CTI ComTec, Inc. PLA-800 User Manual - Page 7

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between the Plate Tuning and Loading Controls. Do NOT exceed the 300 ma. current on
the plate meter in CW.
DO NOT DRIVE AT THIS CURRENT LEVEL FOR MORE THAN 10 SECONDS AT A
TIME. Release the carrier for 10 seconds and then key down again to optimize output.
Once at maximum output, remove the drive and let the amplifier cool down for about 10
seconds. Increase the drive level on your exciter to show a 500 ma. reading on the Plate
Meter. Screen Current should be at 30 ma. If the screen current is showing a negative
downturn on the meter, the plate loading is too heavy. Simply back off the loading control
until the screen current reads 30 ma. Tetrodes do exhibit negative screen current. This is
OK. Depending upon the load value, it is possible to show negative screen current with
maximum output. You should be at the maximum output of the amplifier.
If there is excessive drive and plate current, the overload circuit will activate. Reset it by
pushing in on the reset button on the Power Supply.
At this point you should be ready to transmit SSB or CW. Place your driver into SSB and
modulate normally. You will see the Plate Meter kick up to 500 ma. and the output will
exceed 1200 watts PEP. If RTTY is used, you must reduce the plate current to 250 ma. as
a maximum operating plate current.

Bias Adjustment

On the rear panel of the PLA-800 is a RED square push button and an ORANGE colored
test point jack. To adjust the bias on the 4CX250Bs follow the steps below.
1.
Remove the high voltage RED connector to the amplifier on the rear panel.
2.
Place the PLATE SWITCH on the front panel to OFF.
3.
Place a multimeter in DC Volts position and on a scale of 0-2—volts. Insert the
negative lead of the multimeter into the orange test point jack. Ground the
positive lead of the multimeter to chassis ground on the amplifier.
4.
Turn the PSL 1000 Main Power Switch to ON.
5.
Push in on the RED square button switch and read the bias voltage on the
multimeter.
6.
To adjust the bias voltage, use a straight blade screwdriver in the pot control on
the rear of the amplifier, next to the test point jack.
7.
You must continue to push in on the red switch to adjust the bias.
8.
The correct voltage for the 4CX250Bs is –57 volts DC.
9.
Once adjusted, release the red button switch and the bias voltage will climb to
the cutoff voltage which will be above –100 Volts DC.
10.
The bias is now adjusted for AB1 service and the amplifier can be operated.
Power Change Over
Both the PLA-800 and PSL 1000 can be used for 115 volt C/50-60 Hz operation or 230
volts AC/50-60 Hz operation. The units are supplied for 115 VAC operation unless
specifically requested.