Hi...all you CRCers! This month I'd like to write about a subject that will likely be of interest to collectors who restore low end 1930 radios - the substitution for defective resistance line cords. To begin with, I want to thank Dick Hagrman and his brother Ray for much of the information in this article. In particular, I want to thank them for all the empirical work they did to distill their substitution technique down to a simple procedure and a few component values.
During the 2nd decade of commercially available radios the cost of owning a receiver began to drop dramatically. One reason of course was that by the beginning of the "Thirties" mass production and volume selling was in full swing. At the same time however, cheaper methods of construction were also being implemented. One of these cost reduction methods was the elimination of the AC power transformer - the most expensive, largest and heaviest component on a radio chassis.
Most of us are familiar with the common method of transformer-less operation. The B+, or "plate" voltage is derived directly from the rectified and filtered AC line voltage. At the same time, the filaments of the tubes are supplied with the proper voltage by placing them in series with the input AC line voltage. It turns out however that, during the early years of this filament supply technique, the available tubes, setup in typical arrangements, could not by themselves handle the entire AC input line voltage. Examine Fig. 1 to see how the tube filaments were arranged with a resistor to properly distribute the voltage among the tubes.
The resistance value of each tube filament, and of the resistor, are designed such that each tube gets it's proper voltage and the resistor gets what is left. The total voltage IS the value of the input AC line voltage. Of course, all of this is under the control of good 'ole Ohms Law.
Until newer tube types became available, and eliminated the need for the series resistor, this system worked pretty well except for one thing - the resistance component dissipated a lot of heat. There were two common solutions to this problem. The more expensive method was to place the resistor in an electron tube style plugable container known as a ballast tube. This "tube", though it got quite hot, was mounted above the chassis and away from most of the other components. The cheaper method was to use a resistance line cord. This cord looked like any other cloth covered AC line cord of it's time, but it also contained a third conductor that was actually a resistive wire (like nichrome) that acted as the required resistor for this type of radio.
There were a few radios of the day that did use a voltage dropping resistor that were typically placed under the chassis near ventelating holes. Even some "modern" tube radios used dropping resistors - particularly where pilot lamp voltage was otherwise hard to create.
The advantage of the voltage dropping line cord was that it would dissipate the generated heat outside of the radio cabinet and eliminated one more component to mount inside the radio. Alas, it had a major disadvantage. These line cords didn't last long due to the effects of heat on the rubber insulation. In fact, it is rare to find any of these cords in good shape today - even unused ones. Since frayed and defective cords of this type are VERY dangerous you will not find them being newly manufactured.
The usual method of repair is to replace the old resistance line cord with a conventional two conductor one and a suitably sized resistor. This method certainly works but it puts us back to square one in terms of the troublesome heat dissipation. Then there's also the problem of where to safely mount a power resistor inside a cabinet not designed for such things.
There is another way!
Simply put, we can substitute the voltage dropping resistor directly with a capacitor. Using Fig 2 let's take a look at how this technique works.
As the input AC current passes through the tube filaments it charges the capacitor, first in one direction then the other. The rate of this charging, and the average value of the resulting current flow, is in direct relationship to the filament resistances and the capacitor value. As mentioned above, the voltage across each tube would be calculated with Ohm's Law as: tube VOLTAGE = capacitor CURRENT times filament RESISTANCE. Since the trick is to calculate the size of the capacitor for a particular tube lineup (not hard but tedious), we tip our hat to the Hagrman brothers for providing us both specific part values and testing information.
The capacitor must be a special type. It's a non-polarized electrolytic. It's non-polarized to handle the AC current, and electrolytic because of the relatively high capacitance value required. Although it's technically possible to use back to back electrolytics in this service, Dick says that experience shows the ready made non-polarized capacitor is the most reliable type.
In the case where your radio has a tube lineup whose total filament voltage doesn't match one in Fig. A, we suggest you use the following testing and capacitor value locating procedure;
By-the-way, this technique works equally as well for defective ballast tubes that can't be replaced (I have however found exact replacements at Antique Supply in Tempe). Once again, you may have to deal with a pilot light shunt in this device as well.
Note: n (6.3 V.) = quantity of 6.3 volt tubes in radio