Tube sockets are electrical sockets into which vacuum tubes (also known as valves) 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 plugged in in only one orientation. They were used in all tube electronic equipment to allow tubes, which failed relatively frequently, to be quickly tested and replaced. In the day of tube radio and television it was common in the US for local drug stores to have vacuum tube testers, and sell replacement tubes.
Throughout the tube era, as technology developed, sometimes differently in different parts of the world, many tube bases and sockets came into use.[1][2] 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 (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, as well as high-voltage regulator tubes.) A few unusual tubes had caps for both grid and plate; the caps were symmetrically placed, with divergent axes.
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The earliest tubes, like the Deforest Spherical Audion[3] from ca. 1911, 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,[4] or the Deforest Oscillion.[5] Type C6A xenon thyratrons, used in servos for the U.S. Navy Stable Element (vertical gyro) 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.
When tubes became more widespread, and new electrodes were added, more connections were required, so specially designed bases were created. But as the world was suffering the World War I, and the new electronics technology was just emerging, everything was 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 (ceramic, metal, bakelite, etc.) with a number of prongs ranging from 3 to 7, 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 4 to 7 pins in a circular array, with adjacent pairs of larger pins for heater connections.
Before AC line/mains-powered radios were developed, some 4-pin tubes (in particular, the very common UX-201A ('01A)) 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 4-pin bayonet base, and the bayonet pin was a live connection. (Five effective pins: It was an electrostatic-deflection gas-focused type, with a diode gun and single-ended deflection. The anode and the other two plates were common.)
An early exception to these types of bases is the Peanut 215 which instead of using prongs had a tiny bayonet base with 4 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 4 to 12 contacts.
In 1935, RCA introduced a new type of tube base for their new metal envelope tubes, which they called an "octal base". As the name implies it had eight pins—more than were usually used previously. Octal bases, as defined in IEC publication 67, diagram IEC 67-I-5a, have a 45 degree angle between pins, which form a 17.45 mm (11/16 inch) diameter circle around a 7.82 mm (5/16 inch) diameter keyed post (sometimes called a spigot) in the center. Octal sockets were designed to accept octal tubes, the rib in the keyed post fitting 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. 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 seal-off tip through which the air was evacuated from the envelope was protected inside the post.
Matching plugs were also manufactured that let tube sockets be used as 8-pin electrical connectors; penurious experimenters would sometimes salvage the base from a discarded tube for this purpose. Octal sockets were used to mount other components, particularly electrical relays; octal-mount relays are still common.
Octal and miniature tubes are still in use in tube-type audio hi fi and guitar amplifiers.
Most octal tubes following the widespread European designation system have penultimate digit "3" as in ECC34 (full details in the Mullard-Philips tube designation article). There is a different, totally obsolete, pre-world-war-II German octal type.
A variant of the octal base, the loctal base (sometimes spelled "loktal" trademarked by Sylvania), 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 mechanically locked (hence "loctal") 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 itself, 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 connecitng 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" 7- and 9-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" (for 6.3 volt types) or "14" for 12.6 volt types. Battery types (mostly 1.5 volt) are coded "1Lxn", where x is a letter and "n" a number, such as "1LA4". European designations are ambiguous; all B8G loctals have numbers in the range 20-29, which are defined as specifically loctal (such as EF22) or 50-59 (special bases, including the European 9-pin lock-in base), but other types are in the same range. There is a different loctal Lorenz.
Efforts to introduce small tubes into the marketplace date back to the 1920s when experimenters and hobbyists made radios with so-called peanut tubes[6] 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,[7] 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.[8] 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.[9][10] 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.[11] 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.[12]
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.[7]
The B7G (or "small-button" or "heptal") 7-pin miniature tubes are smaller than noval, with 7 pins arranged at 45 degrees spacing in a 9.53 mm (3/8th inch) diameter arc, the "missing" pin position being used to position the tube in its socket (unlike octal, loctal and rimlock sockets). Examples include the 6AQ5/EL90 and 6BE6/EK90. European tubes of this type have numbers 90-99, 100-109, 190-199, 900-999. A few in 100-109 have unusual non-B7G bases (e.g., Wehrmacht base).
The 9-pin miniature Noval B9A base (sometimes called button 9 pin, B9-1) offered a useful reduction in physical size compared to previous common types such as octal (especially important in TV sets where space was limited), while also providing a sufficient number of connections (unlike B7G) to allow effectively-unrestricted access to all the electrodes even of relatively complex tubes such as pentodes, double triodes and triode-hexodes. It could also provide multiple connections to an electrode of a simpler device where useful, e.g. 4 connections to the grid of a conventional grounded-grid UHF triode (e.g. 6AM4) to minimise the deleterious effects of lead inductance on the high-frequency performance.
This base type was used by many of the US and most of the European tubes (e.g. 12AX7, EF86, EL84) produced commercially towards the end of the era before transistors largely displaced their use.
The IEC 67-I-12a specification calls for a 36 degree angle between the 9 pins of 1.016 mm thickness, in an arc of diameter 11.89 mm.
European tubes of this type have numbers 80-89, 180-189, 280-289, 800-899, 8000-8999.
The Rimlock (B8A) base is an 8-pin design with a pin circle diameter close to noval, and uses a nub on the side of the envelope to engage with a guide and retaining spring in the socket wall. This provides pin registration (since the pins are equi-spaced) and also a fair degree of retention. Early tubes with this base type typically had a metal skirt around the lower ~15mm of the envelope to match the socket wall, and this offered a degree of built-in screening, but these were fairly soon replaced by 'skirtless' versions which had a characteristic widening in the glass to compensate physically for the absence of the skirt. In the European naming scheme, rimlock tubes are numbered in the ranges 40-49, 110-119 (with exceptions), and 400-499, e.g. EF40. Although virtually unknown elsewhere, this was a very common base type in European radios of the late 1940s through the 1950s, but was eventually displaced by the ubiquitous B7G and noval (B9A) base types.
By 1935 new tube technologies were required for the development of radar and telecommunications. UHF requirements severely limited the existing tubes, so radical ideas were implemented which affected how these tubes connected to the host system. Two new bases appeared, the acorn tube and the lighthouse tube, both solving the same problems but with different approaches. Thompson, G.M. Rose, Saltzberg and Burnside from RCA created the acorn tube by using far smaller electrodes, with radial short connections.[14] A different approach was taken by the designers of the lighthouse tube, such as the octal-base 2C43,[15] which relied in using concentric cylindrical metal contacts in connections that minimized inductance, thus allowing a much higher frequency.
The Nuvistor was so small physically, reducing stray capacitances and lead inductances, and its base and socket so compact that they were widely used for UHF TV tuners.
There are many other socket types, of which a few are:
A remarkably wide variety of tube and similar sockets is listed and described, with some informal application notes, at a commercial site, Pacific T.V. [1], including nuvistor, 8-pin subminiature, vidicon, reflex klystron, nine-pin octal-like, 10-pin miniature (two types), 11-pin sub-magnal, diheptal 14-pin, and many display tubes such as Nixies and vacuum fluorescent types (and even more). As well, each socket has a link to a clear, high-quality picture.
Sometimes, subminiature tubes with solid but flexible wire leads all exiting in the same plane were connected by subminiature inline sockets.
Some low-power reflex klystrons such as the 2K25 and 2K45 had small-diameter rigid coaxial outputs parallel to octal base pins. To accommodate the coax, one contact was replaced by a clearance hole.
Vacuum tubes for high-power applications often required custom socket designs. A jumbo four-prong socket was used for various industrial tubes. A specialized 7-pin socket (septar) with all pins in a circle with one pin wider than the others was used for transmitting tubes. Subminiature tubes with long wire leads, 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 transistor reliability became established. This also happened with early integrated circuits; IC sockets are now usually only used for devices that may need to be upgraded.
