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

[Mike Watterson]

I presume their filaments are coated similar to what the cathode of an indirectly heated valve would be to improve emission?

All Battery valves since about 1925 used coated filaments. Uncoated filamentary cathodes are what are called Bright Emitters. The insulated cathode (indirect) was much later and enabled RF, IF, Oscillators and preamps on an AC supply. I think the formulation or process to coat filaments used by Mullard / Philips changed in about 1928.

It's plausible that low emission after excessive filament current is caused by evaporation of some of the surface. It would be interesting to measure the hot and cold resistances of an undamaged filament and suspect but still continuity filaments. Note that the "hot" resistance is nearly double the cold resistance for practically all valves. The temperature coefficient of tungsten. Which is why a filament lamp gives more voltage regulation than a resistive dropper, but may need a thermistor. TVs need a thermistor. A barretter is a primitive regulator using iron in a hydrogen atmosphere to give even steeper regulation for a series current chain.

The coating is a mixture of Barium Oxide and other "rare earth" oxides. Some specialist valves even have a slightly radioactive coating.