All of the microwave oven supplies have a pretty ingenious setup. The magnetron is run with the anode grounded and the heater-cathode supplied with negative voltage, There's a diode and capacitor, and the whole mess runs as a sort of voltage doubler. By simply rearranging the diode and capacitor, a half-wave supply can result. No good for an amplifier, but perfect for a simple tester. Put a Variac in the primary circuit and a couple of well-insulated meters in the secondary, and you've got something.