sentences:  The diode is taking on the current surge to protect the 22 Ω resistor, however the peak over current in the diode can only sustain for a short duration.The 6550A version usually idles with roughly 700 V DC on the plate and 350 V DC on the Screens (though during full clean sine wave 300 W power output, operating voltages will dip to roughly 650 V DC on the Plate and 325 V on the Screens).The output transformer plate load is 1.6 K at 4 Ω tap and 1.75 K at the 2 Ω tap.