The type 71A vacuum tube was a power amplifier triode intended for use in the final audio stage of battery- or alternating current-operated receivers. It began life, under the old style of tube designation, as the UX-171 and was announced by the Radio Corporation of America (RCA) in May 1926. Operated at its published conditions, it was able to supply nearly 800 milliwatts of audio power, a reasonable amount for a single tube. It was superseded in November 1926 by the improved UX-171A, which featured a lower filament current.
The UX-171 and 171A were first issued with the “globe” style bulb, similar in appearance to an old-style light bulb. In the early 1930s, the bulb was changed to the “ST” shape (also known as the “coke-bottle” shape), and given the new designation of 71A.
Radio engineers used it extensively in broadcast band radio receiver designs. The 71A provided more audio power than the UX-112A, and the tube was less expensive than the UX-210 or UX-250 tubes. The 71A's only real competition, for design purposes, was the UX-245 power triode.
Today, the 71A has a small following amongst high-end amplifier builders and hobbyists. It was a high-production tube and is relatively easy to find from various tube suppliers (and on eBay).
John W. Stokes, 70 Years of Radio Tubes and Valves
: New York
, The Vestal Press, 1982
Gerald F. W. Tyne, Saga of the Vacuum Tube, Second Edition
, Sams Publishing, 1987