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Further,
any
amateur
can build
the
SSB
Jr.
rig
and
be
assured
that
his single-sideband signal
will
be
second
to none
in
quality. Performance
has not been
sacrificed
in
the interest of simplification.
The peak
power
output
is
5
watts and the total
power
input, not including filament
power,
is 18
watts
(300
volts
at
60
ma.). The
SSB
Jr.
rig
features
a
self-
contained crystal oscillator
(or buffer for VFO
operation),
40
db. sideband suppression, and
me-
chanical and electrical ruggedness
that
make it ideally
suited as
a
complete portable, mobile, emergency
transmitter,
or as an exciter for
a
home
transmitter.
The system
used
in
the generation
of
the
single-
sideband signal
is
a
simplified phasing method
that
is
daringly direct and
effective. Inexpensive and easily
-
available components are used throughout.
All
of the information necessary to
construct
and
adjust the
SSB
Jr.
rig
appears
in
this article. Tech-
nical details
on
the
new phase -shift network and
the
new
modulator
design are explained
in
the Designer's
Corner section of this
issue.
With reference to the circuit diagram, Fig.
2,
the
first
tube,
a
12AU7,
is a
twin -triode, combination
speech amplifier oscillator.
A
12AT7 serves as
a
twin
-
channel amplifier
in
the
output
of
the phase -shift
network, and the
final
is a
6AG7
pentode.
Starting
with the audio circuit, an input gain
con-
trol potentiometer
feeds
the
grid of
the
self -biased
input tube,
which
is
one-half of the
12AU7
miniature
tube. The output
of
this tube
is
coupled into
a
newly designed audio phase -shift
network
by means
of
transformer
T1.
The outputs
of
the phase
-
shift network
feed
separate triode sections of the
12AT7
miniature tube. These
two
tube
sections are
transformer
coupled to two balanced modulators
each of which employs
a
pair of germanium crystal
diodes.
The balanced modulators are
also
supplied
by
r
-f
signals from the crystal oscillator, which
is
the other
half
of
the
12AU7.
These
r
-f signals are picked
up
by
separate
link windings
on
L1
and
L2,
which comprise
portions
of
a 90
degree
r
-f phase -shift
network
in
the
plate circuit
of
the oscillator. The balanced modulators
work into
a
balanced load circuit
(L:,,
C21, C21)
which
is
link coupled to the grid circuit
(L4,
C17)
of the class
AB' linear power amplifier tube,
a
6AG7.
This power amplifier works into
a
conventional
tank
circuit
(LU,
Cm)
that
is
link coupled to the load.
All
circuit tuning
is
accomplished
by
adjustable
slug
-tuned
coils
wound
on
Millen No. 69046 powdered
-
iron coil forms.
Sideband switching
is
accomplished by the reversal
of audio polarity
in one of
the audio channels (switch
S).
Provision
is
made
for
equalization of gain
in
the
audio channels, this equalization
being necessary
in
order to achieve normal sideband cancellation. In
addition,
a
semi -fixed
control
(R7)
is
provided
for
phase -shift network
adjustment.
Use of this control
eliminates the need
for a
special
transformer,
or
the
need
for
two
non-standard
precision resistors. Stable
modulator balance
is
achieved by the balance/buffer
resistors
R16
and
R17
in
conjunction with the
ger-
manium diodes.
The audio characteristic of the
SSB
Jr.
is
designed
to emphasize the intelligence -bearing frequencies
from
300
to
3000 cycles per second.
This feature
is
obtained jointly
by
the action of
C1
and the audio
transformer
T1.
Low
differential phase -shift
is
main
-
tamed
in
audio circuits
following
the phase -shift
network
by means of
lightly loaded
output
trans-
formers which are
shunt
-fed
to reduce harmonic
distortion
caused by direct current
in
their windings.
A
5
by
7
by
2
inch chassis provides ample space,
with
good access, for all
component parts.
A
cabinet,
as shown, may
be
used,
although this
is
not essential.
It
is
recommended
that
parts layout
shown
in
the
sketches and the photographs
be
followed
exactly.
Obviously other layouts
will
work,
but the layout
shown has been carefully made and many layout
problems have been eliminated.
Before
starting
work ón
the main chassis it
is
advisable to make the audio phase -shift network
board. This
is
diagrammed
in
Fig.
3.
The
base ma-
terial may
be
thin bakelite
or
any insulating material.
The dimensions are
4
inches by 2% inches.
Note
that
one
corner
is
cut
off
to permit access to
the
12AU7
tube. This board uses four
fixed mica
condensers
which are
padded with
four
adjustable
mica trimmers,
and four precision resistors
(Continental
Nobleloy
X-3
,
plus or minus 1% tolerance). In the
unit
shown R, and
R9
are as specified,
that
is,
they
are
Continental Nobleloy
100,000 ohm
resistors. How-
ever,
the
133,300 ohm
resistors
were
made
by
taking
two 150,000 ohm precision
Continental
Nobleloy
resistors and paralleling each of them with
a
one-half
watt
1.2
megohm (plus or minus 10%
tolerance)
resistor. Careful selection of the
1.2
megohm
units
will
permit
close
adjustment
to the desired value
of
133,300 ohms.
A
convenient
way to
mount the
1.2
znegohm
resistors
is
to slip them inside
the
hollow
body of the precision
150,000 ohm
resistors.
The phase -shift network sub -assembly
is
mounted
on
three half
-inch long spacers
under the chassis
directly
below
transformers
T1
and
T2.
It
is
best to
dress the leads from these transformers flat against
the chassis to clear the phase -shift network. Time
will be
saved by installing the network sub -assembly
as the last step
in
the construction.
Mount the
phase -shift network elements as shown
in
Figs.
3A
and
3B.
The dashed connections should
be
omitted initially,
since
the detailed alignment
procedure described later presumes
that
these
con-
nections
will be
made
at the proper time
only.
21x4
K4" TExT .ITE
THICK
C7
-
0.002
MICA WITH
170-780
MME
TRIMMER.
(2430
MME
TOTAL)
8
-
0.0043
MICA WITH
170.780
MME
TRIMMER.
(4860
MME
TOTAL)
9-
0.001
MICA
WITH
50-380
MME
TRIMMER.
(121S
MME
TOTAL)
C10-
500
NNW MICA
WITH 9
T0(80
MME TRIMMER.
(607.5
MME
TOTAL)
A
I
7:
pa
.
5
_`
(
-
+7
'
; y
B
.
Fig. 3.
(A)
Mechanical arrangement of
the
audio
phase
-
shift network.
(B) Detail
view of
the
audio
phase
-shift
network
3
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