afcsman wrote:
spamhog wrote:
Dull, black, heat resistant paints
have been used to help cool engines for ages.

It is not done to cool the engine, it is done to make the
engine look cool.

It would be cool (literally) if one could
spray and heat-cure unshielded tubes
and improve their heat-shedding

Is there any indication that such paints,
or some vacuum-tube specific types,
would help keeping tubes cool
by improving heat radiation?

I'd love some factual info, if it exists, or educated guesses,
rather than uninformed blind guesses,
as I am awfully good at doing uninformed blind guesses already! :-)

One might think that the metal shields would "catch" the radiated heat
after it has left the tube. I feel that the black paint would act as a
heat insulator, preventing efficient transfer of the filament heat to
the outside.

Empirical evidence that most everyone has witnessed, shows
that black objects absorb light from the sun, and get hot.

So, that is exactly what you should expect to happen with
painting a tube's envelope black. The paint will absorb
the radiant heat from the plate, and conduct it to the
glass envelope. The glass envelope will then get much
hotter than it would have if the radiant heat had been
allowed to escape through the glass and radiate out into
space.

The black EMC tube shields cool the tube envelope by conducting
the glass's heat to the metal shield. This is done not to
make an unshielded tube cooler, but rather to make a tube that
must be shielded less hot than it would be in a conventional
shield that lacks the heat conducting structure.

Unless you can put a thermocouple in the tube, it will be
hard to know how hot it gets Inside.

It is a hard vacuum, vacuum doesn't get hot or cold. You could
measure the temperature of some of the tube's elements, but why
would you care if they get hot? As long as they don't get hotter
then the yellow heat they were heated to when they were evacuated,
there is nothing to be concerned about.

(Yes, it can be done, despite the
"how to build a triode" nonsense. Tubes were fabricated and evacuated
with mercury pumps by amateurs in the 19 'teens and 20's).

True, but I would bet that you can't do it! Building a triode
requires a wide variety of knowledge and capabilities. The French
guy that did it in one of the videos built every piece of equipment
that he used in making the triodes, and successfully built a nice
little hard sealed glass triode too. It was impressive, whether or
not it impressed you.


And will it make any difference? Most equipment was designed to operate
over a rather wide range of ambient temperature. Heat dispersion might
be important with power tubes (rectifiers, audio/rf amplifiers), but
most of that is due to the power inefficiencies of operating the
tube,(see the red or white-hot plates!), not from mere filament heat. In
that case fans or liquid cooling would be a better alternative. Try
painting a 3-500 tube black and fire it up!

It would melt the pyrex glass envelope, particularly around the already
highly stressed filament pins.

-Chuck