sentences:  In the event of a tube short failure or simply a transient overload condition the diode will conduct once the current in the 22ohm screen resistor reaches 30mA and beyond, preventing further burning of the PCBA.The diode will clamp the current in the 22ohm screen resistor to 30mA, so preferably the plate resistor will blow, since the plate resistor is acting as a "fuse".Plate resistors should be kept off the circuit board by approx 1/2" min to prevent PCBA burning.