Older radios that do not have an input transformer sometimes utilize a line-cord resistor. These radios are easily identified by their two-prong power plug with a three-wire line cord. The third wire is a resistance element providing a series voltage. The line-cord resistor is used in series with filaments, and is often brittle or broken preventing the radio's tubes from operating (they won't light up). In some situations, where originality is desired, it might be possible to fix the broken line resistance. Due to their age and the fact that they heat up, I would consider replacing the line cord with a two wire cord (or for an original look a 3-wire cord with the 3rd wire not connected). Method 1 discussed replacement for the damaged line cord using an in-line power resistor.
As seen in the Method 1 example, the power resistor dissipates significant heat, and is not useable in many applications such as: plastic case radios, very small enclosures with little or no air flow and situations where original appearance of the radio chassis is desired.
Resistive Line Cord
[Click on Image to Enlarge]
[Click on Image to Enlarge]
This second method avoids the heat dissipation problem by utilizing a capacitor in series with the tube filaments.
To determine the capacitor size perform the following calculations:
(1) Determine the filament voltage and current of the tubes used in the radio.
(2) All the tubes in the series string should have the same filament current.
(3) Sum the voltages (Vf).
(4) Determine the vector voltage (capacitor voltage is 90 degrees out of phase with the line voltage) like this: SQRT(120*120 – Vf* Vf). Trust me, this yields the dissipation voltage. I use 120V for the line voltage since the original design was based on that value.
(5) Use Ohm's law to calculate the reactance of the Capacitor (Xc = Dissipation voltage/Filament Current).
(6) Calculate the capacitance: 1 / (2*PI*f*Xc)
For the same example radio from Method 1 (RCA T4-10), we have the following values:
Step 1: Filament Current = 0.3 Amps
Step 3: Voltage Sum = 6.3 + 6.3 + 6.3 + 6.3 = 25.2 Volts
Step 4: Dissipation Voltage = sqrt(120^2 - 25.2^2) = 117.3 Volts
Step 5: 117.3 Volts / 0.3 Amps = 391 ohms
Step 6: 1 / (2*3.14*60*391) = 6.78 micro-farads
So we would use a 6.8 micro-farad capacitor. The voltage rating of the capacitor must be greater than .707*120 = 85 Volts (standard values are 100 & 200) I personally use 200 Volt mylar (polyester) capacitors.
In-Line Cap to Replace Line Cord[Click on Image to Enlarge]
We don't want to reduce the life of the tubes.Carefully measure your total filament voltage and adjust the capacitance value for your application. It is better to run the tubes at a lower voltage than a higher one. Do not exceed the sum of the filament voltage of the original design, this will shorten the tube life.
The capacitor can be mounted anywhere since there is no heat dissipation concerns.
Note: you CANNOT use polarized electrolytics for this application. Mylar capacitors rated at 100V DC and greater work quite well (I prefer the higher DC voltage caps to stay on the safe side).
