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

[Kalee20]

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...

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.

Well, as an infinite impedance detector, you'd get good linearity and low distortion. I think it's a good suggestion, probably not one often seen because of the extra, floating, battery. But on the experimenter's bench, anything is possible!