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    Ameritrons 1306 Amplifier Information

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    General Operation
    This linear amplifier covers the 160, 80, 40, 30, 20, 17, 15, 12, 10, and 6-meter amateur bands. Up to 100-watts exciter power is applied to relay RLY1 on circuit board RLY. With the main power OFF, the STANDBY/OPERATE switch on STANDBY, with a fault warning LED
    illuminated, or with the rear panel RELAY jack ungrounded, RLY1 bypasses through RLY2
    directly to the antenna port.

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    When power is ON, the STANDBY/OPERATE switch in the operate position, and the rear panel
    RELAY control line held low (below 1 volt), exciter power is routed through RLY1 to the PD8
    power divider board.

    Power Division
    The PD8 power divider board attenuates the exciter input signal, and divides exciter power
    equally, between two 600-watt power amplifier modules. It is 50-ohms on all three ports, with
    8.2 dB nominal attenuation to each output port.

    The PD8 circuit board consists of a conventional magic-T power divider, components T2 and
    R7. This T divides drive power into two equal-power 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 50-ohms and provide an additional 10 dB of input port isolation between
    the two PAM’s. With a 50-ohm source, in excess of 30 dB port-to-port isolation occurs between
    PAM 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
    PAM. The 50-ohm termination and input port isolation results in unconditionally stable PAM’s.

    PAM-606 (power amplifier modules)
    Power amplification comes from two 600-watt power amplifier modules. Each PA module
    (PAM-606) uses four MFR-150 field effect transistors. Each MRF-150 has 100 mA to 300 mA
    quiescent current. Transistor conduction angle is slightly over 180-degrees, providing linear
    class-AB operation. While the precise value of quiescent current has little effect on linearity, it isvery important to adjust all eight FET’s to the same quiescent current. Target current is typically150 mA in this design.

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    Normal dc drain operating voltage is approximately 50-volts. Be aware bias control rotation is reversed in CB2 boards, as compared to the previous generation ALS-1300’s CB1. When servicing any solid state PA, always verify function of bias and set for minimum bias before applying drain voltage.

    Unlike standard Motorola based modules, the PAM-606 modules use two diametrically
    opposed push-pull pairs of 150-watt MOSFET’s. The 300-watt push-pull pairs drive
    balanced VHF striplines. The balanced striplines combine at a matching transformer. The
    linear RF power FET’s mount on a forced-air-cooled aluminum heatsink.

    Two dc fans cool each PAM-606 module. Two thermistors (PAM-606 R2) sense power amplifier
    transistor temperature. Transistor temperature thermistor R2 regulates bias voltage, reducing bias voltage as transistor temperature increases. This bias feedback system keeps transistor quiescent current stable independent of transistor junction temperatures. PAM-606 thermistors R2 also feed a comparator that removes drive when transistor temperatures approach unsafe levels.

    Bias voltages for the PAM-606 modules come from the CB-2 control board assembly. Each transistor has an individual bias adjustment, with minimum bias counter-clockwise from the top view. This is opposite the control function in older CB1 assemblies.

    A second set of thermistors (PAM-606 R1) monitor heatsink temperatures. Voltages from
    thermistors R1 regulate fan speed, increasing fan speed and airflow as the heat sink warms.
    The PAM-606 modules employ significant negative feedback to reduce gain, improve gain
    flatness, improve linearity, and ensure 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. Push-pull operation, negative feedback, and linear biasing of FET’s provide significant pre-filter harmonic suppression.

    The characteristics of linear high-voltage FET’s are very much like those of triode vacuum tubes. While this amplifier will run more than 1200-watts PEP output, linearity might suffer.

    Ameritron recommends running 1200-watts PEP or less for maximum linearity, although most amplifiers will remain clean above 1200-watts PEP. Following these instructions, this amplifier will have IM performance comparable to the best vacuum tube linear amplifiers.

    Band Decoding
    The CB2 board contains band-decoding systems. It also has a sensitive embedded frequency
    counter system. The frequency counter system in all ALS-1306 amplifiers, regardless of band
    selection mode, automatically disables operation between 25 and 28 MHz. This embedded logic function cannot be disabled or changed.

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    The CB2 contains protection logic for predetermined levels of antenna reflected power, filter
    reflected power, PA module balance, and PA transistor temperature. In the event of a safety
    fault, the transmit-receive relay is disengaged in a normal receive transfer and a proper
    warning indicator is given. The normal sequence is remove bias, remove exciter relay,
    remove antenna relay, and illuminate warning LED. Reset requires removal of the fault condition and placing the STANDBY-OPERATE switch in the STANDBY position. If faults are cleared, operation will resume upon placing the STANDBY-OPERATE switch in the OPERATE position


    Harmonic suppression comes from push-pull operation of linear devices, followed by high-
    quality 5-pole low-pass filters. Many amplifiers use inexpensive ceramic disc or mica capacitors. Lead inductance of mica or disc capacitors reduces high-order harmonic suppression. This amplifier uses quality multi-layer high voltage chip capacitors.

    Spectrum Analysis Of Low Pass Filter Board

    Last edited by KD8MJR; 09-14-2014 at 04:49 PM.

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