450 watts / 3.5-30 MHz Linear amplifier

Posted on Aug 25, 2012

This is the circuit of a 500 watt linear amplifier, based upon a design by Frits Geerligs, PA0FRI, who has his own homepage at http://home.planet.nl/~fhvgeerligs. The circuit uses four PL519 TV line output valves in a very simple circuit that will deliver over 450 watts at 3.5 MHz (350 watts at 30 MHz). PL519 (40KG6A) is a more robust replacement for the earlier PL509 (40KG6) tube. Both valves will work well in this circuit. The input drive power is about 50 - 100 watts so it is compatible with most amateur radio HF transmitters. Not shown in the circuit is the cooling fan that is required to force air around the valves to cool them. In operation the 1K0 pot is adjusted to set the total valve anode current to around 50mA to 70 mA.

450 watts / 3.5-30 MHz Linear amplifier
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450 watts / 3.5-30 MHz Linear amplifier - image 1
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T1 is a 4:1 balun wound on a 5cm ferrite rod. 9 + 9 turns. Connect the end of the first winding to the start of the second to form the center tap. L1 is 9 turns of 3mm Dia wire, wound on a 25mm Dia, 60mm long former. L2 is 18 turns on a toroidal former. Use two length of 2mm Dia wire, one with 11 turns and the other with 7 turns. The 50 watt 100 ohm resistor recomended by PA0FRI is formed by two 50 ohm 25 watt non-inductive TO-220 resistors in series, bolted beside the fan. I use 100 x 10K carbon resistors aranged 10 x 10 between two pieces of 0.1" matrix wiring board (veroboard). My method is cheaper and avoids the need to mount input circuitry above chassis. All inputs are kept below the chassis whilst the valve anode terminals and output circuitry is kept below the chassis. The 100pf trimmer capacitor is adjusted for best VSWR from the driving transmitter at 29 MHz. All four valve heaters (40 volts each) may be wired in series and connected to the 220 volt mains via a 6uf 250vAC capacitor for 50 Hz (5uf for 60 Hz). I personally favour the use of a 40 volt transformer winding, on a home-made transformer, to run all the valves heaters (in parallel) as well as the 40 volt fan. This places less strain on the cathode/heater insulation of old tubes that may have been kicked around in junk boxes for years. Winding transformers can be quite involved and I am writing an article for this on another page. But, here is the...

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