Refers to a type of vacuum tube that was developed in the late 1930s and ran through most of the 1940s, primarily in automobile radios and radios manufactured by, but not limited to Philco. They are distinguished from other tubes,primarily the Octal by their nickel plated metal base, with a keyed pin in the center of the base that locks into the tube socket. Around the center pin were 8 smaller pins that provided connections for the heater, cathode, grids, and plate elements of the tube. The locking socket was a major selling point, in particular for companies building equipment for mobile use, such as automobile radios and military equipment.
Another distinguishing feature of locktal tubes was the voltage scheme. Many common tubes of the era used 6.3 volt heater elements, and the usual convention for naming tubes was to assign the heater voltage of the tube as the first number. A 6H6 Octal tube had a 6.3 volt heater. So did its locktal equivalent, the 7H7. Locktals gave way to miniature tubes during the 1950s for most small signal uses due to their smaller size and lower power consumption. Miniature tubes had the annoying habit of shaking out of their sockets, so it became necessary to cover them with shields which used a bayonet mount type of mechanism to keep the tubes in their socket. Most cheap radios omitted this feature to save money.