The Radio Ether Blog

If you have been following this recent set of articles, you certainly know the dangers presented by AC/DC radios. What is an AC/DC radio, you may ask? Well quite simply: it was an early (and long-lived) attempt to produce inexpensive tabletop radios for the masses. These radios omitted the voltage-dropping power transformer due to its cost and weight. In its place they provided cheap, sometimes dangerous alternatives.

Note: AC/DC radios present a very real shock hazard in most of their incarnations due to the direct connection between the chassis and the 120VAC source.

One alternative used a resistive line cord, often called a “curtain burner” for its habit of getting very hot and igniting some unsuspecting nearby combustible. This technique uses a resistive wire connected to the 120V source on one end, and the tube filament circuit on the other. These resistive wires dissipated between 20 and 30 Watts depending on the radio design. For an idea of how hot that would be, place your hand near a 30W light bulb. Due to the heat dissipated by the wire, the wiring would get brittle and cracks would develop. Now think about laying paper or cloth (like those ceiling to floor curtains) on it and preventing adequate ventilation for heat dissipation. If enough heat builds up — whoosh — flames and smoke! Or, if you were lucky, the wire would break (open circuit), the radio would cease to operate, and a costly repair ensues. A previous article addressed the resistive line cord, its dangers and safe, suitable repair and replacement alternatives. Other manufacturing methods, to reduce tube filament circuit line voltage, included the use of a power resistor and best of all was the ballast tube. The remainder of this article discusses the use of ballast tubes. Ballast tubes are of three types: The current regulator, voltage regulator, and line ballast.

The current regulator tube generally found use in battery operated radios where a constant current was required for parallel connected filaments. It provided some regulation of the battery voltage at high voltages but, as the battery voltage drops, it had the additional property of automatically adjusting its resistance to maintain a constant current to the tubes.

The voltage regulator, as the name implies, is designed to maintain a constant voltage drop regardless of current variations within the set. These are not generally encountered in home broadcast receivers.

Line ballasts represent the most frequently encountered type of ballast tube for the antique radio enthusiast. These ballasts are found in many of the common AC/DC radios built during the tube era. Their first, and most important job is to provide the necessary voltage drop to properly supply the series-fed filament circuit. They reduce the line voltage via a resistive element within the tube. This resistive element has an important additional characteristic: as the line voltage increases, the element (and the tube) temperature increases, which in turn increases the element’s resistance, providing an additional voltage drop. As the voltage decreases, the element cools, the resistance decreases, and the voltage drop decreases. This characteristic provides a nearly constant voltage to the filament circuit.

The RMA Ballast Code Standard

The introduction of the drop-in ballast resistor to replace the resistive line cord resulted in each manufacturer making his unit slightly different from other manufacturers. Some use 4-pronged bases, while other use octal bases. The was no standard numbering system. This non-standardization meant that many different versions were manufactured which made it difficult for the radio service man to service receivers with these units.

The RMA Code was an effort to alleviate this non-standardization. It was an attempt to standardize the circuit arrangements and reduce the number of different circuits.
The RMA system of type numbers consists of three main parts and a possible supplementary prefix letter and suffix letter. These are:

• First: a ‘letter’ (or letters) designating the type and current rating of the pilot lamp (or lamps) and the line current the unit is designed to be used with.
• Second a ’series of digits’ indicative of the overall voltage drop across the entire resistor system comprising the unit (including the pilot lamp or lamps) when 300 millamperes (0.3 amp) flow through it.
• Third: a ‘letter’ indicating the circuit arrangement of the resistor elements comprising the unit and the base pins of the unit (see figures a-k below) .
• Fourth: if the unit provides ballast action on the dial lamp section, the prefix letter ‘B” is used at extreme left.
• Fifth: if the unit is of the octal-based glass type, a suffix letter ‘G’ is placed at the extreme right. No suffix is used when the unit is of all-metal construction.

More information on ballast tubes can be found on this site at:

All About Ballast and Resistor Tubes
Radio-Craft, January 1939

Caution: Antique tube radios carry potentially lethal current levels at very high voltages.

Method 1 - Using An In-Line Replacement Power Resistor

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. One wire acts as the standard return. The second wire acts as the typical “hot lead” for the high voltage circuits. Finally the third wire called a resistive line-cord, is connected on one end to the “hot lead” and provides a series resistive element and it’s associative voltage drop to feed the lower voltage tube filaments.

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 .

Resistive Line Cord

Determining The Power Resistor Value

(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 (RCA T4-10) we discover the following values:

These values can be determined either from schematics, online sources or from a tube handbook. For series connected tubes, the filament voltages of the tubes may vary however, all of the filament currents for the tubes will have the same value.

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

The actual line cord identified in the schematics is 315 ohms. An example replacement power resistor would be a resistor with 315-325 Ohms, and a 30 Watt power rating. 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.

Derating A Power Resistor

Derating this power resistor to account for surges and aging would increase the power rating and cost. Typically, in power ratings, a derating value of 50% is used however, a 75% derated resistor would have a power rating of 30W / 0.75 = 40 Watts.

Method 2 – Using An In-Line Input Capacitor

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.

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 filament 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 capacitive reactance:

(Xc = Dissipation voltage / Filament Current).

(6) Calculate the capacitance:

1 / (2*PI*freq*Xc) ; PI = 3.14159, freq = 60 Hz

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 an DC rating greater than the equivalent RMS voltage across the capacitor ( 0.707*120 = 85 Volts RMS). Standard voltage ratings on capacitors are 100 & 200 VDC. I use the resilient 200 VDC mylar (polyester) capacitors.

In-Line Cap to Replace Line Cord

Don’t reduce tube life
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 considerably shorten the life of your radio’s tubes.

This 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 >100 VDC and greater should work quite well (I prefer the higher DC voltage caps to stay on the safe side).