Palstar Commander VHF-1200, Owner'S Manual

Page: 12 / 20

12
Circuit Description and Adjustments
The Commander VHF-1200/2000 uses Genuine Eimac 3CPX800A7 ceramic/metal triodes)
in a class AB
2
grounded grid configuration. Nominal drive power of 35 to 50 Watts will deliver
full rating of clean RF output power.
Metering Functions
The Commander VHF-1200 and VHF-2000 have two illuminated panel meters. The Grid
Current meter provides a continuous reading of the 3CX8QQ-A7s grid current. The meter scale
is 2 mA per division and the range is 0 to 100 mA. DO NOT EXCEED 60 mA OF GRID CUR-
RENT PER TUBE
(60 mA on the VHF-1200; 120 mA on the VHF-2000) . OTHERWISE,
DAMAGE TO YOUR TUBES COULD RESULT. Under typical operating conditions the grid
current will be 20-45 mA on the VHF-1200; 40-90 mA on the VHF-2000.
Plate voltage and Plate current are shown on the second meter. A function switch on the
front panel switches the meter from plate voltage (Vp) to plate current (Ip). Plate current is
shown on the top scale. Each division is 20 mA, and full scale is 1500 mA. (1000 mA VHF-
1200)
The typical plate current under nominal rated output should range from 900 to 1100 mA.
(500 to 600 mA on VHF-1200) with an absolute maximum of 1200 mA. for SSB. (600 mA
VHF-1200) Plate voltage is indicated on the bottom scale with each division reflecting .06 kV.
The scale has a range from 0-3.0 kV. The nominal no-load plate voltage should read approxi-
mately 2,600 V. Plate voltage under nominal full load should read approximately 2,300 V.
Control
An adjustable automatic level control (ALC) circuit limits the peak output power. When
properly set, this circuit insures that the amplifier can not be over driven. Rear panel access al-
lows for easy manual adjustment . A sample of the RF input derives the ALC voltage. Addition-
ally this amplifier has an RF negative feedback resistance in the cathode circuit to help cancel
excessive RF drive without reducing the amplifier's gain.
A.L.C. Adjustments
Your transceiver's internal ALC will maintain linearity. The amplifier's ALC will prevent
over driving the amplifier. The VHF-1200/2000 ALC circuit was designed for negative going
ALC voltage. Proper adjustment is as follows:
1. Use an insulated tool when making these adjustments.
2. Tune the amplifier for operation on the desired frequency for full 1500 W output.
(1200 W on the VHF-1200)
3. With your transceiver set for SSB operation, set the transceiver's microphone gain
for normal operation as specified in its owners manual.
4. While speaking louder than normal into the microphone, adjust the ALC control on
the rear panel through the access hole. Adjust for 1500 W (1200 W on VHF-1200)
maximum output as indicated on an external peak reading wattmeter. If an average
reading wattmeter is used, adjust for approximately 600 W output on voice peaks.
(400 W on VHF-1200)