August 1935 Radio-Craft[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Radio-Craft was published from 1929 through 1953. All copyrights are hereby acknowledged. See all articles from Radio-Craft.
If this Radio-Craft article article is accurate, it was sometime around 1935 that the 8-pin glass-encased vacuum tube base came into existence. The glass-metal designation refers to these being glass enclosed equivalents to otherwise metal encased vacuum tubes. Evidently, the relatively new (and expensive) line of metal tubes sported 8-pin bases so these glass tube designs had to conform in order to be suitable substitutes.
Design engineers and Service Men now are all agog about the latest in electronic devices - a line of tubes having 8-prong bases, and with identical characteristics to the new 8-prong metal-envelope tubes, but having a glass envelope. Two of these new "glass-metal" tubes are illustrated; they are the 6K7 and 6C5, electrical and mechanical counterparts of which (as to characteristics and bases), except for the envelope, are found in the "metal" series, under the same designations.
Design engineers are using these glass-metal tubes until such time as it is convenient to substitute metal tubes as direct replacements. Service Men probably will use the glass-metal tubes for some time to come as service replacements of metal tubes (which new sets are being designed to use), until such time as the production of metal tubes reaches the point where they offer greater competition to the new, relatively low-price glass-metal tubes.
Posted January 20, 2016