[Mondial]

The voltage on the AVC tube cathode should not change much with the localizer control because it is set by the tapped voltage divider resistor. The slider of the localizer control should only change from around -64 V to about +12 V ( -64 from power trans center tap line and +12 from AVC tube cathode). Are all sections of the tapped resistor good and within tolerance?

The cathode of the 24 AVC tube is grounded through the 290 ohm section of the tapped resistor so there never should be 50 V on the cathode. Cathode should be at about 12 V, so if you have 50 V there something must be wrong with the tube or the tapped resistor.