Ninety years ago this month, the November 1928 issue of Popular Mechanics showed how to put together this trickle charger for your set’s “A” battery (the one supplying filament voltage, which appears to be 6 volts in this case).

The voltage was dropped over two lamps in parallel, and the current was rectified by a homemade rectifier consisting of a strip of aluminum (as pure as possible) and strip of lead, in a solution of borax. The article noted that the charger wouldn’t do much good for a completely dead battery, but by hooking up the trickle charger when the set was not in use, it would keep it topped off.