[R-390] Re: Tube shields? We don’t need no steenkin' tube shields!

[Barry Williams]

This is what I saved from Chuck Rippel's site, but I think I may find 
more after searching an old computer. I had forgotten about the Collins 
service bulletin.



IERC Heat Dissipating Tube Shields

About IERC Tube Shields -

The R390A uses 5 different sizes of heat dissipating, black, IERC or 
"WPM" labled tube shields.  Installing the correct type and part number 
shield can dramatically decrease the operating temperature  of the 
vacuum tubes and in turn, increase their service life.  Collins 
addressed this back in the early 50's in service bulletin #303 which 
graphically compares the performance of various types of tube shields 
and not using shields at all.

The proper tube shields can easily identified.  They are anodized black 
(or deep purple), have an open top with a series of tabs folded over a 
thin, octagonal metal tube inserted longways inside the shield.  They 
are also plainly labeled "IERC."  There is a unique model number stamped 
on the outside of the shield denoting which size it is designed to fit.  
Refer to this number when obtaining the shields.

Below is an inventory with individual quantities of the 5 different part 
number IERC tube shields used in the R390A:

      (1)     6025-B   Tall 9 Pin, used for the ballast tube
      (9)     6020-B   Medium 9 pin, used on 5814A's and 26Z5W's
      (2)     5015-B   Short 7 pin, used on the 5654's
    (13)     5020-B   Medium 7 pin, used on 6BA6's, 6C4, 6AK5, etc...
      (1)     5025-B   Tall 7 pin, used on the OA2

Black tube shields labeled "WPM" my also be found.  While I personally 
don't feel these are quite as effective as the IERC design, they are far 
and away better than the shiny types described below.

Radios which still have shiny, nickel-plated tube shields should have 
them replaced with the above IERC shields as soon as possible.  Even if 
they have been painted black on the outside, these shields have no 
provision to grip the tube bottle and sink the heat away from it.  Also, 
the bright internal surfaces of the shield actually reflect the heat 
back into tube and on to its dark internal plate structure which could 
then cause the tube to over dissipate and shorten its service life.



Barry