Vacuum Tube - History and Development - Miniature Tubes

Miniature Tubes

Early tubes used a metal or glass envelope atop an insulating bakelite base. In 1938 a technique was developed to instead use an all-glass construction with the pins fused in the glass base of the envelope. This was used in the design of a much smaller tube outline, known as the miniature tube, having 7 or 9 pins. Making tubes smaller reduced the voltage that they could work at, and also the power of the filament. Miniature tubes became predominant in consumer applications such as radio receivers and hi-fi amplifiers. However the larger older styles continued to be used especially as higher power rectifiers, in higher power audio output stages and as transmitting tubes.

Subminiature tubes with a size roughly that of half a cigarette were used in hearing-aid amplifiers. These tubes did not have pins plugging into a socket but were soldered in place. The "acorn" valve (named due to its shape) was also very small, as was the metal-cased nuvistor, about the size of a thimble. The small size supported especially high-frequency operation; nuvistors were used in UHF television tuners until replaced by high-frequency transistors.