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). A simple replacement for the damaged line cord is to use an in-line power resistor .
[click on image to enlarge]
To determine the correct size power resistor do the following:
(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.
(4) Subtract the total filament voltage (step 2) from 120 volts. This yields the dissipation voltage.
(5) Use Ohm's law to calculate the resistance of the power resistor (R = Dissipation voltage/Filament Current).
(6) Calculate the power dissipation of the power resistor (P=Dissipation voltage*Filament Current). The replacement resistor must have a power rating of this value or higher
For an example radio we might have the following values (RCA T4-10):
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 = 120 - 25.2 = 94.8 Volts
Step 5: 94.8 Volts / 0.3 Amps = 316 Ohms
Step 6: 94.8 Volts * 0.3 Amps >= 28.44 Watts
Power Resistor To Replace Line Cord[click on image to enlarge]
The actual line cord identified in the schematics is 315 ohms.
An example replacement power resistor would be a resistor with
315-325 ohms, 30 watts.
Mount the resistor where it can get air flow and the heat from
the resistor will not adversely affect other components.
Note that many radios cannot dissipate the additional heat
generated by the power resistor.
