The screen grid resistor is attached to the wrong location on the PCB it should be on the other side of R25 (junction R25 R26 and C16). This will lower the screen grid voltage closer to (right at) its maximum rating of 300 volts. Best if you have the pcb out is to also replace R25 with a 7.5 to 10 k ohm resistor (approx 4ma across R25).