HF linear amplifier circuit
Posted on Feb 6, 2014 6254
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.
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.