October 1935 Radio-Craft
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
While working on vacuum tube
based USAF air traffic control radar and radio systems, and having seen many tube
television and radio sets I never recall seeing one of these form-fitting metal
shields like this one appearing in a 1935 issue of Radio−Craft magazine. All the ones I've seen are simple cylinders that slide over the tube and
either twist into a receiving rim slot like a bayonet type lock or they have spring metal fingers that grab
the glass envelope. One kind of scary feature of this shield is that there is a
tab with a hole in it to go around a grounded pin on the tube, requiring that
the installer pay close attention to how it goes on. Putting it on the wrong pin
could cause serious problems like shorting out part of the circuit if the shield
also happens to contact a nearby ground, or it could inadvertently broadcast (or
pick up) a signal, or likely put a hazardous voltage on the shield. As you might guess, utilizing a metal shield around a tube for
anything other than a low frequency application like an audio amplifier or poser
supply requires circuit design that takes into account the capacitive effects of
the large, usually grounded, metal plates.
Glass-"Metal" Tube Shield
Glass-"metal" tube shield.
Amazingly, I happened to find a photo of what looks to be
this snap-on tube shield.
Fitting tightly around the glass, these shields are designed for use with the
new glass-"metal" tubes, making them interchangeable with the metal tubes in sets
designed for the latter. The shield is in four parts, the two main shell pieces
being held in place by the top and bottom pieces, which snap into place. The bottom
piece has a tab which springs over the ground pin on the tube, and thus grounds
the shield as in the metal tubes.
Posted July 28, 2022
(updated from original post on 11/3/2015)