Glass-"Metal" Tube Shield
October 1935 Radio-Craft

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

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.