Powering Your Antique Battery Radio

420 28/08/2021

Powering Your Antique Battery Radio

Many antique radios run on batteries. These include

tube portables

, such as the Zenith model K-401 shown below, and

"farm" radios

, which were used in rural areas where there was no other source of electrical power.

This article gives you some history on battery use in old radios and advice on how to power them with modern batteries. It also gives plans for two inexpensive battery eliminators that you can build, with additional suggestions and technical data.

History of Battery Development

All early radios used batteries—as many as three batteries in the earliest sets. These batteries were known as A, B, and C. Radio engineers soon designed circuits to eliminate the C battery in a typical radio circuit. That left two battery supplies, A and B.

Rechargeable nickel-cadmium battery packs didn't exist in the "good old"days, but some owners of "farm" radios used rechargeable leadacid batteries, of the type still used in cars. Often, the only availablebattery was taken from the family truck or car. When the battery ran down,the farmer could simply hook up the battery to a generator and recharge it.For the portable radio user, the price of mobility was a large, heavy,disposable zinc-carbon power pack.

Early battery sets had several drawbacks. A dead battery could leave you radio-less in the middle of a crucial broadcast. Lead acid cells could leak acid, which might drip out of the radio cabinet onto your lovely Persian rug. Worst of all, if you accidentally reversed the A and B battery connectors, you could fry your radio's precious tubes.

Recognizing these problems, radio makers, many of which also made and sold tubes, sought to develop battery-less radio sets. Perhaps more radio tubes could do part of the job of expensive disposable batteries.

Radio tubes offer two important features. A tube can act as an amplifier,taking a tiny voltage, such as an incoming radio wave, and increasing itsufficiently to be heard through a headphone or speaker. A tube also canact as a diode, which changes alternating current (AC) into a series ofhalf-cycle pulses that approximate the direct current (DC) that flows froma battery.

Alternating current became increasingly available in homes during the 1920s, and radio engineers soon developed new radio tubes, called rectifiers, which could convert AC to DC. Soon, the stores were filled with battery-less radio sets using rectifier tubes. Everyone loved these new radios, except the battery manufacturers.

Having recently lost their bid to develop electric cars for the auto industry, battery makers now faced another blow: the loss of the radio industry. Many manufacturers closed their doors. Others developed smaller zinc-carbon batteries to replace bulky lead acid cells. Smaller batteries made possible small hand-held flashlights, which became very popular. Everyone wanted one or two or three, and these new products used lots of batteries—flashlight batteries.

Soon, consumers were carrying flashlights to places where there was no electricity, such as a cabin in the woods or a treehouse in the backyard. When you got to such a destination, wouldn't it be nice to have a radio to listen to?

Radio manufacturers seized the opportunity to offer something new. On the technical side, new radio tubes were developed to operate on the lower voltages supplied by flashlight batteries. On the marketing side, stylists developed radios with a new appearance.

Many new portables were disguised as expensive luggage. Their cabinets were made of lightweight wood covered with colorful airplane cloth (see


), canvas, leather, or leatherette (see


). Others were made of metal, Bakelite, and other early plastics (see

Tube Portables


The results were spectacular. The new portable radios sold in great numbers, and many still exist today. To use a battery radio in the 1990s, of course, you need the right batteries, or some substitute for them.

Using Modern Batteries in Old Radios

Battery radios offer great bargains for modern collectors. Not everyone knows how to power them, so they often sell for less than their AC-powered counterparts. Many of them are still in amazingly good shape. Daunted by the expense and inconvenience of using large, costly, non-reusable batteries, many owners simply stored their radios in a closet or shelf, where they remained untouched for years.

Some times, you may find an original battery still inside an old radio. Although nice for display purposes, these batteries are invariably dead and cannot be recharged. This website shows a few examples of such

old batteries


Battery radios that use two batteries (A and B) rather than three usually have polarized connectors, which prevented users from making wrong connections. If you are powering such a radio with some other source, such as the battery eliminator described below, you may be fastening wires to the connector with alligator clips, or bypassing the connector entirely.

Understanding A and B

To avoid costly mistakes, work slowly and carefully when you power up a battery radio. It will help to understand a little about the difference between the A and B power supplies.

The A supply provides low-voltage DC to heat the filaments inside the radio tubes. It can be as low as 1.5 volts.

The B supply provides higher-voltage DC for the "plate" circuits of the radio. The B supply can be 22.5, 45, 67.5, or 90 volts.

Why the difference in voltage between A and B? The answer has to do with the way that tubes work.

When you connect the A battery, the filament of the tube is heated torelease negatively charged electrons. When the B battery is connected, itputs a positive charge on the plate of the tube. Electrons travel throughthe partial vacuum inside the tube, flowing from the filament to thepositively charged plate. Many tubes also have small structures, known asgrids, between the filament and the plate. The grid regulates the number ofelectrons that strike the plate.

Thus, every radio tube must be supplied with two different voltages (A andB), and most will need three (A, B, and C). The A voltage heats thefilament to release electrons. The B voltage gives the plate a positivecharge to attract electrons from the filament.

The C voltage lets the gridregulate the flow of electrons from filament to plate. As noted earlier,modern radio designs eliminate the need for a separate C battery. If yourantique radio requires a C battery, check out the building plans near theend of this article.

The voltage required from the B battery depends on the size of the charge needed at the tube's plate. Circuit designers calculate the needed charge using formulas such as Ohm's law (V=I*R), which says that voltage pressure is equal to the product of the flow of electrons through a known amount of resistance.

The plate current of a tube is very small compared to its filament current.That is why filament (A) batteries, despite their lower voltage ratings,often are much larger than plate (B) batteries, which have higher voltageratings. As a result, the filament batteries are exhausted more frequentlythan the plate batteries. If your battery portable quits working, tryreplacing the A battery before the B battery.

Connecting an A Battery

Many battery-powered tube radios require only 1.5 volts for the A supply, which you can provide with ordinary 1.5-volt "D" cells (flashlight batteries). If more than 1.5 volts are needed, connect additional batteries in series. Two 1.5-volt batteries in series will provide 3 volts DC, and so on. Radio Shack and other retailers, sell inexpensive holders that simplify connecting multiple batteries.

If onboard space limits the size of your A battery, consider using smaller-sized "C" cells rather than "D" cells. "AA" cells can also be used, but you may be disappointed with their short operating life. They are too small to power a radio for more than a short test period.

Connecting a B Battery

Although you can still buy manufactured B batteries (see our

Parts page

), these large cells have several drawbacks. They are expensive, costing anywhere from $10 to $30 each, and they don't last long. Under normal conditions, you may get no more than four or five hours of service from a standard zinc-carbon B battery. Batteries also contain hazardous, corrosive chemicals that make safe disposal an environmental issue.

If you connect a new B battery to your radio, be sure to make the right positive and negative connections, especially if the battery terminals don't match the connectors in your radio.

A simple substitute for the traditional 90-volt B supply is to connect ten 9-volt number 216 "transistor radio" batteries in series. These batteries are small and easily obtainable. Their low-current output is quite suitable for B supply applications. If purchased in bulk quantities, their operating cost can be less than buying two 45-volt batteries or one 90-volt battery. The first photo in the next section shows such a homebrew battery pack, with several 9-volt batteries taped together. Although simple to construct, this battery supply will not last long and you will still have the problem of disposal.

Building a B Battery Eliminator

For about $10, you can build a simple circuit that converts 120-volt AC household current into the DC current needed for a battery radio's B supply. This battery eliminator is friendlier to the environment than disposable cells and its cost of operation will be next to nothing. It is designed by Walter Heskes, who has contributed several radio projects and restoration articles to this website (see


). Walter also suppliedall of the technical information for this article.

The following schematic diagram shows the circuit.

All needed parts are available from

Antique Electronic Supply

. If you order everything from AES, substitute a type 1N4005 diode for D1. AES doesn't offer a type 1N4003, but the 1N4005 works exactly the same. Or, you can buy a pack of several type 1N4003 diodes from Radio Shack for about one dollar.

The next photo shows the completed B power supply on the right and a homebrew battery pack on the left. As you can see, the B supply takes up less space, allowing it to fit inside many portable radios. (Click the thumbnail image to see a larger view.)

Parts placement for this simple circuit is not critical.


: this project requires basic electronic construction skills. Any project involving 120-volt household current holds the potential for dangerous shocks. If you are not comfortable in your ability to safely build this project, find a mentor who has the necessary expertise.

In addition to the parts listed in the schematic, you'll need an AC line cord and some kind of connectors to connect the supply to your radio's B battery terminals. As the next photo shows, you can salvage connectors from an expired battery. (Use extreme caution if you dismantle a battery. Battery chemicals can be toxic!)

If you don't have an expired battery with the right size connectors, you'll need to improvise some other sort of connectors, such as alligator clips. Using color-coded leads, red for positive (B+) and black for negative (B-), will help prevent confusion when connecting the supply to your radio. If you're in doubt about where to clip these leads, you'll need to get a copy of the schematic diagram for your radio.

Antique Electronic Supply

also sells schematics.

The following detail shows how Walter used the circuit's bleeder resistors to create the structure for his connector. On the right is the new connector and on the left is the original, matching connector from the radio.

Since the circuit connects to your house curr

ent, you should build it in a small plastic box or other suitable enclosure to eliminate shock hazards. Walter's prototype is enclosed in a small cardboard box that once held a Clarostat variable potentiometer. Ideally, your enclosure should be small enough that it, as well as the line cord, can be contained in the space where the original battery was stowed.

For additional protection while the battery eliminator is in use,you can power it through an isolation transformer. Suitable transformers are availablefrom Antique Electronic Supply and other suppliers.

You'll need to find or make a small hole somewhere in the radio cabinet forthe power cord. Many battery-only portables have no such opening. If yourradio has the common "clamshell" design, one solution is to file a smallrectangular notch in the bottom of the rear cabinet piece, just largeenough to fit the line cord. The next photo shows the notch in Walter's radio.

This is a one-way modification, so keep inmind that making any changes in a collectible radio may diminish its futurecollector value to purists. Some collectors might appreciate the extraconvenience, of course. If you don't want to make an opening, you cansimply leave the cabinet open wide enough to admit the cord when in use,and close it up at other times.

Using Your Battery Eliminator

The next photo shows the completed battery eliminator tucked into the old battery compartment in Walter's radio. The B supply is housed in the small blue box at lower right. It's a snug fit, but the battery eliminator, including power cord, fits completely inside the case when not in use.

The final photo shows Walter's radio ready to plug in and use. This simple addition greatly increases the usability of this fine old radio without diminishing its portability.

When you power off your AC-powered battery set, always remove the plug fromthe AC electrical outlet. The switch in the radio disconnects the A and Bsupplies from the circuit, but it does not remove power to the silicondiode and the filter capacitors, which can remain fully charged. Removing theplug from the AC outlet removes all power to the battery eliminator.

Use an Isolation Transformer

Adding this battery eliminator introduces a risk of shocks that was notpresent when the radio was powered only by batteries. Depending on which way youplug the cord into the wall, the radio chassis may become "hot." If youtouch the bare metal chassis while touching a water pipe or other grounded object, youcan get a shock.

To eliminate this risk, you can plug the battery eliminator intoan

isolation transformer

. Ready made isolation transformers are readily available, although new ones maybe rather expensive. A cheaper solution is to find a used RCA Isotap, which wasdesigned for servicing radios and TVs. I got mine for a few dollars at a radio swap meet.

If you're able to add your own plug and socket, Triad makes an inexpensive


suitable for small radios. You can also build an isolation transformer from scratch,a topic that has been discussed often in the

Antique Radios


Incidentally, the same risk is present in old "AC/DC" ("series string")tube radios that did not use a power transformer. Elsewhere in this websiteis an


explaining how to modify those radiosfor safer operation.

How the Battery Eliminator Works

This circuit is a solid-state version of the classic power supply used incountless AC-powered tube radios. Here is a brief description of how it works.

Capacitor C3 provides a small amount of AC line filtering. It will preventline noise, such as that caused by AC motors in refrigerators, fromentering the B+ supply of the radio and interfering with radio signals.Capacitor C3 can be omitted if line noise is not a problem in your house.This was a bigger issue in the days before polarized plugs and groundedoutlets became common. Without this capacitor, if a user encountered linenoise, the common solution was to reverse the plug in the wall outlet.

Resistor R1 offers a small resistance equivalent to the internal resistanceof a selenium rectifier. (Selenium rectifiers are early solid-statedevices, used in many 1940s and 1950s radios). The extra resistance offersa small margin of surge voltage protection to the capacitors in the R-Ccircuit. The surge occurs when the power supply is first powered up. R1 canbe omitted, if desired.

The diode D1 acts as a rectifier, converting your household AC power into DC.

Resistor R2 is an essential part of the R-C network that filters theincoming pulses of current from the diode. Electrons encounter resistanceas they try to enter R2 and most of them are forced into C1, which fillswith electrons until it reaches its capacity. Then, the remaining electronsare forced through R2. The inertia of the R-C network absorbs the impact ofthe pulsing current and produces a steadier voltage that has the propertiesof DC.

Capacitors C1 and C2 eliminate ripple from the rectified DC voltage. C2 iscalled the smoothing capacitor.

Resistor R3 is a bleeder resistor that places a load across the power supply, reducing the output voltage to the right amount.

Modifying the Eliminator for 22.5 or 45 Volts

The schematic shown above gives the values for a 90-volt or 67.5-voltsupply. Some radios require 45 or 22.5 volts of B+ current. To lower thepower supply voltage to the desired value, you'll need to make a voltagedivider. Adding a pair of 5-watt wirewound or carbon resistors in seriesacross the output from B+ to B- easily does this. You'll need to use amultimeter to determine the correct values for these resistors.

First, build the circuit as shown in the schematic.

Next, follow these steps to determine the values of the two resistors:

Power up the supply.

Measure your actual output voltage. For example, you may read 160 volts.

Divide the required B+ voltage by this voltage. Say, you want exactly45 volts. 45/160 = .2813.

Multiply the dividend by 10000. In this example, .2813 * 10000 = 2813.

Find a 5-watt resistor as close to this value as possible. In thisexample, find a 2800-ohm resistor. (Or, find a 2000-ohm resistor and an 800-ohm resistor and wire them in series; both must be rated at 5 watts.) Callthis resistor R3.

Subtract R3 from 10000. In this case, 10000 - 2800 = 7200.

Find a 7200-ohm 5-watt resistor. (Or, find a 7000-ohm resistor and a200-ohm resistor and wire them in series; both must be rated at 5 watts.).Call this resistor R4.

Connect R3 in series with R4.

Attach the R3 and R4 resistor assembly across the output of the power supply (that is, wire them between the points labeled B+ and B- in the above schematic).

Attach your DC voltmeter across resistor R3. You should read the desired voltage (in this example, 45 volts).

If the voltage is correct, attach the leads for your power supply across resistor R3.

That's all there is to it!

Further Suggestions

After the first publication of this article, I got more suggestionsfrom members of the


newsgroup.Peter Weick offered several interesting ideas:

1. Put two heavier diodes in series. Then, if one fails, the other will act asa back-up. Feeding AC to any battery radio is not so great an idea. 2. A polarity-indicating LED (the kind that is green in one direction, red inthe other, and yellow when seeing AC) would be very helpful at the output endof this circuit as well.3. A neon indicator lamp to let you know when there is current flowing is alsohelpful. 4. An On/Off switch fed from a POLARIZED cord/plug helps.5. A "fast" line-fuse would be helpful. It would not add any personal safetymargin, but it MIGHT protect the downline equipment against diode failure.6. Running the whole shebang off of a small isolation transformer would be thebest personal safety precaution of all. My little local junk shop has a fewsuch rated at about 600 ma @ 125V (75 watts) for about $8. They are smallenough and cheap enough that even three of them in a box would not be too-toomuch given the added safety. BTW, my battery eliminator is made up of a box with two of these isolating asimilar series of dropping resistors. I have also added regulation and zenersfor the filament-string supply(s) as I use it on some older battery TRFs aswell. A couple of toggle-switches on top of the box to vary the outputvoltages depending on the radio... and there I am. Of course, this is no longer a $10 item.... more a $25 item. Still cheap at theprice.Peter Wieck

Building an A/B/C Battery Eliminator

Neil Sutcliffe provided some other interesting information. The first is a two-pagetechnical sheet for the RCA CV-12 power supply, which dates from around 1940.The CV-12 unit provided both A (1.5-volt) and B (90-volt) power; if you are an advancedexperimenter, you might enjoy building a modern replica. Here are images of theCV-12 technical sheets:

Finally, Neil provided a schematic for his own battery eliminator, which providesan A supply of 1.5 volts, a B supply that you can set for 22, 44, 66, or 88 volts, and an optional C supply. His design notes appear after the schematic link.

Design notes on Battery Radio Power SupplyIn a large majority of battery operated sets, the A, B and C (if used) batteriesmust be electrically isolated from each other to allow for the many differentconnections of A and B battery polarity required by various radios.This supply was originally designed for dry battery "farm" radios from themid-1930s and later. In those radios, it was most common for the B- voltage tonot return to the chassis, but to be "lifted" by about 560 ohms to develop the small negative C bias voltage required by the AF output tube. This was done because the direct heated cathode could not be lifted from ground as done in AC operated sets.Therefore, when building or adapting this design, be sure that each output circuit is mutually isolated from each of the others. Remember, too, that the heat sink tab on the LM317 is connected internally to the circuit and must be insulated from any circuit connection.While the Hammond 229A12 is a good choice for the power transformer, it can be a bit pricey. There are options if one wants to use the contents of the "junqueboxe" or wants a bit more output power for tubes like the '01A or '99. For morefilament power, one can choose a filament transformer (low voltage rectifier transformer) with a "resistive" rating of at least twice the DC current neededand about 4V (or more) higher voltage rating than the desired filament voltage.Even though the specified transformer is rated at 6.3VAC per secondary and the ideal rectified voltage appearing on C1 should be 6.3 x 1.414 = 8.9V, one must allow for the forward drop in the bridge rectifier diodes as well as the IR drop in the transformer and still have ample overhead voltage for the LM317 even at the trough of the ripple. Also consider that the excess voltage betweenthe raw DC on C1 and the output voltage must be dissipated in the LM317 regulator. Thus, as the load current requirement increases, so does the need for proper heat sinking of the regulator.Another source for inexpensive isolation transformers for the B+ section is the transformer used in old bathroom razor outlets. These are commonly foundat garage sales for a couple of dollars.I have added an optional isolated "Bias" supply to the schematic at the requestof a restorer wanting to power a Radiola III. Such a supply is also needed ina good number of other early battery radio sets, well into the 1930s. Its circuit is identical to the filament supply except that the voltage setting resistor is selected for the desired voltage up to about 5V.VoutRselect 2 160 3 360 4 560 5 760These values also apply to the filament supply if one wants to increase the output for 2V tubes such as the '30 and 1H4G, but much over 2V will require anincrease of the transformer secondary voltage as mentioned above. If you increase the filament voltage from 1.5V, omit the three "safety diodes" acrossthe filament output.Neil Sutcliffe, (C) Copyright 2010

Thanks to Peter and Neil for offering these additional ideas and informationsources.

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