WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
If you scrap an old MW/LW valve radio for its (large) variable air-spaced capacitor, you are likely to have a 2 x 365pF or 2 x 500pF capacitor that is ideal for quickly determining optimum feedback capacitor value.
Bung a square wave up it, and two seconds find the answer. I was quite surprised. You can put a variable resistor in series with the capacitor, if you like, and waggle both.
