I wanted to measure the "Plate volteage" of my DSL40CR. Googling, the closest I got to was "pin 3". Have no idea where that is.
Plate (or anode) - is a pin 3 of EL34 power tubes in your amp.
Warning: there is lethal voltage inside your amp, so DON'T open the cover when device is connected to wall socket, unless you are experienced with high voltage devices.