If you have a tube output in the radio, it will have an output transformer, the resistance of the output of the transformer should be the same as the speaker you should use. If you ie measure 2ohms across the output transformer, then measure the resistance of a 8ohm impedance speaker, for comparson (resistance may be around 4ohms) which will give you an idea as to what impedance (AC resistance) speaker to use, this will allow the output tube to deliver the maximum power possable. May also be an idea to check the condition of the output tube(s) and any paper capactors for shorts.
