According to Ohm's Law the current through the cathode resistor, flowing from the cathode to ground, is

19V / 470Ω = 40mA

Assuming the screen current is negligible, the plate current is also 40mA. This passes through the output transformer, flowing from the 305V plate supply to the plate, so the voltage across it is

(40mA)(50Ω) = 2V

In a clockwise loop starting at the cathode, there is a voltage rise equal to the unknown plate voltage, then a 2V rise across the output transformer, then a 305V drop from the plate supply to ground, and then a 19V rise from ground to the cathode. According to Kirchhoff's Voltage Law, the sum of the rises is equal to the sum of the drops:

V_P + 2V + 19V = 305V

which means

V_P = 305V - 2V - 19V = 284V

The 6V6GT plate voltage, measured from the plate to the cathode, is thus 284V.

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