Ameritron ALS-1300 HF 1200 WATT NO TUNE TMOS-FET AMPLIFIER
The ALS-1300 is an amateur radio multiband radio frequency linear power amplifier. This linear amplifier covers the 160, 80, 40, 20, 17 and 15-meter amateur bands. Up to 100 watts exciter power is applied to relay RLY1 on circuit board RLY. When power is onand the standby switch is in the operate position,and when the rear panel RELAY control line is held low (below 1 volt), exciter power is routed through RLY1 to the PD8 power divider board.
The PD8 power divider board splits the signal path equally between two 600-watt power amplifier modules. The PD8 circuit board consists of a conventional T power divider, components T2 and R7. This T splits the signal into two equal level signals. Each signal path has a 5 dB attenuator consisting of high power resistors R1 through R6. The 5-dB attenuators on each output port terminate the T in 50ohms and provide an additional 10dB of input port isolation between the two PA modules. With a 50-ohm source, in excess of 30 dB port-to-port isolation occurs between PA module inputs. A minimum of 16 dB isolation occurs regardless of input port termination. The attenuators also work in concert with the magic-T to provide a 50-ohm input termination for each PA module. The50-ohm termination and input port isolation results in unconditionallystabile PA modules.
Power amplification comes from two 600-watt power amplifier modules. Each PA module (PAM-600) consists of four MFR-150 field effect transistors. Each MRF-150hasbetween 200 mA to 400 mA quiescent current. Transistor conduction angle is slightly over 180 degrees, providing linear class AB operation. The normal dc drain operating voltage is approximately 50 volts with four FET’sper module in push-pull parallel. Both modules employ significant negative feedback to improve linearity and stability. The FET’s have direct resistive voltage feedback across each individual transistor from drain to gate, as well as push-pull transformer (T2) coupled feedback common to the push-pull circuit. These feedback circuits improve gain flatness, linearity, and stability. Push-pull operation, negative feedback, and linear biasing of FET’sprovide significant pre-filter harmonic suppression.
Cooling and Temperature
The four power FET’s in each module mount on a forced-air cooled aluminum heatsink. Four dc fans cool the heatsink.Two thermistors (R1 heatsink, R2 transistor) sense the temperature of the power amplifier transistors and their heat sinks. Transistor temperature sensor R2 regulates bias voltage, reducing bias voltage when transistor temperature increases. This bias feedback system keeps transistor quiescent current stable independent of transistor junction temperatures. These thermistors also feed a comparator that removes drive when transistor temperature becomes unsafe.
The second set of thermistors, R1, monitor heatsink temperatures. Voltage from thermistor R1 regulates fan speed, increasing airflow as the heat sink warms.
Each PAM-600delivers a rated maximum of 600 watts peak envelope power into a 50-ohm load. Each PA module feeds a 50-ohm high power combiner CMB that isolates the two outputs. A 50-watt 100-ohm power resistor dissipates signal level errors between the PA modules.
Voltage step down transformer T1 senses voltage across combiner dump resistor R1. T1 is located on the 2KWF lowpass filter board. This voltage, representing PA combiner unbalance,appears on the front panel multimeter as a “PAB” (power amplifier balance) indication. PA unbalance sample voltage also feeds a comparator on the CB1 control board. This comparator disables the PA in the event the power amplifiers become significantly unbalanced, and illuminates the PA front panel light.
2KWF Lowpass Filter Assembly
The output of the high power combiner goes into the 2KWF lowpass filter assembly. Power enters through a directional coupler consisting of T2, C28, 98 and 99, and R2. This system detects power amplifier termination errors. Any significant error will trip the power amplifiers off. A comparator on the CB1 control board monitors directional coupler termination errors. Such errors normally come from selecting the wrong filter for the exciter’s operating band, or having a poor load SWR on the amplifier.
5-pole lowpass filter 2KWF further reduces harmonics. Harmonic levels are significantly below levels mandated by FCC rules governing amateur radio service.
The CB1 control board contains all protection comparators (U1) and latches (U3). In the event of an operational fault, latches lock out the amplifier and illuminate a warning light on the front panel. CB1 also contains bias, relay sequencing, and fan speed controls.
Frequencies above 21.5 MHz are disabled in units sold in the United States and possessions.
The SWR board is a standard 50-ohm directional coupler. It samples line current and line voltage, vectorially summing them before detection. The resulting voltages represent forward and reflected power, or mismatch of the ideal 50 ohm load.
This amplifier provides the following standard features:
- Eight conservatively rated, very linear, 50-volt MOSFET transistors
- Operational in a few seconds, no long filament warm-up time
- Clean layout with easy-to-service modular construction
- Front panel indicators including wrong band and other fault errors
- Quiet variable-speed forced air cooling system
- Power module balance metering
- Power module current and voltage metering with LED illumination
- PEP forward output power and PEP reflected output power metering
- Reflected power protection
- Bandswitch error protection
- Standard negative going ALC output with front panel adjustment control
- ALC metering and an ALC LED indicator
- Fully regulated external power supply
- Compact size 17.5” deep x 7”highx 10.5” wide
- Weight amplifier section23pounds