QRO Technologies HF-2500DX Instruction Manual page 21

(Transmit Mode). RY2 closes and the RF energy supplied by the transceiver passes though
RY2's closed contacts to relay RY3. If you have selected 160, 80, or 40 meters on the band
selector switch SW3, the signal passes through normally open contacts and to R607 the 50 ohm
100 watts input swamping resitor. R607 is connected to ground. The rf signal is supplied to the
control grid of the tubes at the input point of swamping resistor R607. Coupling capacitors C57
& C58 located at the tube control grids isolates the tubes from the input circuit. If you have
selected 20, 15, or 10 meter on the band selector switch CW3, relay RY3 goes to its normally
closed position. Depending on the band you have selected, one of the following relays will latch
to their normal open state. Then a necessary amount of reactance is applied to cancel the
capacitive input reactance of the tubes. The rf signal then goes to RY3 and on to the R607
swamping resistor and the tube control grids as outlined above.
TUBES
The amplifier's tubes are connected in a class AB1 grounded cathode circuit. RF driving power
is applied to the control grids in the normal grid driven configuration. Pins 2, 6, and 6 of the tubes
are internally connected together and are connected B- circuit. The B-1 circuit is not at chassis
ground. Pins 3 & 7 are connect to the filament supply. Pins 1 are connected to the rf input and
grid bias circuit. Pins 5 are connected to the screen bias supply circuit. Parasitic chokes PC1 &
PC2 are connected to the plate lead to suppress any VHF parasitic oscillations. The positive side
of the power supply (B+) is connected to the tube's plate through RFC3 (Plate Choke). Capacitor
C56 provides a low impedance path to ground for any RF energy that my get through RFC3.
Capacitors C54 & C55 are DC blocking capacitors. They allow the RF signal to enter the output
tank circuit, and they block the DC high voltage from entering the tank circuit. Fan B1 circulates
cooling air through the cooling anode fins of the tubes.
RF OUTPUT CIRCUITS
The tuned output circuit of the Amplifier is a pi-l network using a 4:1 transformer (L3). The pi
circuit transforms the plate load impedance down to 200 ohms. The 4:1 transformer further
transforms the 200 ohms down to 50 ohms. L3 has a secondary function to provide a broadband
L coil for harmonic suppression. It is always at DC ground potential. Therefore, L3 will prevent
high voltage from going to your antenna if the blocking capacitors C53 & C54 should short. The
positive high voltage would go to dc ground causing the line fuse to blow which would shut the
amplifier down. Band Switch SW3C progressively shorts out the unused portions of tank coils L1
& L2. The tank coil turns in use are tuned to resonance by Tune Capacitor C51. Load Capacitor
C50 is tuned to complete the impedance match between the tube and the 4:1 transformer L3.
Transformer L3 is then connected to RF Output connector J1 through T/R relay RY1. Capacitor
C52 & C53 provides additional capacitance for C51 on 80 & 160 meters. Capacitors C54 & C55
provides additional capacitance for C50 on 160 & 80 meters. Again, if DC plate voltage should
enter the tank circuit due to a short in either DC Blocking Capacitors C54 & C55, the DC voltage
has a path to ground through the 4:1 Xfmr (L3). This will short-circuit the high voltage supply
which will blow the AC line fuses. Also, DC plate will be prevented from entering the Antenna.
ALC CIRCUIT
ALC Adjust Control (R13) located on the front panel allows you to set the ALC threshold to match
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