2. Overview:
Medium-power 144MHz VHF amplifier is a based on a grid-driven, single 4CX250R
(7580W) forced-air cooled ceramic-metal power tube. RF output capabilities are 400
watts SSB & CW with 8 watts of input drive. Higher output upwards to 500 watts can be
achieved when driving the tube with a greater input power not to exceed 10 watts.
Amplifier Circuitry:
Refer to amplifier schematic Figure 1.
Amplifier input tuning consists of series tuned LC circuit (L2, C2 & C3 parallel
combination) combined with tube input capacitance. Input RF is link coupled to L2
through L1. During operation, C2 (Input) is adjusted to achieve a minimum SWR of 1.5:1
seen from the exciter transmitter.
Output tank circuitry comprises of a quarter-wave strip line inductor (L3) tuned by an air
dielectric "flapper capacitor" (C9) fabricated with two brass plates spaced approximately
3/8". C9 stator plate is fixed against the tube anode cooler while the rotator “flapper"
plate is coupled mechanically to a 5:1 vernier reduction drive. The vernier drive allows
for smooth and precise tuning during operation. A mechanical stop is incorporated on the
flapper capacitor shaft to prevent accidental short circuit of C9. This capacitor assembly
serves as the Plate tuning.
Load tuning (C10) is accomplished with a 15 pf air variable capacitor link coupled (L4)
across the top of the strip line inductor (L3). Output RF is then transferred to the antenna
thru J2.
High voltage DC for the tube anode is fed via the strip line through a 1.72 mH, 600 mA,
RF choke (RFC1) and DC plate blocking capacitor (C8) integral to the strip line inductor.
The blocking capacitor is produced by sandwiching a strip of 0.010" Teflon dielectric
between the underside of the strip line and chassis ground. Without the DC blocking
capacitor, High Voltage DC would be present at the antenna.
Screen, control grid (bias) and filament voltages supplied by T1 are fed through
individual bypass capacitors located at the tube input compartment. Grid bias voltage fed
through Rswamp heavily swamps the tube to ensure stable amplifier operation
(preventing tube oscillations) and raises the input drive requirements to 8 watts. Screen
voltage is fed through R2.
Forced air cooling delivered by a squirrel cage blower enters the bottom tube input
compartment and exhausts through the tube anode cooler fins. A Teflon chimney
mounted at the lower tube seal directs air flow through the tube and exhausts through a
screened port at the tank circuit compartment cover. Prior to operation, forced air cooling
is a requirement prior to energizing tube filament and power supplies. Interlock features
are designed into the screen power supplies to prevent operation without cooling flow.
3. Power Supply Circuitry:
Refer to high voltage power supply Figure 2A and low voltage power supply Figure 2B.
The amplifier high voltage supply is built inside the lower portion of a 6' x 19"
communications rack cabinet. The High-Voltage supply is capable of delivering a
maximum output of 4,000 volts DC at 400 mA. For the 4CX250R tube, high voltage
requirement is 2,000 volts per data sheet. Voltage adjustment is accomplished via a 60A,
120 VAC autotransformer (i.e., Variac) connected to the primary winding of the HV
transformer.
The HV rectifier circuit comprises of four (4) 14kV, 1A diode blocks arranged in a full-
wave bridge. The HV filter capacitor bank is made up of ten (10) 400µf/450VDC
electrolytics connected in series to achieve approximately 40µf and maximum voltage
rating of 4500 volts. 40KΩ bleeder resistors are shunted across each electrolytic.
Step start delay circuitry is designed into the HV supply and located between the
autotransformer and HV transformer primary. This circuit limits inrush current that could
potentially damage the HV rectifier bridge during initial powering as the electrolytic
capacitors are initially discharged and look as a dead short. A RC time constant circuit
allows a 1 second time delay when switching on the main HV power relay to allow the
filter capacitor bank to charge. Once the filter capacitors charge to 5T, a second relay
closes allowing full primary voltage (120 VAC) to enter the HV transformer.
Analog metering circuits for HV voltage and current are included and calibrated to 0-
100V and 0-1A respectfully. 300KΩ of series resistance is connected to the HV voltage
meter and wired across the last filter capacitor (C10) adjacent the B- line. This lumped
resistance scales the meter to register 100 volts full-scale when reaching maximum output
of the HV supply. The meter scale calculates out to 200 volts/division. HV current meter
shunt resistance is not incorporated due to the meter factory calibrated 0-1 amp.
Screen, Grid and Filament supplies are built onto a sub chassis directly above the HV
supply. The supply delivers regulated 325 VDC to the tube Screen, -60 to -200 VDC to
the Grid and 6.3 VAC, 3A for the tube filament. Grid bias current adjustment is featured
to accommodate variations of the 4CX250 series tube. Individual metering is included for
Screen Current, Grid Current and Filament Voltage.
Parameters:
Plate Voltage (No Load): 2200 VDC RF Input Power: 640W
Plate Voltage (Load): 1600 VDC RF Output Power: 390-400W
Plate Current (No Load): 90-100 mA Drive Input: 8-10W
Plate Current (Load): 390-400 mA Plate Dissipation: 240-250W
Screen Current (Load): 30 mA Class of Operation: AB1
Grid Current (Load): 0-1 mA Efficiency: 62.5%
