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In AC/DC receivers, the tube filaments are connected in series. Since the voltage drop across all the filaments in series may be less than the full 117 volts from house’s line voltage, it is necessary to get rid of the excess voltage There are three different means for dissipating this excess voltage:

  1. Power Resistor
  2. Line-cord Resistor
  3. Ballast Tube

Usually it is an ordinary power resistor getting rid of the excess voltage Find a replacement with the same resistance and power ratings Higher power ratings can be used for more reliability. Power resistor is a good replacement for the hard to find line-cord resistor.


Usually a two-prong power plug with three-wire line cord. The third wire is a resistance element to get rid of the excess voltage. Since a line-cord resistor is used in series with filaments, if it is broken the radio’s tubes will not light up In some situations it might be possible to fix the broken line resistance When it is not possible, a power resistor can be used to replace the damaged line-cord resistor. The following steps will help you to find out the specifications needed to replace a line-cord resistor for a power resistor.

  1. Look up in a tube manual the filament volts and current of the tubes
  2. Add up their voltages
  3. Subtract the total filament voltage from 117 volts to get the excess voltage
  4. Use Ohm’s law to calculate the resistance (R = E/I) of the power resistor
  5. Calculate the power dissipation (P=EI) of the power resistor
Example: Radio RCA Moldel 4X3
6A7G 0.3 amperes 6.3 volts
6F7G 0.3 amperes 6.3 volts
43 0.3 amperes 25 volts
25Z5G 0.3 amperes 25 volts
_____________ _____________ _____________
Calculations 0.3 amperes 62.6 volts
E = 117 - 62.6 = 54.4 volts R = 54.4/0.3 = 181 ohms P = 54.4 x 0.3 = 16 watts

Thus, 181 ohms and 16 watts are the specifications of the power resistor. In practice a 20 to 30 watts resistor can be used to increase realiability. If necessary, combine two or more resistors to get the correct values. For example, a 300 ohms at 15 watts in parallel with a 470 ohms at 15 watts will yield a total of 183 ohms at 30 watts.

Last modified: Mar 6, 2021 21:49
Cape Old Radio - Since Nov, 1996
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