Hi Alan,
according to the information for the 5642 HV rectifiers the valves have directly heated cathodes. The line output transformer will have separate highly insulated windings for each valve. The filament voltage is only 1.25V so it's possible only one turn is all that's required on the transformer.
From the Radiomuseum:
https://www.radiomuseum.org/tubes/tube_5642.html
From R-type.org:
http://www.r-type.org/exhib/aai0016.htm
DFWB.