Why is the high voltage called B+? What are A, B and C voltages?
These letters once refered to battery classes for circuits. (You’ll notice that tube texts generally show batteries as power sources for the various DC voltages – they meant it!) A batteries were for the heaters (filaments). B batteries were for plates and other (relatively) high voltage supplies such as screen grids. C batteries were for control grid bias supplies. The B voltage is positive (B+). The C voltage is generally negative (C-). The A voltage polarity generally doesn’t matter, but if it’s DC should most likely be positive. Today, B+ is about the only one of these terms in common usage. I say classes instead of sizes – you could buy a B battery in a variety of voltage and current ratings. The first piece of gear I ever disassembled was a portable, all-tube AM radio (about 8″x6″x2″, and it weighed several pounds). It had a steel chassis with 1.5V filament tubes, and three batteries.
