Rather than short control grids of tubes to B- with metal screwdriver to isolate noise producing amplifier stage, we can use 40 mfd. capacitor and clip negative lead to ground and other lead is insulated probe wire This will short any signal, hum, or noise voltage and not effect direct current voltages in circuits.
We can trace audio signal back to stage where hum or noise first starts. Close SPST switch to connect probe in simulator.
