[RodB]

The 250 ohm resistor in the power supply is to bias the 42 grid at -10 volts. The way you explained it, it sounds like the 42 is now wired for cathode bias. To me, 500 ohms is too high to produce 10 volts. But you can check the 42 tube characteristics to get the right value. Probably in the range of 250-300 ohms and it should have a bypass cap of 5mfd 16v.