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The P.A. tubes are air cooled by pressurizing the area below the
anode and leaving a space above the
angde to
the ambient air
pressure of the room.
 blower mounted in the base of the amp-
lifier cabinet is capable of supplying enough air for,operation
up to 7500 feet,
and ambient air temperature up to 45
%°
C and 50
cycle operation.
àir enters the cabinet through a filter at the
back of the cabinet and is then blown past the grid seal,
screen
seal and out through the radiating fins of the anodes and is
exhausted through the top of the amplifier cabinet.
Complete
air cooling of all parts of the tubes and components associated
with the power amplifier is accomplished in this manner.
It
will be noticed that the blower is ducted directly through the
back of the cabinet and all air intake for cooling the final
tubes is taken through a filter,
so that all air cooling the fi-
nal amplifier cabinet is fresh filtered air.
 disposable type
filter is used.
In case the disconnect box supplying power to
the transmitter would open, there is enough inertia in the blow-
er itself to cool the tuba down to a safe value,
and there is no
need to worry about heat rising within the tube, thus causing
overheating of any of the tube seals.
Of course,
standard prac-
tice would be to shut down the blower with the off delay relay
and let it run for approximately three minutes after the trans-
mitter,
itself,
is shut down.
Metering of all important stages is accomplished with meters
located on the sloping panels at the top of the two end cabinets.
Metering of the Ë.A. consists of filament voltmeter, and a
switched multimeter for total grid current and individual screen
currents,a switched total and individual plate current meter,
a plate voltmeter,
and a switched power output and VSAR meter.
Also located on the front panel of the power cabinet are meters
for screen voltage and grid bias voltage.
Similar meters are on the driver cabin- t to determine driver
operation.
The control panel for the dri•rer portion of the FM-20Bis located
in the first cabinet.
This panel consists of the off- on switches
for the line,
and the off- on switch for the driver plate 'voltage,
and various indicator lights, the uloCal-te;loteswich
e
'ind the
-ntune=oPerato"
switch.
The control panel foethe power ampli»
fier is in the right c
.
,binet and consists'ef.the'
ll
off-on" "
switch fti.:the power azplifiier plate supply'a switch for.fila-
:lent voltages,, a switch for grid and screen' earrents
4.
. ;-nswitch .
for plata currents and the various lights indicating the ópQration
of. the amplifier.
All over current and under current relays in the FM-20BTrans-
mitter are telephone type relays.
The plate overload of the
driver and the plate and screen overload of the amplifier along
with the grid over drive relay in the amplifier control circuit
6
/
22/61
-3-
FM- 20B
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