Heaters, 2,7 are good, the plate and screen are good, 3, 4. Are you sure the 20 VDC is on pin 5 and not pin 8? The cathode needs 20 VDC to show it is drawing current properly. Measure resistance from the cathode, pin 8, to ground. Does it measure 470 ohms like it should? You can calculate current draw from the cathode voltage using ohms law, V = IR or I = V/R If you really have a 236 voltsge drip accross a 470 ohm resistor then the current draw thru the 6v6 would be: I = v / r = 236 / 470 = 500 mA, about 10-12 times too high. Surely the tube would be red plating. Something is off with your measurements, I believe.