+-
~ ///

The inside of the tube is a hard vacuum, so the only way heat will pass the
void is by conduction (through mica, and leads...), and direct radiation.
Direct radiation is where the lion's share of the heat goes. Since the filament
is surrounded by the plate, it should see as much radiant energy as the tube
radiates out through the glass.

-Chuck