The 6AC7 American metal pentode was designed for high gain amplification. Since it has a sharp cut-off it is unsuitable for domestic radios with AVC. The intended application is as an IF or video amplifier for wide-band receivers, e.g. radars. Bearing in mind that the RCA design of metal valves was pinch-less and had very short lead-outs well shielded from each other by the metal envelope.
Type 6AC7 can be thought of as the American answer to the EF50. It consumed 50% more heater power but provided 50% more gm as a result. It was used in enormous numbers in American WWII equipments. Presumably the accessible suppressor grid might be used to 'gate' the signal output if required.
