I remember reading about very substantial increases in lifetime for
tubes shrouded in black, heat-radiating shields.

I have an early series R-390A, I don't care about keeping it pristine,
and I want to introduce all the official service mods, + a few more
aimed at improving reliability.

Its tube shields are all unpainted. I am tempted to paint them black,
using engine type heat resistant paint, but there are some unknowns.
___________________________________________

- Are "proper" black shields so due to black-anodization treatment or a
paint layer?
Are they blackened BOTH inside and outside?

- Is engine paint appropriate?
(Most types are said to improve heat radiation).

- Should one paint shields both outside and inside sauf the electrical
contact area?
(I could e.g theorize that on the inside all transmission is by direct
heat transfer through glass-metal and air-metal contact, not going to
improve with a paint layer).
Are "proper" shields blackened both inside and outside?

- Would the same reasoning hold for at least the _top_ dustcover of a
radio?
___________________________________________

To do a proper job I should first check for temperature changes at the
tube surface in the bare/bare, bare inside / painted outside, and
painted/painted cases, but I am sure there is some knowledge floating
around about this, enough to skip the experimental bit.

Hints, o brethren?

On 13 Sep 2006 03:38:00 -0700, SpamHog wrote in . com:
I remember reading about very substantial increases in lifetime for
tubes shrouded in black, heat-radiating shields.

I have an early series R-390A, I don't care about keeping it pristine,
and I want to introduce all the official service mods, + a few more
aimed at improving reliability.

Its tube shields are all unpainted. I am tempted to paint them black,
using engine type heat resistant paint, but there are some unknowns.
___________________________________________

- Are "proper" black shields so due to black-anodization treatment or a
paint layer?
Are they blackened BOTH inside and outside?

- Is engine paint appropriate?
(Most types are said to improve heat radiation).

- Should one paint shields both outside and inside sauf the electrical
contact area?
(I could e.g theorize that on the inside all transmission is by direct
heat transfer through glass-metal and air-metal contact, not going to
improve with a paint layer).
Are "proper" shields blackened both inside and outside?

- Would the same reasoning hold for at least the _top_ dustcover of a
radio?
___________________________________________

To do a proper job I should first check for temperature changes at the
tube surface in the bare/bare, bare inside / painted outside, and
painted/painted cases, but I am sure there is some knowledge floating
around about this, enough to skip the experimental bit.

Hints, o brethren?

I _think_ I remember that the black is from anodizing, but I Could Be
Wrong.

The best tube shields are the IERC shields with the inner contact
fingers to maximize heat transmission from the envelope to the shield.

I Seem To Recall that The Fine Manual for the 390 and the 390A says to
run with the dustcovers off where possible. Yep, I was right. In
The Field and Depot Maintenance Technical Manual, TM_11-5820-358-35, I
find this:

4. Checking Unpacked Equipment
...
e. Remove the top and bottom dust covers by
removing the 16 screws (TM 11-5820-358-10) and
lockwashers that secure the covers to the main frame.
f. Inspect the subchassis on the upper and lower
decks of the receiver for' loose connectors, loose tube
shields, and broken tubes as follows:
...
(2) If the receiver is to be used in a fixed
installation, remove the shields from all
tubes (fig. 2 and 3) except V201 through
V206, V505, and V701.
(3) Unless extremely dusty conditions are
expected, do not replace the dust covers.
(4) Do not replace the dust covers if the
receiver is to be installed in Cabinet CY-
979/URR or Cabinet CY-917/URR.

So you may be able to save a fair amount on tube shields. A fan to
cool the rig down also is A Very Good Idea. My humble opinion is that
the bottom side of the 390A needs the fan more than the top does, as
the heat tends to rise and get trapped underneath.

--
Mike Andrews, W5EGO

Tired old sysadmin

Although the finned heat shields are excellent at moving heat away from a
particular tube, they do not remove the heat from the cabinet. Heat still
remains in the cabinet. The best long term solution is to remove the heat
from the cabinet. Often, this involves a fan running at reduced speed (for
the sake of noise). Some of the modern small computer fans would do
wonders.

That may require cutting holes in cabinets. Some people are not willing to
do that. Little things can help. Reducing the line voltage fron 122 volts
to 110 or 115 will reduce heat a bit.. Even a cabinet that encourages
natural convection flow will help.

Regarding the tube shields themselves, shiny material causes the heat to be
reflected back to the tube. So, black inside and outside can help - even if
painted. Having the black finned material between the shield and the tube
can help.

You can purchase an infrared heat sensor and measure the temperature of the
tubes. Also measuring the temperature of the case will tell you if you are
making progress.

I have found that reducing the internal temperature of equipment that tends
to run warm has greatly increased the reliability and reduced tube
replacement to a minimum.

73, Colin K7FM

COLIN LAMB wrote:
Although the finned heat shields are excellent at moving heat away from a
particular tube, they do not remove the heat from the cabinet. Heat still
remains in the cabinet. The best long term solution is to remove the heat
from the cabinet. Often, this involves a fan running at reduced speed (for
the sake of noise). Some of the modern small computer fans would do
wonders.

That may require cutting holes in cabinets. Some people are not willing to
do that. Little things can help. Reducing the line voltage fron 122 volts
to 110 or 115 will reduce heat a bit.. Even a cabinet that encourages
natural convection flow will help.

Regarding the tube shields themselves, shiny material causes the heat to be
reflected back to the tube. So, black inside and outside can help - even if
painted. Having the black finned material between the shield and the tube
can help.

You can purchase an infrared heat sensor and measure the temperature of the
tubes. Also measuring the temperature of the case will tell you if you are
making progress.

I have found that reducing the internal temperature of equipment that tends
to run warm has greatly increased the reliability and reduced tube
replacement to a minimum.

73, Colin K7FM

The A number 1 thing to do to reduce the heat in a R390/390A is to turn off the
ovens.

-Chuck

Often, this involves a fan running at reduced speed

Try wiring two "whisper" fans in series. Even quieter than a whisper :-)

Common sense tells me that anything which moves air over the chassis will be
much more effective than some marginal change in the covering of tube
shields.

Phil Nelson

Thank you guys!

Colin:
Heat still remains in the cabinet.

True.I find rather amazing how many old design types, esp. for consumer
tube gear, were oblivious to heat. I think ppl had a "machinery" or
"engine" paradigm in mind. If stove and charcoal iron are hot, why not
a radio?

WRT boatanchors, I always tried to improve ventilation, with a
suspicion that in so doing I might move frequency-defining parts off
the expected temperature range, but the '390 seems to me quite well
temperature compensated. When Collins folks were designing this, bitter
cold in Korea must have been a fresh memory.

Right now that radio is coverless, perched mid-air on a rack-on-wheels.
I run a room fan aimed at it, and the difference in temperature is very
remarkable. The previous owner, I2FZX (sk) used it for decades with
covers on, sitting almost directly on a surface, with an HRO-500 right
on top, and never had problems with it. Maybe the HRO did.

Longer term, I plan to put it in a desktop wooden cabinet, stacked with
some SSB/synchro-AM device and a speaker (and a thermometer??), with
plenty of forced airflow. I am considering flowing air from the bottom
- out the lower side holes - into the upper side holes - out the top -
sucked out the back by a bank of slowed-down, underfed 120mm fans. I
also want to do the ss-rectifier mod, and introduce a NTC resistor for
inrush current limiting, both of which will cut heat.
_____

Back to the original question, I'll do some experiments with engine
paint and a meat thermometer. If it's good for telling if meat is
cooked, it may help avoid cooking tubes. :-)

SpamHog wrote:
I remember reading about very substantial increases in lifetime for
tubes shrouded in black, heat-radiating shields.

That may be true for tubes that run really hot, but most of the tubes
in a R-390A are running at modest power levels, except maybe for the
rectifier tubes and the audio output tubes. They're all running at
well under 100 degrees C, cool enough to touch. That's many many
multiples of 10 degrees C away from their limit.

So I wouldnt worry about it.




I have an early series R-390A, I don't care about keeping it pristine,
and I want to introduce all the official service mods, + a few more
aimed at improving reliability.

Its tube shields are all unpainted. I am tempted to paint them black,
using engine type heat resistant paint, but there are some unknowns.
___________________________________________

- Are "proper" black shields so due to black-anodization treatment or a
paint layer?
Are they blackened BOTH inside and outside?

- Is engine paint appropriate?
(Most types are said to improve heat radiation).

- Should one paint shields both outside and inside sauf the electrical
contact area?
(I could e.g theorize that on the inside all transmission is by direct
heat transfer through glass-metal and air-metal contact, not going to
improve with a paint layer).
Are "proper" shields blackened both inside and outside?

- Would the same reasoning hold for at least the _top_ dustcover of a
radio?
___________________________________________

To do a proper job I should first check for temperature changes at the
tube surface in the bare/bare, bare inside / painted outside, and
painted/painted cases, but I am sure there is some knowledge floating
around about this, enough to skip the experimental bit.

Hints, o brethren?