2A3

2A3 is a directly heated power triode vacuum tube used in audio amplifiers. Developed in the 1930s by RCA, it became a staple in single-ended triode (SET) amplifier designs. It is valued for its warm, natural audio response and relatively low distortion at modest output levels.

Construction and operation: The device is a directly heated triode. It typically uses a 2.5-volt heater and draws about 1 ampere. The plate (anode) voltage rating is around 250–300 V, with plate currents of several tens of milliamperes. In audio use, it operates in class A in a single-ended configuration, delivering roughly 3–4 watts into high-impedance speakers.

Variants and production: Over the years, RCA and other manufacturers offered glass and metal envelope versions (for example 2A3, 2A3-G, etc.). The tube is used by enthusiasts in vintage equipment and in modern SET builds seeking its sonic character. Availability today comes from specialty audio tube suppliers, with some new-production units available as part of boutique tube lines, while many original specimens are in vintage equipment.

See also: Vacuum tubes, single-ended triode amplifiers, triode vacuum tube amplification.