Filament voltage for 20's radios: the light came on...

[Reece]

I recently had need to power a typical 1920's radio for testing. I eventually will build a power supply for these sets and have plans to do so, but needed power right away. I could steal B+ power from another AC set I built, and C power from a few flashlight cells. But I had trouble finding 5 or 6 volts DC for six 01A tubes. They require 5 volts at 0.25 amps, which would be 1.5 amps total. Back in the day of 6 volt car batteries I would have been set. What I needed to do was drop half the voltage of a 12 volt car battery. The resistor would have to be R = 6/1.5 = 4 ohms and would have to dissipate P = 6 x 1.5 = 9 watts. The closest power resistor I had was 8 ohms at 20 watts. I thought to put some other power resistors in parallel with the 8 ohm to lower the resistance, but none of the others that I had were suitable in either resistance or wattage.

Then I thought of the beleaguered incandescent light bulb. We use them for dim bulb testing on AC set where they act as great shock absorbers, so why not as a simple resistor? They do work as resistors, but aren't so simple. As they heat up, their resistance goes up.

I tried one 100-watt bulb in parallel with my 8-ohm resistor, but that wasn't enough, nor were two. Finally three bulbs worked fine. The resistor got hot, but not too hot, because I gently chucked it in my big bench vise as a heat sink. The bulbs glowed just enough to barely show the filament through the bulb frosting, and they got warm, too. I had several DVM's monitoring voltage everywhere, and was able to adjust the radio's filament rheostat to properly illuminate the tubes and get the radio to work.