No matter how careful you are with a valve amp, you can still get caught out.
Happened to me today.
Here is a Marshall DSL 201, unplugged from the mains. HT voltages on the valve bases were measured insignificant, I checked that.
Yet the main HT cap doesn't have a bleed resistor, and the full HT voltage remains on the standby switch.
Normally I check directly on the PSU caps themselves, but this has PCB mounted radial types so you can't access them directly.
Ouch, that hurt when I grabbed the amp to turn it over and my thumb hit the standby switch with my other hand on the chassis.