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Well folks, I am up to a 3 tuber. I have revised
the circuit since I built this is 2002.
The first tube is a type 32 used as an untuned RF amplifier, another
32 as a regenerative detector and a 31 as the audio output. With 90 volts
on the plates it drives headphones nicely. With 135 volts, it will be able to
drive a speaker.
I call this an "All Wave" radio as it has plug-in coils for
the various bands. Pictured is the broadcast band coil. This is the
largest. I also have one that covers 1.8 to 4 mhz and a 5 to 10 mhz coil.
The coil forms are old as the tubes and sockets.
The rest of the items are newer, some even brand new.
You will notice this radio does have a little bit of metal.
I have found from a previous shortwave radio that if you put your hand
near the tuning capacitor, the frequency will shift. This is due to
my hand capacitance changing the circuit values.
This can be minimized by putting a metal plate (copper
in my radio) between the front panel and variable capacitor. This plate
is grounded to the capacitor. This radio has very little hand capacitance
problems.
The building of this set was pretty traditional. Not much
planning and just building on the fly. I designed the circuit myself as
I have in most of my sets. I don't consider this a great feat as the
design is pretty universal anyway. This radio is designed around what
is in my junk box. I hope you are doing the same!
The tubes have 2 volt filaments and draw a combined current
of 240ma. I use two D cell batteries with a 3.3 ohm dropping resistor.
The high voltage is 90 volts for the audio output stage and 45 volts for
the rf amplifier and detector. This is my first receiver to use a "C"
battery supply. I am not talking about a 1.5 volt C cell, but a power
source that is for grid bias. I used a pair of kind of semi dead 9 volt
batteries to give me a 15 volt grid bias. The current is nil and
the batteries can stay in the circuit all the time. I really didn't
understand C battery supplies until I measured the output tube current.
It was nearly 18 milliamps! This would flatten my battery in short order.
The two early stages draw about 2 ma at 45 volts, which is ok. By
putting a negative bias on the grid (through the volume control) the
plate current is reduced down to about 2 ma at 90 volts. The audio is
still the same strength and there is no more distortion with the bias
battery.
Another new feature of this radio is the use of an interstage
auto transformer. This is much like the interstage transformers of the
old days, but this only has one winding. I use a coupling capacitor
to connect to the grid of the audio output stage. I used a
commonly available Bogen T-725 speaker matching transformer.
The main problem I had was getting this set to regenerate.
After all this is where the gain multiplies and also allows me to
receive cw and ssb signals. It turned out that the circuit using the
32 tube is about as sensitive as a marine drill sergeant. I had to
add a lot of extra windings on the tickler coil to make this regenerate. My thanks
to Mike Peebles
for giving me the hint.
What can I hear? This last weekend I sat here listening to
W1AW doing the code practice and
earlier I listened to W3DUQ and a couple other 75 meter AM stations.
Sure was pretty neat! By the way, am I ruining the sets with those
Brother Label maker labels? I try to use them only when necessary.
I feel they are important to label where the voltages connect. How
about the front panel? The reason that I ask is I am getting ready
to purchase a label maker that connects to my PC. I will be able to
make very special labels. But if I am ruining the effect, maybe
I should use my dymo. :-)
73, Dave - N2DS
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