Hello from Ireland

[Mike Watterson]

I think I'll get some 1j37a valves and have a play.

The 1j37a are high filament current, 45V to 60V g2 and 45V to 100V anode. The 1j42a are later generation or last generation design (samples I have were made in 1980s, but maybe late 1960s early 1970s design).  Ural Spirit is selling them and not much more expensive than 1j37a. They are a very reasonable price.
Any radio would need a crystal earpiece or transistor final audio stage, like the car radios that used ECH83 (really an ECH81) with 12V H, no vibrator or inverter.

The dual grid amplifies common mode signals more and differential signals less (the opposite to an op-amp or balun driven amp). The Rod Pentode is a short cut off inherently, you can only do much AGC by varying g2 (20V to 55V is good for most types). The 1j37a and 1j42a allow simulation of remote cut off / variable mu. They can be used for AGC, detectors, frequency doublers, voltage controlled amps. If you tie the two g1 grids together, then the 1j42a is a sharp cut off regular "pentode" but with low max Vg2 and Va of 6V to 10V.

Unlike DF96 or DF97, where "triodised" mode (e.g. DF97 as a DC90 VHF mixer Osc) is g2, g3 and anode tied, the rod Pentodes you tie g2 and anode but g3 is either at +f or -f, or thereabouts.  Like real pentodes there is limited g3 modulation possible, but at less than f+ voltage.
They are really filamentary electron guns with electrostatic lenses.