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

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)

On Sat, 19 Jan 2008, spamhog wrote:

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 at very thin layers, it is negligible. Look at house insulation. R-30
is 1-1/2 feet thick plus. A single pane window (single strength) is more
like R-1. Glass is a great insulator (compared to, say, copper) but at 1/8
inch thickness its almost not there. Vacuum tube glass is even thinner.

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.

You have to compare conduction, convection, and all the mechanisms.

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.... :-(

There may be some small effects, and it might be more tied to how many
times a tube is warmed up from cold, cooled off to cold than actual
temperature (in many applications, folks would turn them on and leave
everything running [eg. computer monitors, even today]).

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?

That black-body radiation works in both directions, not just absorption.
And, it may also be spectrum-dependent so that would have to be measured
with instruments, not our (human) eyes.

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:


Its good that you did some google searching, but on the whole I think you
are worrying too much about cooling. And, if you do manage to cool the
cathode, then emmission would surely suffer. As the ultimate wacky
suggestion, you could immerse the tubes in liquid air/nitrogen and really
keep them cool (cost a lot of money), but then I'll bet you couldn't
"light up the tubes" (with filament voltage) at all.

===== no change to below, included for reference and context =====

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)

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).

On Thu, 17 Jan 2008 04:27:06 -0800, 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! :-)

What I know is that the glass will pass a proportion of the IR energy
being generated by the outside surfaces of the plate. Depending on just
how great a percentage, you may get more heating of the glass from the
paint capturing the radiation from inside than you get cooling from the
paint re-radiating it to the outside.

What I don't know is what will actually be the case.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html