1Ж42А. As a mixer/Frequency changer????

[Mike Watterson]

As a suggested method if you wanted to construct a sensitive infinite impedance detector for the radio using those valves, could you use a floating supply for the filament? A separate LT cell for each valve wouldn't be a hardship....but then why fix what isn't broken?

I can't see how that does anything much as the filaments are the cathodes and the input impedance is much higher than regular valves.
They were used as the input stage in an Electrometer using also 1j24b due to very high input impedance which may be enhanced by the low current, thus low space charge.
https://www.radiomuseum.org/forum/1j42a_1j42a.html#2
The g1a and g1b tied together.

@Trevor I can post a couple of modern 1N60 letter post if you don't want to wait for China or wherever. PM, or better email address. You have mine, I think I got the plotter off you that I've only recently fixed. A tale for another thread?

Well, Philips and Radioshack/Tandy/Realistic had some nice innovative stuff, up to and including the FM Superhet IC with only 70kHZ IF & Op-Amp based RC IF filter (It uses the PLL detector as a tracking filter to reduce the FM deviation). Occasionally bonkers like the floating diode, or the Philips Valve set Unicontrol, an ergonomic disaster. Also Philps persevered with complex TRF using tunable Q/bandwidth (a TRF unlike superhet has a bandwidth varying across the band) and glass coil formers to get superhet performance.