[R-390] Heat Death

[Tisha Hayes]

Somewhere in my collection of lost postings I did a scholarly write-up on
the temperature differences of tube bulb temperatures with no shields,
shiny shields and IERC types. There was even some comparative literature on
MTBF and component aging with temperature increases.

Generally, get the heat out of the radio. The IERC shields did lower
envelope temperatures from 10-60 F. The same tube type would have
significant variations, depending upon the application and circuit design.
Not all 6BA6 tube applications were created equal. The IERC tube shields
had a disadvantage of transferring heat to the chassis as the bottom edge
of the shield would grab the tube collar. Bad shields acted like an
insulator and kept the heat from convective transfer to the air (hot air
rises and cools the tube).

Applying even a little bit of forced air to the chassis made a tremendous
difference. At first this caused me considerable headaches in recreating
the measurements as the slightest change in airflow around the receiver
would throw things off. I ended up painting flat black dots on the tube
envelopes and the chassis so I could use an optical pyrometer for
measurement. It would take more than a day for temperatures to finally
settle down as the entire chassis had to heat-soak.

Interestingly there is one application where I found the IERC tubes were a
very bad idea. On the Hammarlund SP-600, any of the JX models that have the
crystal deck, there is a tube on the backside of that module. It is
sideways mounted. IERC shields make that tube run almost hot enough to melt
solder (it will definitely melt the skin off of your finger).

I suggest using surplus computer fans running at half voltage to move air.
At half voltage you can find fans that are virtually silent. In my radio
console (6' x 3' x 7') I have about twenty fans running constantly and I
can still use a boom mic without the fans being audible. In that mass of
radios the R-390A runs at around 120 F on the chassis. Without the fans
things get hot in there real fast. Ironically it is a Cubic R3030 that
approaches nuclear reactor melt-down temperatures, the tube stuff is fairly
benign. The R-390A (and the 390) have nice "portals" to blow air in from
the sides of the receiver, and I have the fans mounted in the racks.

-- 
Ms. Tisha Hayes/ AA4HA

"In this denial of the right to participate in government, not merely the
degradation of woman and the perpetuation of a great injustice happens, but
the maiming and repudiation of one-half of the moral and intellectual power
of the government of the world." -- Fredrick Douglass, 1848