[R-390] Alternative to IERC tube shields

[Tisha Hayes]

If it was a choice between using the evil silver shields (for mechanical
protection from shrapnel, or clumsy techs) and using no shield at all I
would rather have no shield but add forced air cooling.

A couple of surplus computer fans will do wonders to circulate air through
the chassis, also helping the transformer, chokes and resistors in their
efforts to dissipate heat. Just gotta remember to let convection work for
you and to force air in at the bottom, out through the top (as much as
practically possible).

The shiny silver shields with the metal spring and the bayonet style base
does provide a minimal amount of RF shielding between closely spaced tubes.
The mechanical connection to the chassis is definitely not finger-stock
grade RF proof but it does help some. The worst thing about those shields is
that they act sort of like a dewar flask to keep the hot tube envelope from
dissipating heat to the freely moving air.

Of course in the days of endless supply depots and every Allied Electronics,
Radio Shack or corner drug-store with a tube tester (and a supply) is long
gone. Other than very questionable Russian or Chinese new(er) manufactured
lots we are stuck trying to glean good tubes out of an ever diminishing
reserve. We have two forces working against us;

1. Evil audio-phools who think tubes are the shiznat and decide to mis-apply
tubes to different applications and look at the data-sheet and think the B+
values are the recommended minimum voltage.

2. Our own usage, albeit from a fairly small pool of folks (us) who restore
and use boatanchors or survivalist types who have decided that every solid
state device will go up in a big poof when NKorea NEMP's our country out of
spite. I guess you can make the distinction between an enthusiast and a
survivalist by how many hundreds of spare tubes you have and if they are
stored next to your 7.62 mm ammunition (I keep them in separate rooms
myself).

It is cool to be cool (our generation said that first, the heck with the
iPhone generation). As it has been mentioned before, you may be putting more
stresses on your gear by turning it off and on. (the transformer momentarily
tries to become a motor and rotate by the evil forces of EMF), Instant on
filaments where we drop 6.3 V across a dead short until the filament
resistance rises to some semblance of stable and the horrors of capacitors.

The trade-off is a bizzare form of calculus where the cost of leaving the
power on vs. pranging the ballast, tube filaments, etc vs. How often do you
use your gear all need to be considered.

I have a housekeeper who comes over a few days a week to do the cleaning. To
keep her on her toes I just leave the radio on, tuned up to an EAM
frequency, with a moderate amount of volume. She cannot get into that room
but the sound of EAM messages resonate through the house on an irregular
basis when I am not home (also scares the cats, gotta keep them on their
toes). I think she is afraid to ask what weird emanations are coming from
that locked room upstairs.

All told, with the SP-600's, R-390A's and R-220 running continually (and the
fans) it adds about $50 to my electric bill a month. In the winter the room
is nice and toasty, in the summer it can be uncomfortably warm even with the
air conditioning on. For a winter project I may add some ductwork to pull
cold outside air into the radio cabinet when outside temperatures are lower
than inside. Of course in the south, if it gets down to 50 degrees we are
looking for parkas and the glaciers coming over the Smoky Mountains as we
think the northern half of the country is covered under two miles of ice.

I did a short write-up on the temperature differences between silver
shields, no shields and IERC around a year ago. I need to redo the
experiment and add another variable for with/without forced air cooling.
This time I will put it in a spreadsheet and share it around. Give me a few
weeks on that as I have other eggs to fry right now.

Tisha