There are 3 other disconnects that are useful in initial testing of the amplifier.
For example, the filament can be disconnected in order to test other circuits without risking damage to the tube.
The screen and cathode can be disconnected, and the HV unplugged from the rear chassis in order to test the bias circuit.
Note the connection to the plate voltage metering resistors, as far away from other components as possible.
The fan speed control, a part of the cable assembly on the exhaust fan, is also visible here.