[Arran]

I have had the plate load resistor on 6Q7/6SQ7s fail causing this very issue, they either change in value, going high resistance, or go open entirely, a number of Canadian RCA and C.G.E sets have this problem. Another thing that can cause it is the cathode bias resistor on the audio output tube, if the tube uses cathode, or self bias, a lot of current flow through there and they can drift as it heats up, I will have to look up the 37-630 to see if it uses that or fixed bias.
Regards
Arran