linear
operation. The final amplifier
screen
voltage
is regulated
at 210
volts
with
two
OB2
voltage reguiator
tubes in series. The bias
is
then
adjusted
to
give
5 0 ma of resting
plate
current.
Third, a
provision
is made for lifting the
ground
end
of the fixed bias resistor string during
standby
periods
to cut off the f
inal ampli-fier without removing
plate
voltage. This
eliminates
excessive switching of
plate
transformer
primary porver.
SSB
.)
cw-AM
58
6AO5
RF
EXCITATION
TO
S8-
ro
I
I
TO
CENTER
TAP
OF
eL
ntE
lnlp5roaut*
.H
TO
PIN
5
OF
S58
ACCESSORY
SOCKET
Figure
20
Referringto
the schematic diagram
in Figure 20, these
changes are
self-evident.
A DPDT toggle
switch
mounted
on the
right
hand
side ofthe front
panel
above the
chassis
performs
two functions.
One switch section switches
the clamp
tube in and the
VR tubes out of the circuit for Class C
operation
and the clamp tube out
and
the VR
tubes into
the circuit for Class AB1 operation.
The
6146 screendropping resistor becomes the
VR tube
load
resistor
in linear operation.
While
the regulator tubes are not
physically
disconnected from
the 6146 screen during
Class C
oper-
ation, they will not
conduct
since the Class C
screen
voltage is below the firing
voltage
of the
tubes.
The second switch section
grounds
the end of the f ixed bias resistor
string in the
normal
manner for
CW or AM operation
(Class
C)
but allows this function
to be
performed
remotely
in
the SB-10 during
SSB operation
(Class
AB1) by
routing
the
lead
through
pin
5 of the accessory
socket. A 3 KQ
control replaces the
2.2 KA resistor
(between
DDI
and
EE1
in the
DX-100
or
DX-
1008 manual)
in the bias string to
provide
a means of
varying the fixed blas to theproper
value lor
Class ABl operation.
Once set, thls control needs no further
adjustrrrent
since the
rest ofthe
bias requi.red for Class C operation
is developed
by
grid
current
flowing
through the
grid
resistor.
Power
for
the adapter
is
brought
out through the
accessory socket.
Filament
voltage
should be
brought out
through
pin
7
and
Br
through
pin
6. The two
blue
500Q audio
leads on terminal
strip
GG,
already connected
to
pins
6 and
?,
can be connected
to
suitable
points
to ma"ke
this
modifica-
tion. The ratings
of the
DX-
100 and
DX-
100B transformers are conservative
enough to allow this
extra load. If
extended operation on
SSB is
contemplated, it is
suggested
that
the modulator and
speech
amplifier tubes
(1625,
IZBYT, L2AX7) be
removed
to reduce
the
filament
current drain.
The SB-10 reouires
300-350 VDC at 85 ma and
6.3
VAC at 3.5 A.
CIRC
U IT
TO
PIN I
OF
ACCESSORY
Page
51
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