David Satz pointed out that I misread your post. I have therefore revised my reply:
Start with having a qualified technician measure proper supply voltages: 105VDC B+ from the power supply should translate into roughly 32-36VDC at the plate of the tube.
If the B+ generated by the power supply is out of spec, have the technician switch out dropping resistors in the supply to get it into spec (supply voltages measured under load, of course).
If the plate voltage is below 32VDC, the tube is not healthy.
If both voltages are within specs, remove the tube, and re-insert it after applying a drop of Caig De-Oxit or Pro-Gold to each of the eight tube socket contacts.
Please report back after you've done the above.
