tube shields

[Kenneth G. Gordon]

On 13 Nov 2007 at 13:45, EP Swynar wrote:

> Hi Dean,
> 
> An interesting aside re. tube shields...
> 
> There was a piece awhile back ("Electric Radio" magazine...?) where
> some analysis was done on the longevity of common tubes with specific
> shields attached to them...

Although ER may have reprinted that data, it goes WAAAAAY 
back.
> 
> If I recall correctly, the ONLY shield that did not reduce tube life
> due to over-heating was the black-metal military type...

also called the IERC tube shield. These are difficult to get, but can 
be found. I think Fair Radio Sales has some sizes, but not many.

> the common
> bayonet-attached / internal spring jobbies were death to tubes,
> relatively-speaking.

Yes. The reason being that they are very shiny inside and out, and 
thus reflect the heat back INTO the tube.

> I'm sure someone with a memory better than mine can enlighten us all
> further on this...but I guess the bottom line is, for maximum tube
> life, if you don't require a tube shield in your circuit(s), DON'T USE
> ONE.

Well, don't use the shiny ones, at least. 

The Heathkit shields I have used in the SB/HW transceivers are a 
corrugated metal which has been anodized black inside and out. 
Those do radiate a LOT of excess heat away.

In point of fact, the data you mention was gathered by the military 
some time ago. That particular report, if I remember correctly, also 
stated that simply painting the tube shields with black liquid shoe 
polish, inside (especially), but also outside made a HUGE 
difference in the tube heat.

That data was one reason that Heathkit also changed the final 
amplifier cages in their parallel 6146 rigs from a shiny unpainted 
aluminum to a black-anodized material. Radiates heat away much 
better.

I routinely paint the sides of shields which I install around tube 
compartments with black liquid shoe polish, or with flat black 
enamel.

BTW, the temperature difference between black SHINY paint vs 
black FLAT paint was also significant according to that military 
report.

You want to suck the heat OUT of the tube, not reflect it back INTO 
the tube.

Ken Gordon W7EKB 

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