[Radioroslyn]

B+
Dates back to the old days of radio (1920's) when they used batteries for power. Back then you had an A battery which supplied the filament voltage usually a was a 6v car battery. It would run down and would be carried to the local gas station to be recharged. And don't tip it over on the carpet as the acid would spill out a eat up the carpet.
The B battery supplied the high voltage or plate voltage for the toobs. Usually was several 22 1/2 v or 45v or 90v jobs wired in series to provide the proper voltage. Most often 22 1/2v 90v 135v 180v. B+ is a reference to the plus side of the HV source. Whether it is from batteries or an ac power supply. A bad/leaky paper cap can put a load across the HV or B+ line that it can burn out the power transformer and or the rectifier toob.
The C battery used to supply the negative voltage to bias the control grid of various stages of the radio set, mostly the audio output. Sizes are usually 4 1/2v and 22 1/2v
Wellcome to the Phourm!!!
That the long answer to a short question!
Terry