[Arran]

I very much doubt whether any more then one tube would have a short in it, very likely it will be the one nearest that resistor that burned up. Also tube shorts don't come and go that way, if there is a hard short between elements enough to broil a resistor then it will not normally disappear as the tube colls down. In my opinion it is almost a waste of time trying to troubleshoot a set that still has most of the original paper capacitors still in it, the old caps cause 80% of the troubles these set have. If the short comes about several minutes after the set was turned on and playing then it very likely could be a paper cap that intermittently shorts or becomes very leaky as the temperature changes.
Regards
Arran