[Arran]

I would say that 20 uf is awfully low for a filter cap in an Ac/Dc radio, regardless of whether it has a filed coil speaker or not, I would have expected at least a 30 uf or a 40 as an input cap, and that or higher as an output one, 50 uf/50 uf are quite common in those double section toilet roll style condensers. I can recall replacing filter caps in Ac/Dc radios with 22 uf, 160 vdc jobs, and the sets always had some noticeable hum, such that I replaced them a few year later when I found some of a larger value. In any event the 20/20 unit may have been a, incorrect replacement that someone else stuck in there.
Regards
Arran