The designation “6F6” was given to a family of octal-based power amplifier vacuum tubes. The tubes were available in these form factors:
6F6: metal T-8 envelope
6F6-G: glass ST-14 ("Coke-bottle shape") envelope
6F6-GT and 6F6-GT/G: glass T-9 "Bantam" envelope shape
6F6-MG: large MT-10 “metal-glass” construction for special applications
Type 6F6 was among the group of metal-clad vacuum tubes introduced jointly by RCA and Cunningham Radio Tubes in 1935. As with many in that series, it was an octal-based version of an older glass tube, the type 42. The two tubes are electrically identical.
It gained wide acceptance; many manufacturers, such as Philco, redesigned existing circuits to employ the 6F6, usually resulting in “new” model versions of the previous year's offerings. Other companies brought out completely new designs to take advantage of the 6F6's smaller size. This advantage was lost in using the larger glass 6F6-G; but then regained with the 6F6-GT, which required less space than even the metal 6F6. Few radios besides those marketed by Sears, Roebuck and Company used the 6F6-MG; this version was more of a marketing gimmick.
Designers found the 6F6 a more efficient audio amplifier than the older pentodes such as the types 47, 89, and 6E6. In single-tube Class A service, it could provide up to 4.5 watts audio power at 9% distortion. Two 6F6s in a push-pull Class AB circuit, often driven by another 6F6, were capable of nearly 20 watts at only 5% distortion. For those designers who preferred the sound of a triode amplifier, the 6F6 could be operated in triode configuration at slightly less power output.
Today, the 6F6 series of tubes is used mostly for replacement in existing equipment. The tubes have not been in production for many years, so restorers are dependent on remaining supplies. Fortunately, the 6F6 was produced in quantity; existing stocks aren't likely to be depleted anytime soon.
RCA Receiving Tube Manual, 1940 edition
various audiophile websites