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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
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