All this is very interesting.... There's a bunch of things I find
confusing.

1) Radiation vs conduction
On one hand it's obvious that a layer of dull, IR-black paint has a
thermal insulation effect. But the same may be said of ANYTHING
surrounding a tube. Even one of the "good" IERC shields must have an
insulating effect of sorts. Moreover, the copper "fingers" are metal-
shiny, obviously designed to sink heat by conduction, not by absorbing
radiation.

2) Terminal filament temperature
The issue here isn't avoiding failures as much as increasing tube
life. The heat issue in low power devices was much disregarded till
close to the end of the tube era, due to engineering, commercial, and
I believe psychological effects. In the end, microtubes used in the
NORAD systems reached 500k h MTBF, and even before that, repeater amps
in transatlantic underwater phone cables had already made major
advances in reliability. There is a DoD or Collins study on the
matter, claiming that IERC shields improved MTBF over unshielded
tubes, if I remember correctly. As in all stable thermal systems, once
equilibrium is reached the terminal temperature of cathode and
filament can't be independent of what happens at the tube surface. I
know how to scrounge up a Wiener bridge or a shunt and measure small
deltas in heater current - but where do I look up some ideas on how
current is related to temperature, whence what deltas to expect, at
least in order of magnitude? Without a theory, even a rough one, as
Popper pointed out, there's not much to test.... :-(

3) Cooling effect of going black-body in the IR range
Granted, if 90% of the exchange surface eg in an aircooled engine
faces . . . itself (think deep cooling fins facing each other) most
cooling must come from conduction to a moving ambient medium (aka
air). So blackening should not make much difference there. But what
about situation where there IS open space around a hot device?

Right now, I remember that I know a guy who makes heating systems,
including a successful line of radiation heaters:
http://www.sabiana.it/download_pubblici/catgen_en.pdf, see the 1st
product, called Duck-Strip. The name's an inside joke: they were
designed by a Mr. Anatrella - Italian for "cute duckling". The things
run on hot water. At a Volkswagen plant those radiators heat people
from a vertical distance of over 20m. From an economic standpoint I
am not sure it's totally in the company interest to maximize per-
surface-unit radiation, but I believe they took a look at paints. They
also used to have a gas-fired radiation heater running at a much
higher temperature than the water type.

I'll ask him... stay tuned. Moreover, some things I found by
googling words that came up in this thread:


overclocking & paints (uh...)
http://www.overclockers.com/tips684/

irrelevant but funny
http://members.optusnet.com.au/mcdjim/100_4062s.jpg
http://forums.bit-tech.net/showthrea...2a61& t=53048

read what Dampney writes on improving IR absorbtion with "visible and
IR" black paint
http://thurmalox.com/Upload/Products/Products28.pdf

teacher's guide to experiment :-)
http://www.ed.psu.edu/ci/Papers/STS/gac-3/in05.htm
(no word on measuring temperatures in unreachable recesses)