[Arran]

Yes, there is a large paper capacitor, like a .22 microfarad, that will be tied between B- and the chassis, probably in series with a choke coil. That capacitor will also be paralled with a 220,000 resistor somewhere in the circuit. If that capacitor is open circuit the set will motorboat. It's also possible that the set needs an IF allignment, for some reason this can cause motorboating on a Philco. Another cause could be that the leads of the primary and secondary of an IF transformer may be crossed.
Regards
Arran