2 x 813 HF Linear Restoration part 1

The first steps in restoring and updating the old G2DAF linear amplifier commenced with the removal of the 2 x 6U4GT rectifier valves and the associated voltage doubler components / 300 ohm input circuit. This will be replaced with a solid state voltage doubler (probably a 1N4148 diode stack) and a 1:4 unun broadband transformer to give as close a match to 50 ohms as possible. Next step was to solder a 5000 ohm resistor from the 813 anodes to ground. Two 10k resistors in parallel were used (see photo) to represent the load that the valves should see under normal operation. An MFJ antenna analyser was then connected to the pi - tank output and the variable capacitors in the tank circuit altered until a 1:1 50 ohm match was obtained on 80m and on 10m. This checked the output matching circuit.

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