[Arran]

The 6V6/6V6/6V6GT is actually a good, if not better substitute for a 6F6 or 6F6G, but it depends on how the original tube was biased. If the original tube, or tubes in the case of a push pull output stage, are cathode biased (or self biased) then you can use either tube type, but if the tubes use fixed bias then some circuit modification will have to be done as each tube requires a different bias voltage. I don't know which sort of biasing was used in the 37-630, one way to tell is if there is a resistor connected between the cathode of the 6F6G tube and B-, if it has then it is cathode biased, if not then it's fixed biased.
Regards
Arran