[Synchrodyne]

A single ECC85 is suggestive of a circuit not unlike those often used for FM front ends, with say a grounded grid RF amplifier followed by a self-oscillating triode mixer. On the face of it not unreasonable as a simple way for Band III to Band I conversion with oscillator low, but local oscillator radiation could have been a problem, maybe more so than at Band II frequencies. It happened that Band III (174 to 216 MHz for the UK 405-line TV channels) was about twice the frequency of the full Band II, 87.5 to 108 MHz. Presumably the ECC85 was still good at 200 MHz, even though said to be optimized for 100 MHz. The earlier ECC81 was said to be usable up to 300 MHz.

I am not sure if a PCC84 could be used in the same role. The original cascode triode valve, the 6BQ7, was designed so that either triode could be used in grounded grid or grounded cathode mode. But it had an independent shield connection, whereas in the PCC84 the shield is connected to the second triode grid. Maybe the second triode as the RF stage and the first triode as mixer would have worked?

The attached Wireless World advertisement from 1959 shows an evidently more elaborate Band III converter, self-powered, with ECC84, presumably as a cascode RF amplifier, and ECF80, presumably as frequency changer.

Cheers,