Octal
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.
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