2 Tube Microbroadcaster
I've had this little wooden box for around 3 years and I finally found the right project for it. This is a small two tube medium wave (broadcast band) transmitter. I use the word "transmitter" but it only can be heard for about a meter or so. That is enough for me to use my MP3 player to play some of the old tunes through my old radios.
The idea for this project is from my favorite German tube radio site, Jogi's Tube Shack. There are a lot of pictures on his site, so feel free to click around, even if you don't speak the language (hint: Start with "Neu" on the left side). The project of interest is this Test Transmitter. I liked how the author used the two tubes, modulating the screen of the oscillator with a single stage of amplification. His circuit uses a 12 volt wall wart as his tubes are operated in a "space charge mode". My circuit uses tubes that are designed to work at a maximum of 30 volts on the plate. Each filament runs on 1.25 volts at 10 ma. Both filament and plate are powered by a 12 volt wall wart transformer.
The filaments are powered by dropping the voltage and regulating it with a pair of 1N4001 silicon diodes as a shunt regulator. The third (top) diode is for reverse voltage protection. If The 12 volts is hooked up backwards, the filaments will see much voltage on them. This will immediately burn out the tubes. Before making the final connection to the filaments, the voltage should be checked on the anode of the second diode. It should measure 1.4 volts. Please, (bitte, por favor, tevreden, svp) do this test first! The 10 ohm resistor drops the voltage a little further.
The audio from a headphone level signal comes in to the control grid of the first 6418. The signal is amplified on the plate, dropped through the 10k resistor. The plate of the first tube is connected to the grid of the second tube, the rf oscillator.
You astute home gamer builders will notice that the rf section looks a lot like a regenerative detector. Well, you are mostly right. The rf oscillator transmitter is a close cousin to the regen receiver. The coil is an old 2 winding, 4 terminal oscillator coil from an old tube radio. A good place to buy one of these coils is Antique Electronics Supply. Order P-C77.
I used a padder capacitor, 425-1260 pf, but you can use anything you have laying around like that. A 365 pf air variable will work, except it will tune too high for much of the capacitor.
This is one project that worked right away! Within 15 seconds of turning it on, I was able to hear the signal in a very close by transistor radio. Remember that the range is very short with this rig.
The hex box and having the tubes come up through the panel is just for show. I try to give my projects some visual interest.
Important Information This transmitter may not be legal in some parts of the world. In those parts of the world, this might be called a "test oscillator".
73, Dave - N2DS