Tube socket


Tube sockets are electrical sockets into which vacuum tubes can be plugged, holding them in place and providing terminals, which can be soldered into the circuit, for each of the pins. Sockets are designed to allow tubes to be inserted in only one orientation. They were used in most tube electronic equipment to allow easy removal and replacement. When tube equipment was common, retailers such as drug stores had vacuum tube testers, and sold replacement tubes. Some Nixie tubes were also designed to use sockets.
Throughout the tube era, as technology developed, sometimes differently in different parts of the world, many tube bases and sockets came into use. Sockets are not universal; different tubes may fit mechanically into the same socket, though they may not work properly and possibly become damaged.
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 1930s, tubes often had the connection to the control grid brought out through a metal top 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 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 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 A few unusual tubes had caps for both grid and plate; the caps were symmetrically placed, with divergent axes.
Image:Tube 75.jpg|thumb|upright|right|Tube 75 from the 1930s with UX-6 base and top grid cap

The first tubes

The earliest tubes, like the Deforest Spherical Audion from, used the typical light bulb Edison socket for the heater, and flying leads for the other elements. Other tubes directly used flying leads for all of their contacts, like the Cunningham AudioTron from 1915, or the Deforest Oscillion. Type C6A xenon thyratrons, used in servos for the U.S. Navy Stable Element Mark 6, had a mogul screw base and L-shaped stiff wires at the top for grid and anode connections. Mating connectors were machined pairs of brass blocks with clamping screws, attached to flying leads.

Early bases

When tubes became more widespread, and new electrodes were added, more connections were required. Specially designed bases were created to account for this need. However, as the world was suffering from World War I, and the new electronics technology was just emerging, designs were far from being standardized. Usually, each company had their own tubes and sockets, which were not interchangeable with tubes from other companies. By the early 1920s, this situation was finally changing, and several standard bases were created. They consisted of a base with a number of prongs ranging from three to seven, with either a non-regular distribution or with one or two of the prongs of bigger diameter than the other, so that the tube could only be inserted in a certain position. Sometimes they relied on a bayonet on the side of the base. Examples of these are the very common USA bases UX4, UV4, UY5 and UX6, and the European B5, B6, B7, B8, C7, G8A, etc. Tubes in the USA typically had from four to seven pins in a circular array, with adjacent pairs of larger pins for heater connections.
Before alternating current line/mains-powered radios were developed, some four-pin tubes had a bayonet pin on the side of a cylindrical base. The socket used that pin for retaining the tube; insertion finished with a slight clockwise turn. Leaf springs, essentially all in the same plane, pressed upward on the bottoms of the pins, also keeping the bayonet pin engaged.
The first hot-cathode CRT, the Western Electric 224-B, had a standard four-pin bayonet base, and the bayonet pin was a live connection.
An early exception to these types of bases is the Peanut 215, which instead of using prongs had a tiny bayonet base with four drop-like contacts. Another exception is the European Side Contact series commonly known as P, which instead of using a prong, relied on side contacts at 90 degrees from the tube axis with four to twelve contacts.

Octal

In April 1935, the General Electric Company introduced a new eight-pin tube base with their new metal envelope tubes. The new base became known as the octal base. The octal base provided one more conductor with a smaller overall size of the base than the previous line of U. S. tube bases which had provided a maximum of seven conductors. Octal bases, as defined in IEC 60067, diagram IEC 67-I-5a, have a 45-degree angle between pins, which form a diameter circle around a diameter keyed post in the center. Octal sockets were designed to accept octal tubes; the rib in the keyed post fitted an indexing slot in the socket so the tube could only be inserted in one orientation.
When used on metal tubes, pin 1 was always reserved for a connection to the metal shell, which was usually grounded for shielding purposes. This reservation prevented tubes such as the 6SL7/6SN7 dual triodes from being issued with metal envelopes, as such valves need three connections for each triode plus two connections for the paralleled heaters. The octal base soon caught on for glass tubes, where the large central post could also house and protect the "evacuation tip" of the glass tube. The eight available pins allowed more complex tubes than before, such as dual triodes, to be constructed. 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 evacuation tip was protected inside the post.
Matching plugs were also manufactured that let tube sockets be used as eight-pin electrical connectors; bases from discarded tubes could be salvaged for this purpose. Octal sockets were used to mount other components, particularly electrolytic capacitor assemblies and electrical relays; octal-mount relays are still common.
Most octal tubes following the widespread European designation system have penultimate digit "3" as in ECC34. It is also called International Octal. There is a different, totally obsolete, pre-world-war-II German octal type. The Mazda octal is larger.
Octal and miniature tubes are still in use in tube-type audio hi-fi and guitar amplifiers. Relays were historically manufactured in a vacuum tube form, and industrial-grade relays continue to use the octal base for their pinout.

Loctal

A variant of the octal base, the B8G loctal base or lock-in base, was developed by Sylvania for ruggedized applications such as automobile radios. Along with B8B, these eight-pin locking bases are almost identical and the names usually taken as interchangeable. 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 mechanically locked the tube in place. Loctal tubes were only used widely by a few equipment manufacturers, most notably Philco, which used the tubes in many table radios. Loctal tubes have a small indexing mark on the side of the base skirt; they do not release easily from their sockets unless pushed from that side. Because the pins are actually the Fernico or Cunife lead-out wires from the tube, they are prone to intermittent connections caused by the build-up of electrolytic corrosion products due to the pin being of a different metallic composition to the socket contact.
The loctal tube's structure was supported directly by the connecting pins passing through the glass "button" base. Octal tube structures were supported on a glass "pinch", formed by heating the bottom of the envelope to fusing temperature, then squeezing the pinch closed. Sealing the pinch embedded the connecting wires in the pinch's glass and gave a vacuum-tight seal. The connecting wires then passed through the hollow base pins, where they were soldered to make permanent connections.
Loctal tubes had shorter connecting lengths between the socket pins and the internal elements than did their octal counterparts. This allowed them to operate at higher frequencies than octal tubes. The advent of miniature "all-glass" seven- and nine-pin tubes overtook both octals and loctals, so the loctal's higher-frequency potential was never fully exploited.
Loctal tube type numbers in the USA typically begin with "7" or "14" for 12.6-volt types. This was fudged by specifying the heater voltage as nominally 7 or 14 volts so that the tube nomenclature fitted. Battery types are coded "1Lxn", where x is a letter and "n" a number, such as "1LA4". Russian loctals end in L, e.g. 6J1L. European designations are ambiguous; all B8G loctals have numbers either in the range:
  • 20–29, except for early tubes in the series: DAC21, DBC21, DCH21, DF21, DF22, DL21, DLL21, DM21 which have either B9G or octal bases, the change to Sylvania's locktal standard coming in 1942
  • or 50–59, but other types are in the same range.

    Other loctals

  • Nine-pin loctal bases, B9G, include the 1938 Philips EF50, EL60 and some type numbers in the European 20–29 and 50–59 range;
  • There is a different "loctal Lorenz" in the Mullard–Philips tube designation.