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.
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- 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?