Normally on the "B" eliminators (regardless of manufacturer in the twenties and thirties), the output is around 180 volts, detector stage is half of that, and yours has a middle tap that is not always used, but would fall between the final amp and detector voltages. B- goes to ground or common.
If you have knobs for voltage adjustment, they are likely to be carbon pile types, and the adjustment for the final amp will have an effect on the detector voltage, but not the other way around.
It will work for most battery operated sets.
:I have a modern B Power unit Made in Toledo Ohio ser no 5518 . It does not work i would like to know about the voltage it has if i can get it to work. it has b- Det.V.A. F.A pins. ernie