Tube Socket - Miniature Tubes

Miniature Tubes

Efforts to introduce small tubes into the marketplace date back to the 1920s when experimenters and hobbyists made radios with so-called peanut tubes like the Peanut 215 mentioned above. But because of the primitive manufacturing techniques of the time, these tubes were too unreliable for commercial use.

RCA announced new miniature tubes, which proved reliable, in "Electronics" magazine. The first ones, such as the 6J6 VHF dual triode, were introduced in 1939. The bases commonly referred to as "miniature" are the 7-pin B7G type, and the slightly later 9-pin B9A (noval). The pins are arranged evenly in a circle of 8 or 10 evenly-spaced positions, with one pin omitted; this allows the tube to be inserted in only one orientation. Keying by omitting a pin is also used in 8 (submin.), 10, and 12-pin (Compactron) tubes (a variant 10-pin form, "noval+1", is basically a 9-pin socket with an added center contact).

As with loctal tubes, the pins of miniature tube are stiff wires protruding through the bottom of the glass envelope which plug directly into the socket. However, unlike all their predecessors, miniature tubes are not fitted with separate bases; the base is an integral part of the glass envelope. The pinched-off air evacuation nub is at the top of the tube, giving it its distinctive appearance. More than one functional section can be included in a single envelope; a dual triode configuration is particularly common. Seven- and nine-pin tubes were common, though miniature tubes with more pins, such as the Compactron series, were later introduced, and could fit up to three amplifying elements. Some miniature tube sockets had a skirt that mated with a cylindrical metal electrostatic shield that surrounded the tube, fitted with a spring to hold the tube in place if the equipment was subject to vibration. Sometimes the shield was also fitted with thermal contacts to transfer heat from the glass envelope to the shield and act as a heat sink, which was considered to improve tube life in higher power applications.

Electrolytic effects from the differing metals used for the miniature tube pins (usually Cunife or Fernico) and the tube base could cause intermittent contact due to local corrosion, especially in relatively low current tubes such as were used in battery-operated radio sets. Malfunctioning equipment with miniature tubes can sometimes be brought back to life by removing and reinserting the tubes, disturbing the insulating layer of corrosion.

Miniature tubes were widely manufactured for military use during the second world war, and also used in consumer equipment. The Sonora Radio and Television Corporation produced the first radio using these miniature tubes, the "Candid", in April 1940. In June 1940 RCA released its battery-operated Model BP-10, the first superheterodyne receiver small enough to fit in a handbag or coat pocket. This model had the following tube lineup: 1R5 - Pentagrid Converter; 1T4 - I.F. Amplifier; 1S5 - Detector/AVC/AF Amplifier; 1S4 - Audio Output. The BP-10 proved so popular that Zenith, Motorola, Emerson, and other radio manufacturers produced similar pocket radios based on RCA's miniature tubes. Several of these pocket radios were introduced in 1941 and sold until the suspension of radio production in April 1942 for the duration of World War II.

After the war miniature tubes continued to be manufactured for civilian use regardless of any technical advantage, as they were cheaper than octals and loctals.