The 6CY5 VHF tetrode dates back to c.1956-57 and I think it was from Sylvania. It was of the sharp cutoff type whereas the Mazda 30F27 was remote cutoff. Quite possibly Mazda was inspired by 6CY5.
The 6CY5 might have slightly post-dated the 6BN4, which was the first of the new generation of TV tuner VHF triodes. Both developments were aimed at reducing the cost and complexity of VHF TV tuners without losing too much performance. The 6BN4 was in fact half of a 6BZ7 double triode, this having been one of the early American cascode valves, along with 6BK7 and 6BQ7. Attention was paid to lead-out resistance and capacitance, and pin layout in order make the 6BN4 relatively easy to use.
I imagine the argument in favour of the 6BN4 was that although it required neutralization, that was not as complicated as it was in the “old days” when the 6J6 was sometimes employed as an RF amplifier, and it was as quiet as a cascode stage. It had less gain, but that was easily made-up with recent and improved triode-pentode oscillator-mixers and improved pentode IF amplifiers. In favour of the 6CY5 was that it was easy to use as a pentode, no neutralizing required – and before the cascode valve arrived, most makers were more comfortable with using a 6AG5 (or similar) than a 6J6 - and it had more gain than a triode.
The 6BN4 had a “featured” application in the new Standard Coil “Neutrode” turret tuner. I am not sure if the 6CY5 had such a prominent debut. It was used though, including in at least one RCA VHF tuner, the KRK71D/E.
The RCA Nuvistor development program plan included both a VHF triode similar to the 6BN4, and a VHF tetrode similar to the 6CY5.
The next step prominent step with American VHF TV triode-based tuners was the Standard Coil Guided Grid turret, later 1959 I think. This used either a 6ER5 (EC95) or a 6ES5 (might have been from Sylvania). By that time Amperex was making inroads into the US TV set market with Philips valve designs such as the EC95. Both the 6ER5 and 6ES5 were described as semi-remote cutoff. The 6FY5 (EC97) was described as being remote cutoff. These triodes were easier to use in the neutralization department and being of the frame-grid type, had higher gain.
In 1960 RCA claimed that its new VHF tuner using a 6CW4 Nuvistor triode was about 1.5 dB better in noise factor than could be obtained with frame-grid triodes such as the 6ER5 or conventional triodes such as the 6FH5. I am not sure that a consumer-oriented Nuvistor tetrode was ever realized, although there were industrial tetrodes. Perhaps this putative gap between intent and reality reflected the fact that the triode was essentially the valve of choice for non-cascode VHF TV RF amplifiers, and that interest in the tetrode had faded.
By way of a sidebar item, the descriptor “semi-remote cutoff” appears to have been an American term, not much used in respect of European valves. It was used to describe valves, mostly those developed for TV applications, that had grid bases wider than was typical for sharp cutoff valves, but distinctly shorter than those of remote cutoff valves developed for radio receiver use, such as the 6BA6. Early examples were the 6BZ6 pentode and 6BC8 cascode double triode. The adoption of such valves was one solution vector in addressing the cross-modulation and AGC difficulties that were occurring with existing sharp cutoff valves and the scatter in the curves when operating biased well back. Another approach was the issue of sharp cutoff valves with more tightly controlled curves, examples being the 6CF6 (a selected 6CB6, as I understand it) and the 6DE6 pentodes. There was a good article on this topic in RCA Engineer 1955 October-November, page 18ff. This is accessible at:
http://www.americanradiohistory.com/..._Issue_Key.htm.
Returning to the Mazda 30F27, evidently it was released in the UK at about the same time as the Mullard PC97:
Given this timing and the recent events in the USA, I’d say that there was little chance that Mazda would succeed with its new tetrode. Perhaps its view was that it had to innovate in directions other than Philips/Mullard was going. Hence the 30C13 (printed circuit compatibility) and the 30C17 (remote cutoff mixer pentode).
Cheers,