[N2IXK]

Grounding the screwdriver when testing for HV at a rectifier tube plate is definitely a BAD idea.

In most sets, the voltage at the plate of the HV rectifier has a DC component of a few hundred volts, in addition to the high voltage AC pulses. Providing a DC path to ground will take the B+ (from the HOT tube plate), and short it to ground through the HV winding on the flyback (200-300 ohms?) and the (low) resistance of the arc itself. This can easily burn the flyback winding open, as it is not designed to have any appreciable DC current running through it, and it is wound with very fine wire.

A grounded screwdriver should only be used to (briefly) check for DC high voltage at the CRT anode lead, and then only if you don't have access to a real HV meter or probe.