Editing Tube socket (section)

==Octal {{anchor|Octal base}} ==
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| image1    = Heathkit_Vibrator.jpg
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| image2    = Octal_base_crystal.jpg
| caption2  = Because of the ubiquity of the octal socket, many other components used it for their pin configuration including [[relay]]s (not shown), [[Vibrator_(electronic)|vibrators]] (top), [[crystal oscillator]]s (bottom) and small-signal [[transformer]]s (not shown)
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In April 1935, the [[General_Electric#Public_company|General Electric Company]] introduced a new eight-pin tube base with their new metal envelope tubes.<ref>Editors, [https://worldradiohistory.com/Archive-Radio-Engineering/30s/1935/Radio-Engineering-1935-04.pdf "Metal Tubes for Receivers"], ''Radio Engineering'', April 1935, pp. 18 - 19</ref><ref>G. F. Metcalf, J. E. Beggs, [https://worldradiohistory.com/Archive-Electronics/30s/Electronics-1935-05.pdf "All-metal receiving tubes, the manufacturing technique"], ''Electronics'', May 1935, pp. 149 - 150</ref> The new base became known as the ''octal base''.<ref>Editors, [https://worldradiohistory.com/Archive-Electronics/30s/Electronics-1935-09.pdf "Making Metal Tubes"], ''Radio Engineering'', Sept. 1935, pp. 31, 34</ref> 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,<ref>IEC 60067: "Dimensions of electronic tubes and valves. IEC dimensions" (1966). Also published as BS 448-1:1981.</ref> diagram IEC 67-I-5a, have a 45-degree angle between pins, which form a {{convert|17.45|mm|in|frac=16|abbr=on}} diameter circle around a {{convert|7.82|mm|in|frac=16|abbr=on}} 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. This reservation prevented tubes such as the 6SL7/6SN7 dual triodes from being issued with metal envelopes, as such valves need three connections (cathode, grid, anode) for each triode (making six total) 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 connector]]s; bases from discarded tubes could be salvaged for this purpose. Octal sockets were used to mount other components, particularly [[electrolytic capacitor]] assemblies<ref>{{cite web|last=Schmid|first=Kurt|title=R-390A plug-in multi-section electrolytic capacitor kit|url=http://www.schmid-mainz.de/elco.pdf|work=Schmid-Mainz|access-date=20 July 2013}}</ref> and electrical [[relay]]s; octal-mount relays are still common.<ref>A socket similar to the standard otcal but with 11 pins also exists, and is also called "octal". It is an industry standard, developed mainly for industrial 3-phase relays (3PDT or TPDT) in order to accommodate three changeover contacts. It was also used in some older power supplies but was never used for tubes.</ref>

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.{{citation needed|date=January 2013}}

Octal and miniature tubes are still in use in tube-type audio [[hi fi|hi-fi]] and [[guitar amplifier]]s. Relays were historically manufactured in a vacuum tube form,<ref>{{Cite web|title = Amperite {{!}} Time Delay Relays {{!}} Flashers {{!}} Controlling Devices {{!}} Relay Requirements|url = http://www.amperite.com/history.html|website = www.amperite.com|access-date = 2016-01-22}}</ref> and industrial-grade relays continue to use the octal base for their pinout.<ref>{{Cite book|title = Advantages of a Full Featured, Octal 700 Series Relay|publisher = Magnecraft|url = http://www.serelays.com/library/section1/105A_750.pdf}}</ref><ref>{{Cite book|title = Amperite Relay Catalog|publisher = Amperite|url = http://www.amperite.com/assets/Documents/Amperite%20Catalog%20Printed%206.0%20Reduced.pdf}}</ref>
<gallery>
File:6P3S.jpg|An octal base tube, Soviet 6Π3C, similar to the [[6L6]].
File:6SN7 octal base.jpg|Octal base of a [[6SN7]] 
File:Vacuum tube octal socket.jpg|An octal socket
</gallery>