[Arran]

It has been my experience with the farm battery radios that I have owned that the vast majority of them have never been serviced and still have all of the original capacitors and resistors in place. When you think about it other then age the components are never really put under any great stress, the tube filaments run more or less cold since they are low voltage and low current, the B+ voltage typically ranges between 45 and 135 volts, mostly around 90 volts.
They were also never played day in and day out like an AC set, if they didn't want to continuously buy new batteries they had to ration their listening time, though many used a storage battery and rheostat for the "A" supply to get around this. Also because of the rural electrification programs after the war many of these sets were retired for AC operated replacements. Believe it or not in many cases you can hook up the correct voltages to one of these sets and it will play, although some have an electrolytic or two that will be dried out and need immediate replacement.
Regards
Arran