[Arran]

My experience with battery sets is that if they included a small value electrolytic on the B+ line, then it needs to be there. I have encountered these in a number of farm battery sets over the years, usually in a wax sealed cardboard tube, dried up, and likely perished inside, and with it open circuit the set will hum, at least on a battery eliminator, it may actually be a low frequency oscillation. Also the value is not too critical, but I always used 150 vdc caps as replacements, that way I could use them in 1.5 volt and 90 volt sets as well as 2 volt and 135 volt ones.
Regards
Arran