IERC Tube Shields for the R390A
Chuck Rippel, WA4HHG
The R390A uses 5 different kinds of heat dissipating, black, IERC tube shields. Installing the correct type and part number shield can dramatically decrease the operating temperature and in turn, increase the life of the vacuum tubes. Collins addressed this back in the early 50's in a service bulletin which graphically compared the beneficial performance of various types of tube shields with 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. Some may also have a series of internal "fingers" which also serve to grip and sink heat away from the the glass bottle. These are both plainly labelled "IERC." There is also a unique model number stamped on the outside of the shield denoting which size vacuum tube it is designed to fit. Refer to this number when obtaining the shields.
Below is an inventory with individual quantities and part numbers ofthe 5 different IERC tube shields used in the R390A:
(1) 6025-B Tall 9 Pin, used for the 3TF7 ballast tube
(9) 6020-B Medium 9 pin, used on 5814A's and 26Z5W's (6U8A)
(2) 5015-B Short 7 pin, used on the 5654's (6AL5, 5670)
(13) 5020-B Medium 7 pin, used on 6BA6's, 6C4, 6AK5, 6DC6
(1) 5025-B Tall 7 pin, used on the OA2 (6AQ5, 6BF5)
Ever burn you fingers on a 6AQ5 or 6BF5 which are commonly used as audio tubes in Collins receivers? The 5025-B, tall 7 pin shield fits these perfectly and is very effective sinking the extreme heat away from the tube bulb. I use one of these on the audio tubesof this type in each of my Collins receivers.
Black tube shields labelled "WPM" my also be found. While I don't personally feel these are quite as effective as the IERC design, they are still far and away better than the shiny ones described below.
Radios which still have shiny, nickel plated tube shields installed should have them replaced with the above IERC shields or even "WPM" shields as soon as possible. Even if they have been painted black on the outside, these shields have no internal structure to grip the tube bottle and sink heat away from it. Also, the bright insides of the shield actually reflect the heat back into tube and on to the dark internal plate structure which could cause it to over dissipate and shorten tube life.
The very best place to find the IERC type shields is at hamfests.
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15 September 1997