Page 82
FILAMENT AND
BIAS
SUPPLY
Transformer T2 has two secondary windings. One winding
furnishes 5, VAC at 30 amperes for the amplifier tube
filaments. The two
#47
pilot lamps for meter illumination
are also connected across the filament line.
Filament supply is fed to the
two
tubes through RFC 3, a
coil which is bifilar wound on a ferrite core. This coil forms
a choke to raise the tube filaments above RF ground so the
driving voltage will not
be
short-circuited.
The second winding on transformer T2 is used in a half-wave
rectifier circuit for the bias supply voltage, to operate relay
RLI, and to furnish ALC threshold voltage. The AC voltage
from this winding is rectified by Dl6 and filtered by
C4.
This DC voltage is connected to lugs 2 and 11 of relay RL1.
In the receive mode, this voltage is applied through lug 8 to
the center-tap of the filament winding. This positive voltage
increases the voltage difference between the tube grids
(which are grounded for DC) and the tube filaments, which
now carry the positive DC voltage in addition to the AC
filament voltage. The tube grids are consequently biased
beyond cutoff and no plate current flows.
In the transmit mode, the center-tap of the filament winding
is
connected to ground through lugs 8 and 5 of the relay, the
5.1 volt zener diode ZDI, and R3. The plate current through
the zener develops 5:1 VDC operating 'bias for the tubes and
limits the idling plate current.
RELAY
The relay has three sets of singlepole, double-throw
contacts. When the relay coil circuit is open the contacts are
in the receive mode.
Approximately 120 VDC is connected to one side of the
relay coil at lug 11. Lug 10 connects the other side of the
relay coil to the Antenna Relay jack on the rear panel. This
jack is usually connected to normally open relay contacts in
the exciter
(such
as atVOX or
PTI
relay). When these relay
Contacts close,,they must connect the amplifier relay coil
circuit to ground. The amplifier relay will then close and its
contacts will be in the transmit mode.
The function of amplifier relay contacts 2, 5 and 8 was
;
discussed in the "Bias Supply" section.
Relay contact
7
is connected to the RF INPUT connector.
In the receive mode the incoming signal is transferred
directly to the RF lnput through relay contacts
9,3,
1,
and
7.
In the transmit mode, the RF Input voltage is connected
.
through relay contacts 7 and
4
to lug 1 of Band-switch wafer
'
1 F.
In the transmit mode, the RF Output is connected through
relay contacts
9
and
6
to the pi network output circuit of
the Amplifier.
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