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Intermediate Amplifier
The 813 intermediate amplifier
is
located directly above the
oscillator unit
(tube V6) and has its
associated coil and tuning
condenser directly adjacent.
The intermediate amplifier tank
coil
is
center tapped
to
ground with neutralization
of the
final
power amplifier
to the
opposite end from that
of
the grid drive
end.
The screen
voltage
is
obtained through
two
voltage dividing
resistors which
are
normally
set
at
the factory to obtain approx-
imately
350
volts
on the screen,
allowing
a
margin
for
high
line
voltage
and
furnishing ample drive for
the
power amplifier.
The
plate current
of the
intermediate amplifier
is
indicated on meter
M6.
The
intermediate amplifier tunes
to
resonance by capacitor
C24,
in the 813 tank circuit and
is
tuned
to
minimum plate cur-
rent.
On
the
plate coil
L7,
the taps (when used) should always be on
the same
relative tap from the center
tap.
As
much
coil and
as
low value
of
capacity
to
tune should be used, except
to a
point
of
extremely low capacity which will tend
to
result in arcing
in the condenser.
In case of
changing frequency
to
a
new frequency from that pre-
viously supplied when manufactured,
the
following information
will be valuable.
The
tuning ranges will be
as
follows:
For 1200 KC
to
1600 KC use 15 turns on each side of center tap
(first tap
from center
tap).
The second tap from center
(21
turns) will provide
a
tuning range from 1500 KC
to
950
KC.
The
full coil provides tuning from 720
KC
to
1100
KC.
Tuning from
650 KC to 720 KC requires the addition of
a
200 mmfd.
padding
condenser
(C41), and
from
550
KC
to 650 KC,
a
400 mmfd. padder.
Power Amplifier
The
power amplifier consists
of two
810 tubes operating in par-
allel.
The load is
connected through
a
low pass filter
to
the
inductive branch
of the
tank circuit,
a
system which minimizes
harmonic radiation.
The final
amplifier
is
resonated by means
of
capacitor
C33, and
brought
to
exact frequency by the variable
capacitor
C32.
The value
of C33 is
determined by the carrier
frequency and
is
usually properly provided
at
time
of
shipment.
9/11/63
-3-
BC-250GY-1
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