The apparent Philips demarcation between the EF80 as a domestic receiving valve and the EF91 as an industrial valve turns up in the following Pye examples. During the 1950s, Pye used many Mullard valves in its domestic equipment. The FenMan II FM-AM radio receiver of mid-1955 was an example, with 2 x EF80, 1 x ECF80, 1 x ECH81, 1 x EF85, 1 x EABC80, 1 x EBC41, 1 x EL84, 1 x EZ80, 1 x EM80. These were all from the Philips/Mullard noval domestic series except for the Rimlock EBC41, probably used because the European release of the EBC81 was held back, as noted in this thread:
https://www.vintage-radio.net/forum/...d.php?t=159120. In particular the EF80 was used in the positions (FM RF amplifier and FM limiter) where a high-slope, sharp cutoff VHF pentode was required.
On the other hand, on the industrial product side, the Pye Telecommunications PTC145 VHF monitor receiver of the same period, covering 68 to 174 MHz, had the valve line-up (AM version) was: EC91, 2 x EF91, ECF82, 2 x EF92, ECC82, EZ90. Here the EF91 was used in the positions (1st mixer and 2nd IF amplifier) where a high-slope, sharp cutoff VHF pentode was required. Now it could be that the EF91 was preferred over the EF80 for technical reasons – perhaps it might have been better in very narrow-band applications – but the overall valve line-up suggests a preference for B7G types except where B9A was unavoidable.
Cheers,