Space Charged AM
Broadcast and Short Wave Radios
jim@wa6otp.com
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(Winter 2016)
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So, before I
tell you
about the radios, let me tell you about me and what I know and what I
found
out.
First I found out I did not know
as
much as I thought I did. I am not a
radio engineer; I have been
building radios for over 60 years and
am still
learning. All self taught, so there are
many
holes in what I know.
I am not going to
show you all the convoluted math, I do not know how to do it, I do not
know
what
all of the
words mean.
I will show you how
I got my two radios to work, and work well.
It is easy when you
know how. So it goes with all of life.
I wanted to make a
tube radio for AM Broadcast and one for Short Wave.
I wanted to
use 12
volts only.
I have a solar
panel
and 12 volts to the radio bench and it makes more power than I ever use.
I wanted the
radio
to be EMP safe, if that is possible.
I wanted to
have it
be useable in an off grid situation.
I
know I have TRX rigs. But,,,
I looked on the
internet, found many radio designs, some for low voltage some not.
Most were for 6 volt tubes.
I wanted 12 volt tubes for least
amount of
current draw.
I can build regen's,
TRF's and combinations or variances of each.
Regenerative Receivers work very well
but take some constant knob
turning.
I wanted a
Super Heterodyne
Receiver for easy control.
Looking through my
spare parts I found lots of parts but when I started to do a
preliminary layout, one thing
or another made it more and more
difficult. IF cans for one, one is not
enough and I did
not have two alike
or I liked that could be modified.
So to get started I made my own
from
scratch.
This was the
beginning of what showed my ignorance and gave me some concerns.
How much
insertion
loss is acceptable ?
How broad is a
real IF
can?
How to get a
real
measurement of the can? Hook to sig gen
and scope that will do it, right?
Well kind
of, how do I hook it up? What happens if
I reverse the leads? Some circuits have
an IF with one coil
and some with two coils in the
can. Testing parts out of the circuit and
in the
circuit, they act differently.
I go on line
to buy
1700 KC if cans cannot find two the same, or they want $40 for them.
So I build single coil 1700 KC IF
coil on
slug tuned form and they seem OK.
Not sure if they are
too sharp or insertion loss is too high. I set them aside.
I think I will build this in stages
and test
each stage. I build test boards, ten of
them.
LO in the back
and
Front End with parts placed
I build the LO
first
with a 12AU7A using just one triode.
Works great. Stable.
Then I start the Front end with the homemade
1700KC IF cans.
Above is Pictured A
Test Front End with 1700KC IF
These
numbers are for 1700KC IF, the finished radio used 429KC IF.
I build my
first
front end mixer with a 12AU6A, and an IF with 12BA6.
Found a Double gang 7-413pF.
I want to tune from 510Kc to 1550
KC. Well with 200uF I need a cap that goes
from
54pF to 460pF and
it should tune from 520-1561
close enough for front end, I think. Now
how to make the cap 7-413pf be
52-469pF?
I parallel the cap with a fixed cap of 47pF, That will give me
54-460pF. I wrap
a coil on a PVC pipe,
1.82 in diameter and 1.625 long,
I thread it 48PTI and
wrap 78 turns on it, 200pF. I test it
and it is good.
LO was made for 1700 KC IF.
Coil form was
PVC
1.060 diameter and .375 winding space for 38 turns of 31 gauge. Gave me 50 uH.
I added trim cap of 70 pf
in parallel with the LO section of the tuning
cap, and then series it with 130 trim
cap.
I built the audio
amp. I used a pair of 12K5's.
I tried several transformers from the junk
box
and some work so-so.
I broke down
and bought a transformer, Atlas Sound HT-47 for $12.78
from "LowVoltageProducts.com". This amp works very well, with plenty of
volume.
I hooked it up to a crystal set and the sound
was so nice, no distortion.
All this I
accomplished in the first week. Now my
frustration starts and my mistakes begin.
The front end works with a TRX amp, the tube amp
works with a Xtal set,
but they do not work together.
Maybe not
enough drive? Wrong. Maybe impedance is
wrong? Wrong. I test, I change
resistors.
I change tubes. I
change caps. Nothing gives me what I want. I build many variations with different
tubes
and designs. Some work badly and some
not at all.
Things got quite
confusing and I lost track of time and of exactly what I did each and
every day
for the next month. Really
a month, I am
old and retired and its winter and it is raining.
I will make this work! It
should work. So I know that two days were
spent trying to
understand
what happened to the front end, it went into a hole
and I could
barely get anything out of it.
Then I
noticed I had moved my antenna switch and had no antenna for two days.
I moved the
switch
and the radio came alive. I
was
mad to say the least.
I built about 40
individual test rigs. I made the chassis
myself. I scrounged all the connectors
and
made patch cords and built and tested and built and
tested. Some worked but not like they
should.
And I could not match up the IF with the Amp.
I spent
several days
on a detector test rig 12AV6, and it was dead from day one. I built the AVC circuit,
then I pulled it
out. I needed to just make it detect and
pass on the signal. Many 15 hour days
later
I notice a cap that goes from the grid back to the
diode through a
resistor, and to the IF output,
I see I have none. I
put in a cap and the tube comes to
life. I must have built 4 or 5 different
tube
detectors. Now this
detector works.
I put the front end
and IF strip into the detector and the detector into the amp and no
luck.
Blurry old eyed vision, I failed to see that
I have two caps in series, one on the output of the
detector and one on the input
on the amp. I remove one.
The
radio
works. I am amazed. It
is loud.
It tunes 40 stations. It sounds
great.
I call my sister. My
wife will not give me the correct response, after being usurped by a
radio.
My sis says "that Is great"; she
has no idea what I did, or what I have been through.
I don't care I need someone to say it.
Testing Photos
This was the SW test
version with the Front End in the back left and the audio amp back
right,
the LO
is left front and the BFO in the middle and detector front right.
It worked but
not as good as I thought it
should.
More testing tried
an RF amp up front with some success.
Still not happy.
More testing of
different LO
OK enough of
that.
This is the version
I ended up with one for Broadcast and one for Short Wave.
I decide to
change back
to broadcast band and try new IF transformers.
I bought
tubes, I
bought IF cans, I bought an output transformer.
The IF cans I bought were 1.4
MHz. They had too much stuff on the inside
so
I stripped them and made them 429
KC.
IF cans as I got
them. $5.00 each from Surplus Sales of
Nebraska. Item GH2342-12-1
1.5 x
1.25 x 1.9
After I changed
them:
Testing front end
coil.
Finished Test Radio
This radio works
good.
Things of note:
Use a tracking calculator to make the two capacitors
track properly. It makes the trial
and error method obsolete. It also is right
the first time. I wrote the tracking calcultor
using several sources of other's work. It is
a gwbasic program that will run under DOS
the cmd prompt C:\ in a small
window. The print utility will only work with a parallel
port.
Most of the parts found
in a High Voltage Radio are not needed in low voltage radio.
Dropping resistors and most caps
to ground
just limit the radio and add to distortion.
The math on paper
and the real world are different. Wire length and posistoin of
parts
come into play.
You
need to be able to adjust the coils and the caps in circuit to get
things
right.
Open cap and adjust
coil to high frequency wanted.
Close cap and
adjust
trim cap to low frequency wanted.
Do it several
times
to get the best balance.
Coil Data for BC.
Front End Secondary
Coil (final coil)
Form .750
diameter
X 1.2 long winding space 181.986 uH, I put too much on and peeled
it off till it hit the range I
needed.
Primary is 8
turns
over the secondary.
Local
Oscillator
Form .375
diameter
X 1.o long winding space, 89.127 uH same as above
I tried
electron coupled but would not keep running so I went to link coupled.
Capacitor
BC used Capacitor
from junk stereo
8pF to 417pF
Front end
section
needs to be 51pF, 460pF
Parallel 43pf will give
51pF, 460pF
LO section needs
to tune from 979 to 2079 KC
89.127 uH
C2
is
57.86
C3 is
558
To change BC Radio
to Short Wave, change the front end coil and cap, change the LO coil and
cap.
Add BFO at 1.5 KC low of IF, for CW
and SSB.
I will document
short wave soon.
