HF linear amplifier circuit

  
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The PA output Pi section consists of 2 variable Cs (20 - 120pF and a second (home made) 50 - 450pF switched in for the lower bands by a simply modified 12v power relay). The anode choke (200t, 24swg 5` long on 1` dia PTFE tube stock) is mounted directly onto a 3300pF 10kV doorknob capacitor using a small turned brass ferule. The grid compartment.
HF linear amplifier circuit - schematic

The main fan blows into the sealed grid compartment, and exits through holes around the grid bases to flow through the PA chimneys. The heater/cathode connections are again made from thin brass strip. The heater choke is wound on ferrite rod (from a scrap transistor radio, bifilar 28t 10swg). The input circuit consists of a 100ohm non-inductive 50Watt resistor (Farnell - about £3) directly across the input. The grid load is also about 100ohm under normal operating conditions so the input VSWR should be quite good.



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