• 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.

Does anyone use an RCD on the DC Plate HT?

Residual Current Device working only if the AC "live" (sum in 3 phase case) and "neutral" currents differs.
It has current sensors which are exiting electromagnet coil. If the error current enough large, the electromagnet pulls, and break the connection/s/.
With DC it's inoperable.

BTW no such devices generally available for magnitude of kV.