The 6F6 is an audio output pentode valve, designed in the mid 1930s. This type was obsolete by the 1960s but still available in small quantities.
Many American valves of this era were available first in metal, then in large classic glass envelopes (suffix -G), and later in tubular (-GT) shape. In the case of types which ran hot, such as the 6F6, satisfactory -GT versions did not appear until the later 1940s.
The 2-part base caps fitted to some -GT valves were rather wider than the conventional 1-piece moulded base caps fitted to many -G valves so that in some cases an apparently smaller -GT valve would not fit into a set designed for -G valves. Hence it became necessary to distinguish between -G and -GT versions in lists, etc, and to keep both types in production. However, with many (eventually, most) tubular types it was found practical to use a base cap design which fitted all types of Octal socket. Valves fitted with this type of cap were listed as 'GT/G', for obvious reasons. Thus, while type 6F6 is very much a 1930s valve, the -GT/G version is almost certainly a late 1940s or 1950s replacement.
With 315 Volts on the anodes and operated in class A1 push pull, this valve type would generate 10 Watts with 3% distortion.
The wide glass tube envelope is 28 mm in diameter and, excluding the IO base pins, is 70 mm tall.
References: Datasheet, 1040 & 1043. Type 6F6GT/G was first introduced in 1936.
Absolute Maximum Operating Conditions¶
Updated January 27, 2013.