|
Ok - go ahead and scroll down and look at all the pictures first.
I'll wait. It's kind
of like having your desert (eye candy) first before the meat and potato's. Have you finished?
Good. Let us continue.
I am proud to introduce you to my first loop tube radio. I had this in mind for a while
and it took me a long time to get started. The radio design was simple, but how was I to make
this into another show piece radio? Do I make it into a 1920's look, or a rock n' roll 50's
radio? This one is both. The design, and parts are something that you might have seen in a
1950's Poptronics magazine. Then there are some hints that have that definite roaring 20's look.
The loop itself is an example as well as the art deco garolite front panel. The wood base
gives this radio a breadboard look. I sure did it all. This puppy kept me up nights thinking
how I was going to build her. I like that curve that decorates the front panel. I used
a large pizza pan as a guide.
The radio is a simple regenerative type using a 6418 subminiature tube. This is a
pentode tube that has a 30 volt plate maximum rating. The filament runs on 1.25 volts at
10ma. With 18 volts on the plate and a low current filament, the batteries will last a long
time. I built another radio using this same tube.
The most popular way of controlling the regenerative feedback in a pentode based
radio is a variable resistor voltage divider connected to the screen grid. I decided to use a
throttle capacitor this time. The main advantage is there is no need for a double pole power
switch. Since all B+ currents flow through the tube, turning off the filament also shuts
off the B battery current. I had experimented with throttle capacitor regeneration control
before but never made a project this way. Before I built the final project, I used my
breadboard to check the circuit. It worked great.
A standard pair of high impedance magnetic headphones are used to listen to the
music. A crystal earphone may be used if a 4.7k ohm resistor is shunted across the earphone.
Sound powered, or balanced armature headphones work even better.
A dual section, 270 pf variable capacitor is used to tune this radio. A single
section capacitor of around 600 pf total will work. My radio tunes to about 1630 khz, due
to the size of the capacitor and construction of the loop.
Now that I have discussed the more or less easy parts, it is now time to talk about
the special feature of this little radio. That feature is the ability of this
radio to work as a loop antenna for another radio. This means that this project is likely
to have daily use around my house. Here is how it works:
I added a quarter inch stereo phone jack, which is on the lower right corner of
the front panel. This jack has two cut-off switches built in. These would normally mute
a speaker when the headphones were plugged in to the stereo. In this case, the switches
remove the tickler coil turn from the radio circuit and this winding can now be used as
the pick up winding to a radio. This is an automatic feature of this project.
If this is something you don't want to do, I have provided a "lite version" schematic
for your building pleasure.
The loop is the first part of my radio that I built. I used two sticks that
are 25 inches (63,5cm) long. They are 1/2 x 3/4 inches (12mmx18mm) square. They are notched
in the center so they fit together nicely. The mast is 3/4 inches square and is 20 inches
long. There are two 4 inch (10cm) square pieces that hold the loop together. These
can be made from about anything. I used HDPE pieces that were in my bin.
There are 4 end pieces that have notches for the wire. I made these pieces from
1/16 inch (1,5mm) garolite. These pieces are 4 inches (10cm) by 1-3/4 inches (4,5cm)
There should be 14 notches in your design. Four brass thumb nuts make the connections.
If you make one, if it is slightly smaller, the tuning would be perfect. If you disconnect
the loop, make sure your headphones are unplugged. This is in case you short certain
wires, causing battery current to flow.
The 4 wires between the antenna and radio is Belden 8890 miniature test lead
wire. It is rubber covered and very flexible. I made some little wire spacers to keep
the wires neater and to reduce the detuning as the loop is turned. They add a lot to
the looks too. Which reminds me, if you move your body near this loop, the radio will
detune. Normally this could be thought of as a design flaw, but I think of it as a
feature. If you put the loop radio into oscillation, and with a near by radio, beat this
oscillation with a local station, you have a
theremin. Just think, you can make music
while listening to Limbaugh.
Happy building de N2DS!
|
