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Mon October 7, 2013
That's a Radio?
The average person probably wouldn't know what this object was if they came upon it at an estate sale or in an antique shop. They would have less of a clue were it not displayed with the headphones, which are also from the same time period. The two largest knobs are graduated with a scale that reads 0 to 100, the others are not labeled at all. If you were to turn it around, you would see the first clue that it might be a receiver of some type. There are seven thumb screw terminals, two of which are labeled antenna and ground. Three are labeled "B Battery", and the remaining two are labeled “A battery” and "Common". There is nothing to indicate what frequency it receives, such as 540 –1600, or whether it is AM or FM.
To a collector of vintage electronics, this item would definitely warrant a closer inspection. Since this is the first of my articles on vintage radio, I decided to go way back to the beginnings of radio as we know it today, the early beginnings of the vacuum tube era. This radio is a very early set, built with parts which appeared on the scene between 1919 and 1922. It may have been a kit radio, or it may have been designed and built by a hobbyist from plans found in an early wireless magazine. Perhaps it was built to monitor the broadcasts of the first licensed broadcast station KDKA which hailed from Pittsburg PA in 1920. It may have been built to allow its creator to monitor wireless telegraph transmissions between ships and shore stations, or for use by an early amateur radio operator.
There were receivers which predated the age of tubes. An Englishman named John Preece began wireless transmission of Morse code for commercial purposes as early as 1892, and Marconi made his famous transatlantic transmission on December 12, 1901, but I’ll cover those technologies at a later date.
If we examine the set outside of its mahogany cabinet, we see that it has three early (type #12) vacuum tubes and their associated components. Most of the components are mounted directly on a piece of wood. This is referred to as a "breadboard design". If you were to divide the set in half, the actual receiver is the right half, consisting of one tube with a tuning coil (far right) and tuning capacitor (aluminum color, dead center). The remaining tubes and their associated components on the left are primitive audio amplifiers. This is regenerative receiver, based on a design by Lee DeForest
Like all first generation receivers, this radio was powered by batteries. Going back to those thumb screw connectors, The “A” battery was 6 volts and was often a heavy lead acid rechargeable, like a modern car battery. It would have imparted an unpleasant sulfur smell when in use and eaten little holes in the carpet if any of the acid were to spill or bubble out. This battery was used to power the filaments, which are the glowing hot parts of a tube, similar to the filament in a light bulb. I’ll spend a little more time on how tubes work next month. The “B” battery powered the radio circuitry and was probably a 45 or 90 volt dry cell pack. On this set, the builder had a separate “B” battery connection for each tube, but in all likelihood, they would have been connected to the same “B” battery.
Those “B” batteries were expensive and this builder devised a clever way to maximize battery life. Each tube has its own headphone jack and the tube itself can be turned on or off from the front of the radio. For strong local stations, only one tube would be needed, so the others could be shut off to increase battery life. The headphones would be plugged into the leftmost jack (as viewed from the front), located just below the large main tuning knob. Need more volume? Move the headphone plug to the middle jack and turn on the next tube. Need still more amplification for a weak station? Use the right hand jack and all three tubes.
So, just what could you hear?
As I mentioned before, the tuning knobs had scales of 1-100. The set will actually tune most of the AM radio band, FM would not appear for another three decades. There were a few reasons why the knob was not inscribed with the traditional 540-1600 kHz that we take for granted on modern radios. Initially there were only two or three channels allocated for broadcast, so finding the station wasn’t too hard. Next, the radios (both kit radios and commercially produced) were not all that stable. The spot on the dial which might be 540 kHz on one day might be 700 kHz two days later due to temperature and humidity change, or a battery getting weak. Even as circuits became more stable, most manufacturers resisted using actual frequency dials until the late 1920s or early 1930s. There was also a debate as to whether the frequency or the wavelength should be indicated by the dial.
Operating the set was a bit tricky, as the gain (sensitivity) was largely controlled by the regeneration knob which is the second largest knob on the set. Apply too much regenerative feedback and it would break into a squealing howl. The technique was to use the large knob to find a station, then increase regeneration control for maximum volume which occurs just before it starts squealing. The three small knobs turned on their respective tubes and served as a volume control for each stage.
Does it still work?
Yes, connected to a good antenna it can receive most AM stations that you would expect receive on the average modern receiver. Selectivity is fair. The set appears to be about 90% original. The front bakelite face was originally black as the backside of the faceplate remains today, but probably faded to a light brown from sunlight exposure. I replaced one segment of wire which was missing when I found the set. A previous owner replaced the main ‘A” battery switch with a 1940’s vintage one, indicating that the radio saw at least some sort of use up to that time. One item, the white and chrome variable resistor located just right of center was disconnected when I found the set. The resistor's exact purpose is unknown. It may have been the intent of the builder to use it to fine tune the regenerative feedback, but upon completion he or she found it unnecessary.
What is it worth?
It is hard to put a monetary value on a homemade or early kit radio unless you can positively identify the kit manufacturer or who made it. The square lead coated wire and construction techniques indicate that in all likelihood it was constructed when the components were new, not from leftovers a decade later. I have seen similar sets sell for $200-$300 or more.