Re: {Collins} Black tube shields



Virtually ALL heat transport from the tube plate to the outside world is by radiant heat transport to the glass.

The glass absorbs this IR radiation from the plate and the glass gets hot. Blowing air over the tube cools the glass.

Tube shields are NOT put in place to cool tubes. Quite the contrary because convection cooing is always reduced by a tube shield.  

Tube shields are present to alter electrical performance. Shields can affect noise (due to photoelectric effects on the glass and other tube parts from light sources inside a radio) and also act to minimize electromagnetic coupling between one tube and another, and they reduce pickup of electromagnetic fields (like 60 Hx hum). They also reduce radiation of RFI in the case of oscillators.
Shields were NEVER intended to improve heat transport!

Of course, some tubes shields, those with high thermal contact with tubes, high OD surface area and high emissivity help overcome the otherwise negative thermal effects brought about by shields.

Removing shields can have significant negative impact on circuit performance so don’t remove them. The designers had the shields in mind when they designed the circuits and removing them negates this part of the design.

If tube temperature is a big concern for you, add a quiet blower fan to keep things cool but don’t remove the shields!

Gary

W0DVN

> On Nov 5, 2017, at 8:11 PM, CL in NC via Collins <collins@xxxxxxxxxxxxx> wrote:
> 
> Somewhere, there is an old article, possibly about the R390 and 390A, that studied tube shields.  It compared envelope temps vs type of shield.  Of course the black IERC shields with the internal fins did the best, but one painted flat black did a good job compared to the a regular shiny one that actually reflected the heat back in.  I can't say I remember it exactly, but think it said no shield was better that a shiny one, and said painting them black was a good compromise.  The one thing I pondered was the fact that the heat must be radiated to the envelope, but since there is no air to conduct the actual heat of the tube elements how does removing the envelope heat cool the metal inside?  Would seem cooling the glass envelope protects it from melting, but doesn't really reduce the actual plate temp.  Guess it must work though.
> 
> Charlie, W4MEC in NC 
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