CIRCUIT
DESCRIPTION
The following
block
diagram
and
circuit
description
will
show the
builder
how the theory
of the
phasing
mefliod
has bee-n
practically
applied
in the
SB-10 and will
result
in an even
better
under-
stanairig of the
adapter.
This
knowledge
will
aid
in construction
and
possible
trouble
shooting'
and thus
is well worth
digesting.
AUDIO
AND
PHASING
CIRCUITRY
The
12AX7
dual
triode tube
provides
two
stages of
resistance
coupled speech
amplifieation
with
sufficient
gain
for a high impedance
microphone
input.
The design of
the
speech
amplifier
cir-
cuitry favors an
audio
frequency range offrom
300
to 3000 cycles
per
second. The
output
of the
speech
amplifier is
capacitively
coupled through a
gain
control to one section of
a 12AT7
acting
as an audio driver. The
output of the
driver
is transformer
coupled to the
low impedance
side
of a
90o
audio
phase
shift network which
is designed
to operate over the audio range of 300 to
3000 cycles
per
second. By restricting the audio
frequencies
to the stated limits,
errors
in the
90o
phase
shift network,
which
will direc
y
impair
the
suppression
of the unwanted sideband,
are mini.mized.
A
control, known
asthe
phase
balance control,
is
connected across thelow
impedance
(500ohms)
side of
the
interstage
transformer which
feeds the
passive
network. The function
of the control
is balancing
the input circuit to the
network.
The output ofthe
phase
shift network isof a
relativelyhigh impedance
and isconnectedto the two
grids
of a dual
triode 12AT7 tube.
This tube
functions
as the modulator.
The
cathodes of the 12ATT modulator arebiased
with a
variable
and fixed resistor
configuration,
The
purposeofthe
control islor audio balance.
This tube mustdeliver
equal output to the
primary
oI each of the
two
modulation
transformers.
VOICE
CONTROL AND ANTI-TRIP
CIRCUITRY
The
output of
the
speech
amplifier
is
also
capacitively
coupled to one section
of a dual triode
12ATT tube
acting as the vox voltage ampliJier.
The other
half of the 12AT7
tube acts as the
anti-trip amplifier
and
receivesand amplifies thereceiver output,
which is transformer
coupled
to the
Erid.
Figure
Page 7
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