Heathkit HA-14 - Page 45

Page 42
HEATHEIT)
CIRCUIT
DESCRIPTION
Refer
to
the
Schematic
(fold-out
from
Page
43)
while
reading
this
Description,
The
Linear
Am-
plifier
is
basically
made
up
of
three
circuits:
Input
Circuit,
Output
Circuit,
and
the
Antenna
Change-Over
and
CutOff
Bias
Circuit.
Each
circuit
is
described
individually,
INPUT CIRCUIT
Tubes
V1
and
V2
are
connected
in
parallel
in
a
cathode-driven
(grounded
grid),
class
B
con-
figuration,
Driving
power
for
each
band
is
cou-
pled
through
a
broad-band
network,
consisting
-
of
a
coil
and
associated
capacitors,
and
through
i
eae
‘
capacitor
C13
to
the
cathodes
of
tubes
Viand
V2,
Coils
Li
through
L5
with
their
associated
ca-
pacitors
are
used
to
make
up
these
impedance-
matching
networks,
which
reduce
distortion
and
increase
efficiency,
The
correct
input
network,
as
well
as
the
cor-
rect
output
circuit
coil
tap,
for
each
band
is
selected
with
the
Band
switch,
(The
coils
in
the
input
networks
are
factory
aligned.)
To
keep
the
capacitance
of
the
filament
supply
from
shunting
the
RF
driving
power
to
ground,
the
filament
winding
is
isolated
from
the
cathode
circuit
by
a
bifilar-wound
filament
choke,
Also,
this
choke
provides
a
cathode
current
path
to
ground,
OUTPUT CIRCUIT
High
voltage
is
applied
to
the
plates
of
tubes
V1
and
V2
through
RF
chokes
RFC1
and
RFC2
and
parasitic
chokes
PC1
and
PC2,
Tuning
capacitor
C15
is
connected
on
the
input
side
of
tapped
final
coils
L6
and
L7,
C16
is
switched
in
parallel
with
the
Tuning
capacitor
on
the
80
meter
band,
Loading
capacitor
C18
is
on
the
output
side
of
final
coils
L6
and
L7,
Capacitor
C17
is
switched
in
parallel
with
the
Loading
capacitor
on
the
80
meter
band,
Output
power
is
applied
through
antenna
change-over
relay
RL1
and
through
the
SWR
(standing
wave
ratio)
bridge
circuit
to
the
RF
Output
connector,
The
SWR
bridge
consists
of
coils
L8,
L9,
and
L10,
capacitors
C20
and
C21,
resistors
R6
and
R7,
and
crystal
diodes
D2
and
D3,
RF
choke,
RFC4,
is
connected
from
the
output
side
of
the
final
coils
to
ground
to
provide
a
direct
DC
to
ground
path
in
case
capacitor
C14
should
short,
ANTENNA CHANGE-OVER, ALC, AND
CUTOFF BIAS CIRCUITS
Antenna change-over relay RLI is controlled by
a VOX (voice operated transmitter) relay in the
Exciter used with the Linear Amplifier, The
relay in the Exciter is connected to the Antenna
Relay lug (12) on the power plug of the Linear
Amplifier, ‘When transmitting, the VOX relay
grounds the Antenna Relay; when receiving, the
Antenna Relay is ungrounded,