The RCA 5Z3 vacuum tube has base pins that fit the standard four-contact sockets, and should be mounted in a vertical position with the base down. If it must be mounted horizontally, it should be mounted with the pin openings at the top or the bottom, which will make the plane of the filaments vertical. It is a high-vacuum rectifier for radios with high direct-current requirements. This tube is meant to be used with an A-C line through a step-down transformer. The tube has an A.C. filament voltage of 5.0 V, and a filament current of 3.0 amperes. When you use this tube in your vintage stereo, make sure to provide proper ventilation so it doesn't overheat.