Tube socket

Tube sockets were ubiquitous in early electronic equipment to allow vacuum tubes (also known as valves) to be easily removed for testing and replacement. Tubes often failed because the filament burned out, but other failure modes were common. In the heyday of radio and early television, it was common to find a vacuum tube tester in local drug stores, which also sold replacement tubes.

Tube sockets were typically mounted in holes on a sheet metal chassis and wires or other components were hand soldered to lugs on the underside of the socket. In the 1950s printed circuit boards were introduced and tube sockets were developed whose contacts could be soldered directly to the printed wiring traces. Looking at the bottom of a socket, or, equivalently, a tube from its bottom, the pins were numbered clockwise, starting at an index notch or gap, a convention that has persisted into the integrated circuit era.

In the 1930's tubes often had the connection to the control grid, brought out though a metal cap on the top of the tube. This was conected by using a clip with an attached wire lead. An example would be the 6A7 pentagrid converter. Later some tubes particularly those used as radio frequency (RF) power amplifiers,or Horizontal delection amplifiers in TV sets, such as the 6DQ6 had the plate or anode lead protrude through the envelope. In both cases this allowed the tube's output circuitry to be shielded from the input (grid) circuit. In the case of the tubes with the plate brought out to a cap, this also allowed the plate to run at higher voltages, over 26,000 volts in the case of recifiers for colour televison (such as the 3A3).

Contents

1 Octal base 2 Miniature tubes 3 Other socket styles 4 See also

Octal base

Prior to World War II, the most common tube socket for low and medium power applications had eight holes with electrical contacts arranged in a 17 mm (11/16 inch) diameter circle, surrounding a larger (8 mm or 5/16 in) center hole with an indexing slot on one side. The glass envelope of each octal vacuum tube (e.g. the 6SN7) was cemented into a bakelite or plastic base with a hollow post in the center, surrounded by eight metal pins. The wire leads from the tube were soldered into the pins. The pinched-off glass nub through which the air was evacuated from the envelope fit into the post. The post had a protrusion along one side that matched the indexing slot in the socket, so the tube could only be inserted one way. '

Matching plugs were manufactured that let tube sockets be used as electrical connectors, though penurious experimenters would salvage the base from a burned out tube for this purpose. Octal sockets were used to mount other components, particularly electrical relays and octal-mount relays are still common.

A variant of the octal base, the loctal base, was designed for ruggedized applications such as Automobile Radios. The pin geometry was the same as for octal, but the pins were thinner and the center hole had an electrical contact that also clamped the tube in place.

Miniature tubes

Miniature tubes (e.g. the 12AX7) were introduced around World War II. Wires protruding through the bottom of the glass envelope plugged directly into the socket-there was no separate base. The pinched-off air evacuation nub was at the top of the tube, giving it its distinctive appearance. The wire pins were arranged in a circle with one pin missing, so the tube could only be plugged one way. Seven and nine pin tubes were common, though miniature tubes with more pins were later introduced. Miniature tube sockets often had a skirt that mated with a cylindrical shield that fit around the tube. Some shields incorporated a spring to hold the tube in place. These were used if the equipment was subject to vibration.

Other socket styles

Vacuum tubes for high power or UHF applications often required custom socket designs. Subminiature tubes, introduced in the 1950s were often soldered directly to printed circuit boards. Sockets were made for early transistors, but quickly fell out of favor as their reliability became established. The same thing happened with early integrated circuits.

See also

• Nuvistor