08-08-2013, 07:26 AM
Has the set been electronically restored yet or is it as is? If it hasn't been electrically restored, works fine when it starts up, but starts distorting after 15 minutes that's a sign of a bad resistor, probably a bias resistor in the audio stages. Bad capacitors can do that as well but it usually shows up after a much shorter time frame. It could also be a defective tube, one with gas or secondary emissions.
Regards
Arran
Regards
Arran