Batteries and 1920’s AM Radios

Restoring the Freshman Masterpiece AM radio was a lot of fun, but using batteries wasn’t. They’re convenient for testing, but expensive for regular use. The “B” battery alone uses ten 9-volt batteries, and that’s over $20 per set. There are battery simulators available for old tube radios, but they’re not cheap either, and where’s the fun in just buying a plug-in power supply? Wouldn’t it be cooler to use solar power and a 12-volt rechargeable battery?

The Freshman Masterpiece, like most radios of the era, used two batteries: a 6-volt lead acid “A” battery for the tube heaters, and a “B” battery that supplied 45 and 90-volts  for the tube plates. A single, solar-powered device for all those voltages would be better.

A Battery Simulator for the Freshman Masterpiece

Originally, I wanted to use DC-DC converters, inexpensive devices that change DC voltages with switching technology.  A DC “Buck” converter would provide 6-volts and a “Buck-Boost” converter would supply the higher voltages. These devices are super efficient — as much as 95%! The “buck” converter (about $2 each), and the “buck-boost” model (about $13) would have worked well except the radio mixes  positive and negative grounds for tube heaters and plates. DC-DC converters do not provide isolation, and would have resulted in a dead short on the 12-volt battery.

Then I realized that a small inverter could provide isolation for one of the supplies, making it possible to mix positive and negative ground systems. The high voltage supply was the best candidate for this approach since only a couple watts were needed at 90 volts. Looking in my archive, I found several examples of low power inverters that used filament transformers to produce 120 or even 240-volts A/C. The simplest versions used only a few parts. To that, I could add a rectifier and filter capacitor for the  high-voltage supply. Here is the basic circuit:

Refining the Design

Breadboarding was the first step. The schematic I had suggested 2N3055 or similar power transistors, 220-ohm resistors and a 24-volt center-tapped transformer to make an astable multivibrator. I had several small filament transformers in my junk box, and a variety of power transistors. I couldn’t get this first version to work though. It simply wouldn’t oscillate. I tried different transistors and transformers with no success. What was I doing wrong?

A night’s sleep and it finally dawned on me. The transistors I had were much higher gain than the devices of yesteryear. One transistor was turning on and staying on while the other was permanently cut off. I began substituting larger resistors and at about 1 k-ohm, the circuit was working, although with a very odd waveform. I decided to go with it and added a rectifier bridge, filter capacitor and load, and the waveform changed into the desired square-wave.

An littletinkering, and it was operating reliably, although not too efficiently. About 7 watts in to produce 2 watts out. With about 7 more watts to power for the tube heaters, the radio could be powered with a small AGM (absorbed glass mat) battery. Cheap and readily available, AGM batteries are a great choice for 12-volt projects like this and easy to recharge with a 10-15 watt solar panel. One last detail: the Freshman requires both 90 and 45 volts. The addition of a dropping resistor and two 43-volt zener diodes completed the circuit. The dropping resistor value  was determined with the radio connected at 2.2 kOhm, 1 watt.

Building the Battery Simulator

The build was easy and took a couple of hours. Parts were as follows:

• Aluminum “Econobox” (6″ x 3 3/4″ x 2″)
• Prototyping Perfboard
• 120-volt to 12-volt center-tapped filament transformer (Radio Shack 273-1360 — check eBay or Philmore)
• 2, TIP 31C NPN power transistors
•  2, 2.2 k-Ohm @ 1/4 watt resistors (part of an assortment that includes some LEDs)
• 4, 1N4007 rectifier diodes
• 20 uF @ 250 volt electrolytic capacitor
• 2.2 k-Ohm @ 1 watt resistor (part of an assortment)
• 2, 43-volt @ 1 watt zener diodes (part of an assortment)
• DC-DC Buck Converter (package of six)
• SPST toggle switch
• LED (see 1/4 watt resistor assortment, above)
• 300 Ohm 1/2 watt resistor (dropping resistor for the LED)
• circuit board standoffs (to mount the perfboard and DC-DC converter, assortment)
• 3, 1000-1500 pf feedthrough capacitors

There is nothing special about the parts I have listed. A  well-stocked junk box  can help with substitutions. Other power transistors will work in the circuit (eg. 2N3055). A variety of power transformers will work — they just have to be 12-24 volts center-tapped. Boxes, circuit boards, switches, LEDs and so forth can use whatever is on hand.  One tip about DC-DC converters. Since they are switching power supplies, they can produce a lot of radio frequency noise. Not something you want around a sensitive radio. I used a metal enclosure and feedthrough capacitors to trap the  noise inside the box as much as possible.

The Radio Shack miniature transformer I used is just big enough for the job and generates a little heat. To be safe, I decided to use a piece of aluminum sheet, bent to fit in  the case, as a heat sink. A slather of silicone thermal grease between the transformer, sheet and box transfers heat nicely and keeps the transformer a little cooler.

Am Radio Works Great

Late at night, the AM band is really interesting. As I tuned across the band, teenage memories of distant radio stations flooded back. One advantage of our rural home is very little power line noise. The new supply is just as quiet as the batteries had been, and I could hear stations all around the region. New Orleans. Tulsa. Denver. And many others that didn’t identify while I was listening. The Freshman Masterpiece is surprisingly sensitive although not too selective — the nature of its “Tuned Radio Frequency (TRF)” design. Still, a lot of fun. Hearing radio as my grandparents would have in the mid-1920’s while adding a modern twist to the experience. The Magic of Radio.