Despite the protection circuit, power tubes blow, screen resistors opening are common. The system isn't perfect. A plate resistor will act as a fuse. A 10 ohm plate resistor rated at 5W will eventually open with sustained 1/2A. In '77 they used a 5 ohm 5W plate resistor, 1A sustained. There is also a 1 ohm 5W shared cathode resistor. When these blow damage is done, often to the tube and the PCB. The fusable screen resistor also offers some protection. High frequency oscillations are capable of damaging an amp as well. Ideally a plate resistor and a series connected ferrite bead would be good to have.