This linear uses a single 4CX250b forced air cooled tetrode and produces 200 -250 watts output for a drive level of approximately 2 – 3 Watts. The circuit is very simple; the biggest challenge is the physical mechanics of it. The photographs show it all really, as you can see no expense has been spared .The chassis base is a die cast aluminum box, the walls of the case are fabricated from copper clad board and the lid is a piece of perforated steel ( a perforated aluminum ceiling tile would also suffice ). The aperture in the side of the chassis is for a vacuum hose supplying forced air from a squirrel cage blower mounted in a separate box that hangs underneath my operating bench. Looking at the circuit diagram you can see just how simple it is; no neutralisation is used as the control grid of the valve is heavily swamped by the 3K3 resistor in the bias supply. L1 and its tap provides an impedence match from the 50 ohm transceiver output to the 3K impedence control grid of the valve. The linear has proved to be extremely stable and easy to tune when in use. The anode tank coil is made from 2.5mm mains cable with the insulation removed. The output change over relay used was salvaged from old Westminster/Europa sets, these are fine for this power level providing the contacts are closed BEFORE any RF is applied to them and RF has CEASED before the contacts are opened (otherwise the arc produced will destroy the contacts and could also damage the valve), hence the timed delay circuit for RLY2 and the hold circuit for RLY3. VR1 sets the standing bias for the valve; 50mA- 70mA seems optimal for this linear. The anode current meter allows easy tune up and monitoring of the valve. Lethal voltages are used in this circuit so extreme caution is advised.
Barry Zarucki M0DGQ
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