• 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.

Grounding the Gyrator Rev 08?

The PCB's "ground plane" -which is contains mounting holes- tied to the circuit's "GND" output.

The circuit's GND possible connected (via ground loop breaker ... or not) to the safety ground, which is mainly connected to the case.

If you connect this PCB to the heatsink (which possibly is part of the metal case) with metal washers, ground loop may occurs.

If you use plastic washers, this can be eliminated.