5U4G

The 5U4G is a full-wave vacuum rectifier with a directly-heated cathode, rated for up to 275mA output current. It provides a relatively soft, gradual power supply characteristic with moderate voltage drop (~40-50V under load), which contributes to the dynamic, spongy feel in amplifiers that use it. The 5U4G's compliant power supply behavior is part of the sound in classic amplifiers like the Fender Tweed Bassman and many early hi-fi designs. RCA, GE, Tung-Sol, and Sylvania all made reliable examples; it is one of the most commonly encountered rectifier tubes in American amplifier designs of the 1950s and 60s.