[Mondial]

The open osc coil is causing the voltage on the 36 cathode to be very high. Without the coil providing a ground connection for the cathode, the voltage will rise to the plate potential, which is why you measure little difference between P and K. The open coil is a very common problem and will need to be rewound.

The 75 plate voltage is not significant in that it depends greatly on the condition and characteristics of the tube itself. The 75 may be a bit weak, but still should work ok. In any case the 75 plate voltage has nothing to do with the open osc coil.