Tube sockets are electrical sockets into which vacuum tubes (electronic 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 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 (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 color 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. The first tubes The earliest tubes, like the Deforest Spherical Audion from c. 1911, used the typical light