6.5. Simplified Schematic Diagram
Please, see Figure 6-1 Simplified schematic diagram.
The 4CX1000A/8168 ceramic and metal radial beam tetrode (V1) is grid-driven. It can dissipate up to
1000 W when forced air cooled and is specifically designed for class AB1 RF linear amplifiers.
The input signal from the RF INPUT jack is passed through a broadband input matching circuit, which
consists of components in the INPUT PCB and Rsw. This circuit tunes out the tube input capacitance. The
swamping resistor Rsw is a termination load for this circuit and can dissipate up to 100 W of RF drive
power.
Cathode resistors labeled Rc create DC and RF negative feedback, thus stabilizing the gain and equalizing
the frequency response. The varistor VSsg, in the screen grid circuit protects the tube and the screen grid
voltage regulator in the event of an internal tube flashover.
The combination noted as Lp1-Rp1 in the plate circuit is a VHF/UHF parasitic suppressor. DC plate voltage is
fed through chokes RFC1-RFC2 and the capacitor Cb3 blocks it from the output. The output circuit consists
of LP1, LP2, LL, CP1-CP3, and CL1-CL3, which form a classic Pi-L network to suppress any harmonic
frequency emissions. This tank is switched and tuned by S1A-S1C and the air variable capacitors CP1, 2 and
CL1, 2. The output signal is fed through an additional VHF low-pass filter for frequencies above 55 MHz (Lf1,
Lf2 and Cf). Then it is passed through the vacuum antenna relay K1, wattmeter current transformer TA1,
and a high-pass filter RFCasw-Casw for frequencies below 100 kHz, to the antenna switch and the three
outputs. The chokes RFC3 and RFCasw keep track of the antenna relay contacts and together with Casw
prevent the plate supply voltage from reaching the antenna. The RF choke RFCasw shunts the high voltage
to ground should the DC blocking capacitor Cb3 fail. The resistor Rasw protects Casw from electro-static
voltage build-up from the antenna.
The PLATE CAPACITIVE DIVIDER and RF WATTMETER are the main sources of information for the control
circuit of the amplifier during the antenna impedance matching process.
All supply voltages are delivered from the MAINS&LOW VOLTAGE and HIGH VOLTAGE SUPPLY PCBs. The
control grid, screen grid, and plate currents, plate cooling airflow temperature, reflected power, etc. are
continuously monitored by the Control PCB. Many software-derived protections are based on this
information.
Detailed electrical schematic diagrams are available from ACOM or from your dealer on
request.
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