THIS ARTICLE IS WRITTEN WITH THE
ELECTRONIC TECHNICIAN IN MIND
Those of us who have serviced, restored and repaired radios made in the 1930's are quire aware the problems associated with the replacement of AC line cords used in many of the small AC/DC radios made in the early - mid 1030s'. The cords this article is referring to are the ones that have the voltage dropping resistor built into the line cord.
Reason for the resistance line cord. The tubes used in these radio receivers were series strung to eliminate the need of the expensive, heavy and space consuming power transformer. The tubes used were the typical tubes of the era. The line voltage was significantly higher that the sum of the needed heater (filament) voltage so a method of dropping the voltage without the use of a power transformer was in order. One method was to use a dropping resistor. Dropping 60 or more volts resulted in a power dissipation of 18 - 20. This was not acceptable in the small compact cabinets that housed most of these early mantel radios. If you don't think 20 watts worth of heat is much of a problem, see how long you can keep your hand on a lighted 25 watt light bulb.
Origin of the resistance cord? I do not know but somewhere, someone came up with the idea of stringing the resistance element in the power cord. The resistance of 150 ohms or so would spread the heat dissipation out over about 6'. So spreading 20 or so watts over 6 feet would average about equal 3 - 4 watts per foot. That shouldn't be a problem - huh? Well maybe not if folks would read and follow instructions. Who knows how many fires where caused by the line cord overheating because of the cord was buried under a rug or wrapped up tight in a ball? Another reason for the cord overheating would be capacitor failure in the radio putting the full line voltage across the line cord resistance.....and capacitor failure was very common in the early days of radio. In fact I will state that capacitor failure is the single most common problem one will encounter today when servicing these early radios (or any early piece of electronic equipment), more so than tube failure.
My solution: I install a vintage or vintage looking 2 wire AC line cord. I replace the resistor element with a capacitor (or capacitor of the appropriate value depending on he need line voltage drop. You cannot use just any capacitor, electrolytics are an absolute No No! AC type capacitors rated for high current at 400 volts (AC) or more are in order. They can be ordered from Allied Radio or other sources. There is a complicated formula (which I won't supply) for determining the proper capacitance for the need voltage drop. I keep a supply of these capacitors on hand ranging from 1 - 10 uf @ 400 - 600 volts and use the tried and true practical method to determine the capacitance value.
Here's how I do it: After I've cleaned and replaced all the needed parts required, such as all new capacitors, resistors, tubes etc. I alter the AC line cord wiring to accommodate the line resistor within the radio chassis (it was in the line cord). This can be tough on some of the small sets but instead of a large heat consuming resistor, I use capacitance. One place you can usually find room is where the original electrolytics where mounted. The electrolytics can easily be replaced with tiny modern ones under the chassis without concern for space. Now to choose the capacitor size. The capacitors used for the heater voltage drop are a bit large and can look out of place if cosmetics are concerned. I solve this issue by housing them is a box that I fabricate to look like and electrolytic package (see photos below).
Determining capacitor value:
There's a somewhat
complex formula for determining the capacitor's value but I find
it easier to just clip lead in a capacitor of a smaller value and
keep bridging it until I obtain the correct heater voltage on one
of the lower voltage tubes.
The voltage drop needed will vary depending on the radio brand, model and tubes used. First add up the voltage the tubes require. I'm going to use the Emerson model Q-157 as an example. The 'Q has a tube line-up of; 1-6C6, 1-6D6, 1-25L6 & 1-25Z5 (see schematic). So, the tube heater voltage equals 62 volts, this means we have to drop around 60 volts. There's already a 10 ohm resistor in series with the heaters which will drop about 6 volts, so that leaves about 52 volts we need to drop.
Since I have already restored all the electronics it the radio, it's essentially ready for power up if the line dropping device was in place. Next I connect a pair of clip leads in the circuit where the capacitance devise will be wired. I then clip a capacitor of around 2 uf into the circuit and measure the filament of one of the tubes and monitor it's voltage while determining the needed capacitor value by adding more capacitance (if needed). Be sure and allow the measured voltage to stabilize as it take a few seconds for the tube and capacitor to become fully stable.