The type 27 vacuum tube was a general-purpose triode announced in April, 1927 by the Radio Corporation of America (today’s RCA). It, along with the type 24A tetrode tube, was a departure from the filament-type of tubes then in general use. Alternating current (AC) was becoming the preferred method of powering radios, and radio engineers’ new designs required tubes that could be lit from AC.
The 27 was immediately popular with both manufacturers and experimenters. It could be used as a radio- or audio-frequency amplifier, an oscillator, or a detector. The 27 gave solid, stable performance in all those applications. Consumer radios used the 27 extensively until around 1932, and the introduction of the improved type 56 triode.
Some radios, such as RCA’s Radiola 60, used only 27s for all functions except for the final amplifier tube, a type 71A. I have a Radiola 60 in my collection and it is an excellent-sounding radio, even by today’s standards.
In appearance, the first production 27s resembled old-style light bulbs. In later years, the bulb’s shape took on the more familiar “ST” shape, also known as the “coke-bottle” shape.
With the advent of glass audio (high-end amplifier employing only tubes), the 27 has made a small comeback. It was a high-production tube, and remains relatively easy to find today – on eBay, for example.
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: Indianapolis, Sams Publishing, 1987