[R-390] Tube Shields

[Tisha Hayes]

I think what confuses many is the design of the IERC tube shield (and others
of the same family) where the shield has a mechanical attachment (albeit by
spring pressure) to the glass of the tube. Transferring heat by conduction
cools the tube much more efficiently than trying to remove the heat by
radiation. Whenever there is a tube shield that does not contact the glass,
the only heat transfer mechanism is radiative with maybe a small amount of
convective heat transfer by the random air currents inside of the shield.

Depending upon radiative cooling alone would be similar to disconnecting all
of the cooling fans on your car's motor and sitting in traffic for a few
hours. While some heat will be transferred by radiation (as the motor gets
smokin-hot the hood heats up) the motor will sieze up quickly (unless you
believe Castrol television commercials). In a car, the cooling comes from
moving a much cooler fluid through the hot motor, then giving that heat
energy a place to go in the radiator (where it becomes forced convection
transfer to the air) where the cooling fans or driving can dilute the heat
into the atmosphere.

Air is a pretty good insulator of heat, look at how close your finger needs
to get to a soldering iron tip before you get burned. You may feel the heat
at less than an inch (radiative) but there is a dramatic temperature
difference from being 1/4" away and touching the iron (conductive). Tubes
suffer from the same problem where there is this tiny air-gap between the
glass and the metal.

The IERC tube shields actually touch the glass in several places and the
spring fingers are under slight compression. The heat transfer is from the
contact. My crazy experiment was to slightly increase the thermal conduction
of the spring contact area by using a small dab of thermal grease (what they
use on CPU's, commonly sold at Radio Shack) to increase the contact surface
area. Ideally the glass envelope and the IERC shield would be at the same
temperature (yea, glass does not conduct heat evenly, that makes my brain
hurt). BTW, the thermal grease idea makes a mess if you are constantly
pulling tubes and has a tendency to remove tube lettering.

Where the IERC fingers come down and imperfectly "dock" with the
bayonet-socket (from the silver shields) gives another conductive escape
path for the heat to the chassis. To remove heat further would be to
passively remove it from the radio by natural convection (heat rises) or
forced convection (fans).

-- 
Ms. Tisha Hayes

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"I will not recant the truth. I am corn, not chaff; I will not be blown away
with the wind or burst by the flail. I will survive both."
-Walter Milne, 1558