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A
word of
caution about the
coils.
Make sure
that
the hot and
cold ends are as specified on
the circuit
diagram-the
asterisk indicates the end which
is
the mounting end,
that
is,
the end with the long
tuning
screw.
The
links
on
the
coils are wound
over the
cold end,
as indicated
in
Fig.
11.
As a
suggestion, wind the
links with solid insulated hookup wire. This type
of wire
is
convenient, holds
on well,
and makes
a
nice looking
job. Twist the
wires
together
when
running
from one coil to another coil, or to
another
connection point.
A
small
terminal strip may
be
placed under
L,
to serve
as
a
convenient junction
point
for the links coming from L, and
L2
and
going
to the balanced modulators.
The
small fixed mica
tuning condensers
that
con-
nect across
L1,
L2
and
L3
are
mounted
on
the
coil
form terminals. The coupling capacitor between
L1
and
L2
(C6) is
shown
dotted
in
the circuit diagram,
since
the amount
of
actual capacitance needed at
this point
will
depend
on
stray
coupling effects
in
the particular unit you build. More information
will be
given on
this later.
Note
that
the grid connection of the
6AG7
is
above the panel from the hot end of
La
through
a
hole in
the chassis right next to pin number
4
(the
grid
terminal)
of
the
6AG7
socket. Direct strapping
of
terminals
1,
3
and
5
of this socket to the chassis
is
desirable to ensure stable amplifier operation. Note
also
that
a
2
by 2)4 inch brass or
aluminum
shield
is
placed between
coils
L2
and L; below deck.
The unused transformer leads may
be
cut
off close
to the winding and forgotten. The secondary wind-
ings of
T2
and
Ta
have several intermediate taps
that
are not used.
All
leads from the three transformers
are
fed
through
small
rubber grommets
in
the chassis
to circuits on the underside. All,
that
is,
except the
secondary leads from
T3
which remain above chassis.
Twist these
leads
together
before
running them
to the
sideband reversing switch
on
the front panel.
Do
not ground either heater
lead
in
the chassis,
as you may wish to use an
a -c
heater
power supply
or
perhaps run your automobile engine while trans-
mitting
if
the
rig
is
used
for
mobile work.
Ample
mounting
space
for
C1
and
R3
will be
found
near
C2,
the
four -section
electrolytic condenser. With
reference to
C2,
one
20
mf section
is
C2A,
another
is
C2n,
etc.
The heater
leads
that
run from the
12AU7
may
be
cabled
together with the other leads
from
T1, T2, T3.
The germanium
diodes deserve special care
in
handling. Do not bend the leads
close
to the diode
unit
itself.
The diodes are mounted
by means of
their
leads between the
coil
terminals
of
L3
and the
ap-
propriate
ends of
121,
and
R77.
Protect the germanium
diodes from
heat
while soldering by holding
the
lead
with
cold pliers between
the diode itself and the
end
where
the soldering
is
taking
place.
Further,
use
only as much
heat
as
is
necessary to make
a
good
joint.
A
four -wire shielded cable brings power from the
power
supply to the exciter. The
shield serves as
the negative plate supply
lead and should be con-
nected to chassis ground.
A
male plug
at the other
end of
the
cable makes
a
convenient connection to the
power supply.
The power supply
is
not unusual
in
any respect.
Any source of power supplying
300
volts and
60
mils
or more
may
be
used.
It
is
not necessary to
use
electronic bias either, and
a
standard battery
sup-
plying
10.5
volts may
be used for bias.
4
The power supply used with the SSB Jr.
rig pic-
tured
is
shown
in
Fig.
7
and the circuit diagram
given
in
Fig.
6.
A 5V4
-G
rectifier tube feeds
a
single
-
section filter to supply 300 volts, and
a
6H6
tube
acts
as
a
bias rectifier to
supply
10.5
volts. Resistor
R1
adjusts
the
bias
voltage obtainable.
The main
a -c
switch
is
S1,
and the
stand-by switch
is
S2.
Note
that
resistor
R2
acts
as
a
low
resistance
bleeder to drop the positive voltage to zero quickly
when
the
rig
is
turned
off. A
double -pole switch
is
employed with the switch arms tied together,
as
this
arrangement
gives the effect of
a
double break
con-
tact.
There
is
nothing critical about the
power
supply
layout, and any arrangement may
be
used to suit
your convenience.
The
SSB
Jr.
rig as
designed requires
that
a
high
-
output
microphone circuit
be
used.
A
single
-button
carbon microphone, connected
as shown
in
Fig. 8B
is
quite adequate,
even desirable, if mobile
operation
is
contemplated.
On
the other hand, low-level microphones, such
as
the usual type of crystal
or
dynamic microphone,
may
be used if
a
one
-tube preamplifier
is
provided.
A
suggested circuit
is
shown
in
Fig.
8A.
This pre-
amplifier may
be
built
as
a
separate unit
or incor-
porated into the
SSB
Jr.
rig.
Either the preámplifier
shown
or
the single
-button carbon
mike circuit
will
provide
in
excess of
the
2
volt (RMS) signal
level
required
as
a
minimum input signal to the
SSB
Jr.
As
is
true
with many
transmitter
designs,
there
are some component
parts
used
in
the
SSB
Jr.
rig
that
must
be
chosen carefully. Obviously, the pre-
cision resistors specified are
important. If
precision
resistors are not
available-although
you should
try
to get them
if
at
all
possible-you
may use non
-
precision resistors which have been checked on
a
good
resistance bridge. You may
find
that
these
resistors
will
change value
after they have
been
used
for a while,
and
that
is
why it
is
desirable to
use precision resistors initially.
The adjustable mica trimmers used
in
the audio
phase -shift network may
be
any
good
grade of mica
trimmer. Those actually
used are El-Menco mica
trimmers-T52910
for
the
170
to
780
mmf range;
T52510 for the
50
to
380
mmf range; and T52310
for
the
9
to
180
mmf range.
Resistors
R4, Rs
and
R11
are specified as plus or
minus 5% tolerance. This
is
because the values
stated
are required, and these values only come
in
the 5%
tolerance
series.
The germanium
diodes are specified
as 1N52
diodes. Other types, such as 1N48,
1N51
and
1N63
may
be
used instead.
If
possible, select four diodes
which have
about the
same forward resistance. The
forward resistance
is
the
low
resistance
as checked
on an
ohmmeter. To determine approximately what
it
is,
measure the resistance
in one
direction, then
reverse the leads to the diode and make
a
second
I
measurement. The two readings should
be
quite
different. The
lower
resistance
is
the
one of
interest.
I
Make this measurement
on
the
four diodes you
intend
to use to make sure
that
the forward resistance
of
any
one of
the diodes
is
within ten per cent of the
average resistance of the group.
The diodes used
in
the
rig shown
measured ap-
proximately
250
ohms on
a
Weston
772
analyzer
when
the analyzer
was set to the RX10 scale. (Dif-
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