tube shields

[Garey Barrell]

Eddy -

Collins conducted a study of the IERC shields a long time ago, titled 
Collins Service Bulletin #303A.  I think I have a copy here somewhere, 
but a lot of things haven't surfaced yet from the move.  There was a 
synopsis published in the Collins Signal, Technical Data Sheer G-0297.  
Collins conclusion was that use of these shields could increase tube 
life by as much as 55%.  The bright metal shields had the opposite 
effect, which makes the recent tendency to polish the original dull 
finish to a mirror shine as part of a "restoration" even dumber than it 
looks!  The shiny surface merely reflects the heat back to the dark 
colored plate of the tube,  _increasing_  the internal temperature even 
further.  Polishing the outside greatly reduces the ability of the 
surface to radiate whatever heat does escape from the tube.....  
Polishing the surface of chassis shields has the same effect, reflecting 
any heat that did manage to escape back to tubes, transformers and other 
dark objects.

There is more to the IERC shields though than just the black anodized 
surface.  They originally had an insert of beryllium spring fingers 
inside which made physical contact with the envelope of the tube and 
conducted heat directly to the shield where it could be reradiated by 
the black external surface.  Unfortunately these inserts could be 
removed easily, and sometimes were, to allow the shield to fit over a 
"fatter" tube than it was designed for, greatly reducing its effectiveness.

73, Garey - K4OAH
Glen Allen, VA

Drake 2-B, 4-B & C-Line Service Supplement CDs
<www.k4oah.com>



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...
>
> If I recall correctly, the ONLY shield that did not reduce tube life due to
> over-heating was the black-metal military type...the common bayonet-attached
> / internal spring jobbies were death to tubes, relatively-speaking.
>
> 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.
>
> ~73~ Eddy VE3CUI - VE3XZ
>
>   
>   

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