Thanks Pieter, that does help a lot, and puts the whole issue into perspective. One may see that prior to the arrival of the EF80, Mullard was travelling a different pathway to the rest of Philips, one that involved offering both the EF42 and the EF91 for TV receiver applications rather than the EF42 alone. Once the EF80 was available, its superiority for the TV job made it the natural successor to both. Not only that, but the UK valve industry generally came on board as it were by offering clones of the EF80, something that had not happened with the EF42. You could say that in the EF80 era, Mullard crossed over from the EF91 branch line to the EF80 main line.
ECC81 was the European designation for the American 12AT7, which had been introduced by GE in 1947 as a TV and FM mixer-oscillator, with improved performance as compared with the established 6J6, in particular lower microphony. Most data sheets claim that the 12AT7/ECC81 was good to 300 MHz as an amplifier, with some makers claiming higher than this. In US practice, it was used as an FM mixer-oscillator through the 1950. However, its TV role fell away when triode pentodes such as the 6X8 and 6U8 were introduced late in 1951 as being better suited to the new 45.75 MHz standard IF. (The first cascode valves, 6BQ7 and 6BK7, were introduced at about the same time.)
Philips Book IIIC included the ECC81, and showed various circuits including push-pull RF amplifiers, shunt cascode RF amplifier and mixer-oscillator. Lower noise at Band III frequencies was claimed as compared with the EF80. At least according to RCA’s work, the shunt cascode was less amenable to multichannel applications, which is why it developed the series cascode circuit, which it originally named “driven grounded grid”. On the other hand, the shunt cascode was probably fine for single-channel front ends.
The ECC81 did have something of a split personality though. It was often listed as an AF valve, along with the ECC82 and ECC83. Mullard advertised it both ways:
Returning to the EF80 and EF91, this table (from Fisher; VHF TV Tuners) provides some comparative data and I think shows that the EF80 was pretty good for its time. The 6CB6, dating from 1950, was RCA’s best TV pentode of the time, having been developed specially for 40 MHz IF strips.
Cheers,