Silvertone Model 7104
This 1940 Silvertone is a farm radio, which
means it was battery operated. It used a 1.5-volt A battery and a 90-volt B
battery. It has inductive permeability tuning instead of the more common tuning
capacitor. I made a battery eliminator power supply and mounted it in the
battery compartment using an isolation transformer, some diodes, a LM317
regulator and filter caps. It is very sensitive and can be loud considering the
maximum output is about a half watt. The knobs are not original. I made them
from drawer pulls.
Notice the switch centered just below the dial. It is a battery saver feature.
When switched on a resistor is inserted in both the B+ and the filament circuit
to reduce the current draw. Someone a long time ago had bypassed the switch and
apparently had made an AC operated supply. I replaced the switch and wired it
back per the schematic. It works with my battery eliminator just fine. I was
really surprised at the sensitivity of this radio. It picks up stations all
across the dial with only a six-foot wire as an antenna.