Some comment on EF42 performance relative to the EF91 and EF80 was provided in the recent thread “EF91 to EF80”,
https://www.vintage-radio.net/forum/...d.php?t=157399,
I think that the best insight came from the valve choices for the Fitton (Ambassador)-BBC VHF AM-FM comparator receiver. In this both the EF91 and EF42 were used, the EF42 being chosen for situations where its very high slope was advantageous, including the oscillator, AFC reactance valve and the two limiters. It was also used for the RF amplifier, although it was said that had the EF80 or 6F1 been available when it was designed, they would have been even better for that role. The 6F1 was basically a 6F13 with two cathode pinouts and with the internal screen sharing a pinout with the suppressor grid as a consequence. With a single short production run special-purpose receiver, use of several generally similar valve types was not really a problem, whereas for larger-scale production, rationalization on just one would have been highly desirable.
Fitton used the EF91 as mixer. But where an autodyne mixer was required, then the EF42 was probably a better choice because it was a better oscillator. So for a two-valve front end, an EF42 RF amplifier and an EF42 autodyne mixer was logical, with the advantage that the same valve type was used in both positions. Once the EF80 became available, then that would have been the better choice for the RF amplifier, and that being so, it might also have been chosen for the autodyne mixer to achieve commonality, and perhaps on the basis that the RF amplifier improvement was worth more than the loss in mixer performance.
Lowther’s valve choices for its initial FM tuner model were also indicative. It had a 6BW7 (effectively a higher slope EF80) RF amplifier, EF80 IF amplifier and EF42 limiter. From that one could infer that the higher slope of the 6BW7 as compared with the EF80 was beneficial in respect of the RF amplifier (where a two-cathode pinout valve was strongly preferred), but not in respect of the IF amplifier, where stability considerations may have limited achievable gain. (Jason found the EF80 to be a slightly better FM IF amplifier than the EF91 simply because it had lower in-situ interelectrode capacitances, and so a higher stability threshold.) Presumably the very high slope of the EF42 could not be used to advantage in the IF amplifier, but it was useful in the limiter. Given that Lowther products were of the low production “hand-carved” type, valve rationalization would not have been a paramount concern.
Cheers,