Early DIY B Batteries

In the early days of "Stone knives and bear skins" enterprising radio nuts would build their own B "batteries" using mason jars, lead and aluminum plates, and battery acid. These were directly connected up to the house current. Many a young radio nut got his rump blistered when his mother found the acid that would spray out of the top of these home made "batteries" would ruin her beautiful hardwood floor. If the jar ran out of electrolyte the heat could crack the glass, and...all that nice acid would run all over the floor.

What they had built was called an "Electrolytic Rectifier". It's reign was very short lived, fortunately, as copper oxide rectifiers and selenium rectifiers soon appeared.

 This enabled the companies to make a combined A/B/C pack the radio connectors would plug into and the pack was then plugged into the lamp socket or the wall outlet. (Lamp sockets came before wall outlets in many homes, as they were wired for lights only; and you could buy an adaptor that screwed into the lamp socket and had a 2 prong AC connector on it.)
  The fancy combo supplies had switches to set the B+ and C+ voltages, and sometimes the filament (A+) voltages. I did see one that only had B & C voltages and a battery was still used for the A voltage.