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