This may seem a dumb question, but how much current (in ma.) should a power supply deliver from its B+ winding to power a four tube radio? The tubes are a 47, an 80 and two 24A's. I see in the manuals that the 47 requires 31 ma. plate current. and the two 24's each require 4 ma. Does that mean then that the power supply must deliver no less than 39ma? Or do you measure the resistance of the load presented to the power supply and calculate the current requirement from that?

: This may seem a dumb question, but how much current (in ma.) should a power supply deliver from its B+ winding to power a four tube radio? The tubes are a 47, an 80 and two 24A's. I see in the manuals that the 47 requires 31 ma. plate current. and the two 24's each require 4 ma. Does that mean then that the power supply must deliver no less than 39ma? Or do you measure the resistance of the load presented to the power supply and calculate the current requirement from that?
Hi Tom

Your are doing it right. The load is actually in series with the tubes. Your numbers are the currents with a 250 volt B+ supply. With higher voltage the tubes will draw more current. One other item to consider is screen current. The numbers you mention are plate current. The screen will add a few more ma.

The radio should also have a speaker field coil. These fields are usually in series with the radio B+ supply. In that case it will not add current to your B supply. In some radios the field is actually B+ to ground. In this type of circuit you need to allow for the additional current drawn by the field.

Norm

: This may seem a dumb question, but how much current (in ma.) should a power supply deliver from its B+ winding to power a four tube radio? The tubes are a 47, an 80 and two 24A's. I see in the manuals that the 47 requires 31 ma. plate current. and the two 24's each require 4 ma. Does that mean then that the power supply must deliver no less than 39ma? Or do you measure the resistance of the load presented to the power supply and calculate the current requirement from that?
Hi Tom, In some of the earlier radios in particular, the B+ voltage is divided down to lower voltages through a bleeder resistor. This is often a large wire wound resistor connected between B+ and ground with taps along its length at the apropriate voltages. You need to add the current taken by this resistor to ground as part of the total. Depending on the set it could be several milliamps. I would expect the total current drain in a set such as yours to be about 40 to 55 mA. Don.