[Arran]

 If those voltage readings are correct then the output transformer is obviously good, though the proper B- common negative point in most AC/DC sets is the power switch on the back of the volume control. That being said 165 volts (hopefully measured on the DC range) is ridiculously high for the plate and screen grid voltages on the 50L6, as Greg stated if the 150 ohm resistor (R7) isn't open circuit then the tube might be bad, open cathode perhaps. In this set the B- is isolated from the chassis by a .05 uf cap (C20) paralleled with a 470 k Ohm resistor (R11), if you are measuring DC voltages and they are exactly the same between B+ and the chassis as they are between B+ and B- then the .05 uf cap C20 may be shorted.
  The resistor that I mentioned in the last post was labeled R5, it's a 470,000 ohm resistor on pin 6 of the 12SQ7, but given your voltage reading on the plate of that tube I'm wondering if you have your volt meter set on AC volts rather then DC volts since you are also reading 165 volt there when it should read about 40 vdc.
Regards
Arran