Tube socket
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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. Some varieties of nixie tubes were also designed to use tube sockets.
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 tracks. 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 connected 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 deflection 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 isolated from the input (grid) circuit more effectively. 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 rectifiers for colour television, such as the 3A3).
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[edit] Octal base
In 1935, RCA introduced a new type of tube base for their new metal envelope tubes, which they called an "Octal base". True to the name, it had 8 pins on an 11/16" pin circle, with a 5/16" diameter keyed locating prong or "spigot" in the center. When used on metal tubes, pin #1 was always reserved for a connection to the metal shell, which was grounded for shielding purposes.
The octal base soon caught on for conventional glass tubes as well, where the large central prong could also house and protect the "evacuation tip" of the glass tube. The eight available pins allowed more complex tubes to be constructed, including dual triodes, etc. The glass envelope of an octal base tube 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 and 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 in the correct orientation.
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 (sometimes spelled "loktal"), was developed by Sylvania for ruggedized applications such as automobile radios. The pin geometry was the same as for octal, but the pins were thinner, the base shell was made of aluminium, and the center hole had an electrical contact that also clamped the tube in place. Loctal tubes were never that popular, and were only adopted widely by a handful of equipment manufacturers, most notably Philco, which used the tubes in many ordinary table radios. Loctal tubes are often difficult to remove from their sockets (look for a small indexing mark on the side of the base skirt and push the tube from that side to unlock). They are also prone to intermittent connections due to the short, thin base pins, which have a tendency to build up an oxidation layer.
[edit] 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 be plugged only one way. The envelope and pin arrangements could accommodate the inclusion of more than one functional unit within the envelope, with a dual triode configuration being common. Seven and nine pin tubes were common, though miniature tubes with more pins were later introduced and used for the inclusion of up to three amplifying elements for television receiver applications. 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. Sometimes the shield was also fitted with thermal contacts to transfer heat from the glass envelope to the shield and thence to air or to the chassies, as this was believed 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 led to intermittent contact due to local corrosion, especially in relatively low current tubes such as were used in battery operated radio sets. An apparently lifeless battery radio can frequently be 'repaired' by merely removing and reinserting the tubes in their sockets, thus breaking the insulating layer of corrosion.
[edit] Other socket styles
Vacuum tubes for high power or UHF applications often required custom socket designs. A jumbo four prong socket was used for various industrial tubes. A specialized 7-pin socket with all pins in a circle with one pin wider than the others was used for transmitting tubes. 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 where sockets are now usually reserved for those devices that may be upgraded or used for the replacement of firmware.
