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It's summertime which means it's time to build little
tube radios again. The summer propagation isn't as good as in the winter, so
I switch to listening to tube sets more than the crystal radios.
This is a one tube regenerative 3 band radio. It is built around
the triple section 1D8GT tube. This tube has a diode, triode and pentode all
under one glass. This is an almost perfect tube for these little radio projects.
I didn't use the diode section in this project. The triode section
is used as a regenerative detector. The regeneration
is controlled by a pot that controls the B+ voltage. This method turned
out pretty well as I am not a big fan of a variable throttle capacitor to control
the regeneration. Too much control interaction. The main thing I learned is when
the tube gain is low, you better have a lot of turns on the tickler coil.
The transconductance gain of the 3Q5 that I used in my
last radio is 2000 micromhos.
The triode section of the 1D8 is 325 micromhos. This means that more feedback
is needed. Took me a while but with the help of my friend Fred Wise, the light
was shed and corrections were made. I also made the grid leak capacitor 250 pf
rather than the usual 100 pf for shortwave radios.
The pentode section is the audio output stage. To keep down the
plate current, I placed a 9 volt battery in the grid circuit for bias. This is
called in the old days, the "C" battery. The spec sheet calls for -9 volts
on the grid when the plate voltage is 90 volts. You can run the output stage
at 45 volts. If you decide to do this, reduce the bias to 4.5 volts. Old batteries
pulled from the smoke detector will work fine for the bias as the current is
zero.
The coil has three windings, antenna input, main tuning and
tickler. I put more turns on the antenna coil than was probably necessary.
There was a fair amount of raspy sound when detecting cw (morse code) signals
. This was cured by the small input trimmer capacitor in series with the
antenna lead. It depends on the antenna you choose to use.
If your set seems to be over driven, take off some turns from the antenna coil.
The tickler coil has lots of turns as I described above. If you build
one of these and it doesn't work, try reversing the tickler windings. I end up
with it wrong about half of the time. I tack solder the wires until I have it right.
The main tuning coil has two taps for three bands. The tuning range
is from below the 80 meter amateur band to the 25 meter broadcast band
(3.5 - 12 mhz). I used a
three position switch to select where I wanted to tune. This brings up another
point about regeneration. These sets don't like to see a large variable capacitor.
I was going to tune everything in one swoop by putting all the sections of
the capacitor together. When the capacitor was at half value, the regeneration
dropped out. I ended up using the 170pf section only. I then put in the switch
and made a three band tuning radio. My radios don't always end up as I start.
The variable capacitor I used has a vernier reduction drive built
in to the capacitor. If you use a direct drive half turn capacitor, then you
might want to put in a 25pf band spread capacitor in parallel with the main
one for easier tuning.
Another point of interest I would like to point out is the use
of a copper board behind the front panel and on the baseboard. This adds
stability to the radio. If this were left off, every time you put your hand
near the radio, it would detune. Plain metal sheet can be used. If you need
something for this project, I may have extras around.
This little radio is cool. I operate it from batteries and use
a high impedance headphones, just as in the old days. At night the band comes
alive with ham radio stations and foreign broadcasters. If you turn off
the lights, you can see the faint red glow of the filament.
73, Dave - N2DS
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