On Thu, 17 Jan 2008, spamhog wrote:

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

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! :-)


I'm not sure that a layer of black paint (as in exploiting what physicists
call "black body radiation" [or rate of heat transfer is proportional to
wavelenght raised to a power greater than one, and I don't remember the
power]) of is going to help you very much. Yes, mirrors, white surface,
black surface reflect, respectively, high, medium, or low amounts of
impinging radiation but at some point down the time scale, it will all
come to equilibrium anyway.

Several additional thoughts:

1. The layer of paint probably won't act as much of an insulator (silicon
grease, an insulator at high thicknesses, is used in thin layers between
computer CPUs and heatsinks and nobody gets worries about this).

2. A question about heat dissipation would have to involve knowing that
most heat is dissipated (from the plate) as infra red (unless the tube
plate is warm enough to start glowing red). Black paint would have to be
examined in terms of its spectral absorbtion as a function of visible-IR
wavelengths and compared with how well glass (which, IIRC, passes IR but
not UV) passes a given quantity of heat at the same operating
temperature. The passband and transmission spectrum may also be dependent
on any doping (dyes with bandpass absorptions, etc) but I certainly
recall no writings about this in terms of tube cooling.

3. Some tube sheilds were shiny, some were blackened. Good question as to
whether the difference helped or hurt temperature, but some tube sheilds
were advertised as helping with heat transfer (had some kind of slots).
However, all of the metal enveloped tubes had a dull black surface and
there may have been a minor component of contribution to surface cooling
through black body radiation, or it was simply the cheapest surface to
make.

4. Personally, an opaque tube envelope means I can't tell by looking at
the tube if the filaments are lighting up when I turn on the power.

Its not clear to me that you need to worry much about running tubes at a
lower bulb temperature since glass (and even the metal covered glass
tubes) won't melt until you get a way much quite a bit hotter than they
usually run. If you are worried about heat causing a variety of
accellerations of temperature-based aging processes in other components
(transformers, capacitors, etc), then put a small fan somewhere to draw
out the heat or blow in cooler air. If you are thinking about pushing
tubes beyond spec limits, then I'd suggest just not doing that (or, to
get more power, or whatever, put more tubes in parallel or
use bigger tubes, but that did not seem to be part of yor goal).