B batteries provided the high voltage to the "plates" of tubes. I recall 90
volt units.

A batteries provided filament voltage (1.5 volts) to a lot of tubes like the
1EQ5, where the first digiot indicated the filament voltage, and the letters
the type of tube characteristics, and the final digit the number of pins that
were active.
6 AQ 5 was a 6 volt amplifier with two filment connections, a plate
coonection and two grids.
However sometimes there was a cathode too (I'm getting hazy on this stuff),
maybe the filament didn't count as two.