The 6F6 is an audio output pentode intended for domestic and car radios. It dates from the mid-1930s and was a well-established classic long before WWII.
Its output is roughly similar to that of the well-known type 6V6 but it was considerably more sensitive and could be driven directly from the detector output, thus saving one stage of audio amplification. This was convenient particularly in compact car radios but the price of the extra gain was paid in a larger cathode and more heater power than the 6V6. As a result it tended to run hot and, in car radios, a de-rating was normally recommended. Triad have built their version with a wider metal tube and also a longer tube. This would offer a much greater radiating surface than the versions in a 25 mm diameter tube.
The operating values given are for single ended use with an anode load of 7,000 Ohms.
If a pair are used as an audio amplifier in push pull class A, 10.5 Watts are possible with 3% distortion.