GFCI is a suitable replacement for two prong outlets without a grounded wire.
This is absolutely true per the NEC, but the assumption made is that the incoming AC would come into contact with the chassis and trip in time. I’m not sure if this would prevent electrocution if a high voltage DC supply with lots of potential energy in the rectifier caps were to come into contact with the chassis. Lots of things could come into play here and even if the GFCI DID trip those caps may be several seconds away from fully discharging through someone’s heart or through the ground on the interconnect cables.
Seriously though, not grounding the chassis of a high voltage component is hobbyist level BS.
The DC power supply in the amplifier is a separately derived power system and does not have any reference to the AC mains electrical service Equipment Ground system.
The high voltage DC power supply B- for the amp is connected to the chassis. In the world of the internal circuitry of the amp the B- to chassis is the ground. No different than Class II double insulated audio equipment that doesn’t use an EGC.
Looking at the wiring diagram of the amp if B+ comes in contact with the Chassis there will be a short circuit. EGC or no EGC makes no difference. (What if a power tube shorts?) It is possible the AC mains fast blow safety fuse for the amp will blow because the high voltage secondary winding of the transformer would be overloaded which would cause the primary winding to overload and cause the fuse to blow shutting off the AC power to the power transformer.
The Equipment Grounding Conductor (EGC) is designed to allow unwanted fault-current to travel back to its source by way of a low-impedance (unobstructed) electrical path.
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