A Great DC Converter for Mobile Tube Transmitters
A great solid state power supply for mobile tube transmitters
Back in the day . . .
I’ve always been fascinated by tube transmitters. My earliest memories of Ham Radio involve lots of “big iron” and vacuum tubes. The lovely glow of the filaments and the gentle warmth they provided in my chilly midwestern shack tickles the nostalgia bone pretty hard.
Don’t get me wrong, as I became aware them, I lusted after the newest solid state devices like High Fidelity stereo amps, and tiny portable 2-Meter handle-talkies. But nothing was quite as satisfying as a contact made with a tube transmitter.
Back to the Future
Of course today, my shack is all solid state. It works well, and I’ve made lots of great contacts with it. Still, it would be fun to get a “hollow-state” transmitter back online. I’ve got a couple of 50’s era transmitters awaiting restoration, but those are long term projects. Not something one can do in a week or two.
A “starter” restoration project
Recently, I noticed a Johnson Viking Mobile for sale on eBay. I had never even heard of this model, and the price was right. Very small, about the size of a shoe box, it looked like a relatively easy restoration project — a way to “get my feet wet” before diving into the more serious projects.
The original mobile station included a receiver and a dynamotor to provide the necessary high voltage for both units. Dynamotors, small devices with a low-voltage motor driving a high-voltage generator on the same shaft, are not easy to find. In fact, I’ve never seen one on the auction sites.
Power requirements
The Viking Mobile power needs are modest. 6-volts for the tube filaments, 275-325-volts for several miniature tubes and 300-500 volts for the 807 audio drivers and finals. With 500 volts on the plates, it can deliver over 30 watts of AM on 80, 40, 20, 15, and 10 meters.
We can build it better.
My first thought was a simple, transformer-based power supply that would run on 120-VAC. But it seemed a shame to spoil the mobile capability of the transmitter. After all, Mobile is in its name. Also, my entire shack runs on solar-generated 12-volt DC power. It would fit right in.
A DC Converter
Then I noticed a “new old stock” Triad TY-83 toroidal transformer on eBay. I remembered these transformers from back in the day. There were several models, each capable of converting low-voltage DC power into the high voltage DC needed for the mobile equipment of the day. Why the Triad TY-83? Examples of homebrew DC Transformers can be found in old QST (December, 1964, pg. 17) and 73 Magazine (December, 1969, pg. 70) show how to use a filament transformer and a couple of transistors to generate the high voltage needed for tube designs. These were latest in solid-state technology (circa 1960’s) for powering tube-based mobile rigs.
The best solution (in 1979)
The TY-83 was tailor made for my application. What sets it apart from the homebrew designs is its efficiency. Whereas a filament transformer runs at power-line frequencies of 60 Hz., these toroidal transformers run at around 2000 Hz. That makes for a much smaller inductor and more efficient power conversion. Capable of producing 500-volts at 250 mA, or 250-volts at 500 mA, I could see how an “economy” design, where both voltages are produced from a center-tapped transformer and a diode bridge, would answer my needs precisely.
Breadboards and lessons learned
I started by breadboarding the circuit. High-power germanium transistors aren’t common, and I had several silicon power transistors in my junk box. With higher current capacity and gain than the recommended 2N277 or similar, I reasoned it would be a slam dunk.
Still, a couple hours of tinkering and no joy. It simply wouldn’t oscillate. Was there something about the germanium transistors that was critical? Only one way to find out. To eBay . . . !
High Power Germanium is the answer
A couple of days later, I had two kinds of germanium transistors. New Old Stock 2N1358’s, and an interesting pair of gold 2N277 transistors mounted on a gold anodized heat sink. Substituted into my breadboarded circuit, both worked with the first try.
I’m not enough of a solid state engineer to know exactly why, but I theorize that the considerable leakage germanium transistors exhibit might be the reason. In any case, I was now in business. As advertised, the TY-83 was providing over 500 volts with a 20 K-ohm load.
Making it small
I wanted it to be as small as possible, and found an interesting aluminum project box on Amazon. With a slightly larger footprint than the head sink and about 3-inches high, it would make a compact unit.
Many of the parts were found in my junk box, but I resorted to eBay for some 5-watt low-ohm resistors, high voltage filter caps and a couple of terminal strips. Surplus Sales provided an extremely rugged multicore cable and octal Amphenol jack to connect the power supply to the transmitter. Cable strain reliefs came from Amazon.
A “buck” converter for the filaments
Providing 6.3-volt power for tube filaments was a little more challenging. The transmitter was originally intended to run in the 6-volt cars of the 50’s, and I wanted to supply it with 12-14 volts. The solution turned out to be a “buck converter.” Fed with anything from 3.5 to 35 volts, these little wonders can provide anything from 3.5 to 25 volts out — regulated and current-limited. They’re tiny as well, about the size of a pack of gum. Amazon had a model capable of passing up to 4 amps for $10.
The Build
I’ll let the pictures do the talking regarding the build, but suffice it to say, it wasn’t easy to fit everything into the chosen box. I felt more like a watch maker than an electronics tech.
Solid power output
It worked right away, and as we’ll see in the coming posts on the Viking Mobile transmitter provided all the power needed. 550 and 280 volts for plates and gain stages, and 6.3 volts about around 4 amps for the tube filaments. It did get rather hot though.
Is 165 degrees too hot?
The 2N277 transistors can function continuously at up to 95C. I measured the heatsink at about 164F (74C), But that’s hot enough to burn yourself. Maybe I had used too small a case and heatsink. Not having a better idea, I got a small muffin fan and strapped it to the heatsink.
A “muffin” fan solves the problem
Bingo! Amazing what a fan will do. The heatsink stays well below 90F (32C) with the transmitter fully loaded. The 6-bolt buck converter seems to be running at an acceptable temperature as well.
Stay tuned for a post or two regarding the Viking Mobile restoration. It worked out fine, but you’ll have to read the details when I get it posted. Gotta love those retro updates — bringing a 50’s era transmitter into the 60’s!
