In the late 1920s, the first radios powered by household alternating current (AC) started to appear on the market. They were a boon for those listeners who actually had AC power in their homes — "No more messy batteries," read a typical advertisement. However, much of rural America was still not on the power grid. Not wishing to miss out on a huge market, manufacturers continued to produce radio sets powered by batteries, and employing vacuum tubes that were more efficient than those used in early 1920s sets. Collectors and restorers today refer to these radios as "farm radios", since historically these radios tended to wind up on farms and rural households.
There are three general categories of farm radios, based upon the power scheme used in the sets:
Straight Battery sets: These radios used "A" batteries to supply the tube filaments (requiring 2 volts), blocks of 90- and 45-volt "B" batteries to supply the tube plates, and "C" batteries to provide bias voltage for the tube grids. The alphabetical terminology came from the designations for the various voltages used by the radio. By 1940, improved tubes were in use that didn’t require grid bias, and the C battery was no longer necessary. These radios also used the new smaller batteries that had been designed for portable radios.
Vibrator sets: This type of radio was powered by a single 6-volt automobile storage battery. The vibrator was a type of mechanical switch that converted the direct current (DC) output of the battery to 6 volts of AC. This voltage could then be stepped up by a transformer, and rectified back to DC in order to provide voltage for the tubes’ plates and grids. The filaments, of course, were powered directly by the battery voltage. These sets usually contained the same tube types as used in AC sets.
32-Volt Lighting sets: Some farms employed a lighting system based on 32-volt batteries (yes, there were 32-volt light bulbs in those days). Radios were available that could be operated from the 32-volt circuit, complete with specially designed audio output tubes. Though this was a lower plate voltage than usually found in radios, the sets performed quite well and many examples survive today.
Since these radios aren’t powered by ordinary house current, their restoration creates a bit more work for the collector. A power supply must be built, since neither the correct high-voltage batteries nor 32-volt electrical systems are available today. Vibrator sets usually require a new power supply as well; the contacts inside the vibrator are almost always corroded and cannot be restored. Farm radios are today a specialized area of collecting and a labor of love for many antique radio enthusiasts.
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