Farm Radio

So the other night, Elizabeth finds a deal on an old radio on a local Facebook buy/sell/trade page and makes arrangements to pick it up the next day.  Because she is awesome like that.  Upon bringing the bargain home, I gave it a quick look-see.  It turns out to be a 1937 Silvertone model 4419.  Silvertone was the store brand for radios sold by Sears, Roebuck & Co.  The astute viewer of the photo below will immediately realize this is a farm set.

You will note the lack of a conventional AC power cord and that there are two rather heavy leads, in this example one is red and one is black.  Additionally the chassis has a large enclosed metal box atop it.   The two heavy leads are for battery power from a storage battery and the metal box contains a vibrator type power supply to produce the required voltages for the tubes.

Farm radios were sold up into the early 1950’s as many areas, particularly rural farming communities, did not have electricity.  There were basically two types.  Those powered by dry cell batteries, usually a 1.5v “A” battery for the filaments and a “B” battery for the plate circuits that usually was on the order of 45, 67.5, or 90v.  (The “A” and “B” designators are not battery sizes but are merely used to denote the function…A for filaments, B for plate, and if needed, “C” for a bias supply.)  This worked well and resulted in a lightweight set.  Many portable tube radios of the period operated in the same manner.  But the disadvantage is that these batteries eventually run down and must be replaced.  No big deal in a portable set, but on the farm it could require an inconvenient trip into town.  The other method resolves this problem.  Using a storage battery (like a car or tractor battery) that is about 6 or 12v.  (and in some cases, 32v) and a power supply that can convert this to the voltages needed by the tube circuits, the radio can be powered in this manner and the storage battery can be recharged for use again and again.

The vibrator power supply is simply an electromagnet operating a buzzer-like contact that is used to interrupt and reverse the current passing through it…this AC or chopped DC wave is then fed to a transformer and a variety of filter capacitors and choke coils to yield a usable higher voltage DC current.  If the waveform generated is AC, they will often feed it back through another set of contacts on the vibrator in order to chop and reverse it back to DC prior to being filtered.  A sort of mechanical rectifier that works because the oscillation of the AC inverting contacts is perfectly synchronized with the rectifying DC output contacts since they are all mechanically affixed to the same moving part inside.  Doing this reduces the overall parts count by negating the need of a separate rectifier.

Such vibratory power supplies were found not only in farm radio sets, but also in automotive radios, some military and aircraft sets, and some amateur radio gear.  This technology in the power supplies was utilized well into the 1950’s and in some cases even the early to mid 1960’s when transistor radios replaced the tube receivers and then when high power switching transistors began to become available that could perform the functions in applications where tubes were still required, such as mobile transmitters.

Charging the storage battery that ran these farm radios was sometimes accomplished by putting it back into the tractor or car it was borrowed from.  Others had a so-called “farm light plant” that consisted of a small engine powered generator whose job was to charge up a battery bank that would then power the radio, a few lights, and some other household appliances that were built to run on low voltage direct current.  Common system voltages were 6, 12, 24, or 32 volts.  The most ambitious and, depending upon location, cost effective installations included a wind powered generator to charge the batteries.  (and you kids thought off-grid houses were a new thing)

Zenith, Philco, Sears, and others sold their own versions of the small “Wincharger”  and often marketed them right along with their radios.  Larger units such as the 32 volt or 115volt Jacobs machines could power a multitude of items on the farm.

For those lacking the funds (it was still during the depression era) to invest in a wind powered or factory made light plant from Delco or Fairbanks-Morse, there was another popular option.  Build one.

Building a small farm light plant entailed getting a small generator, such as from an old car…and a small engine to power it.  Common at the time would have been the Maytag, Briggs&Stratton, or Sattley engines that had kick-starters and were used on washing machines.  Yes.  Your grandma or great-grandma on the farm was a bad-ass who would kick start the laundry like it was a Harley!  Several years ago, when I was active in an antique engine and tractor club, I re-created a 1930’s farm light plant to run a Philco farm radio that I had restored.  This made for a nifty little hands-on exhibit that did a great job of showing how some of these things worked and were used and then repurposed.  (and you kids thought repurposing and recycling were new ideas)

This re-creation uses parts that would have been found at the time.  Some scrap wood for the base.  A generator from a car (in this case a ’32 Ford) and a Maytag engine from the washer.  In addition, I added an ammeter from a Model T and a porcelain based knife switch.  Then wired it together with cloth braid covered wire.  It not only works, but it looks right.  When connected to a 6 volt storage battery and started up, it chugs right along and provides about 10 to 15 amps of charging current.  More than enough to run a few lights and power the radio.  At shows I used to also bring along a tiny transmitter and could play old radio shows into it and let the programs be tuned in on the restored farm radio.

I still have the old Philco and the homebuilt light plant.  Now, it seems, I have another radio in the queue to restore that will be in need of that light plant as well.