[Arran]

From what I can remember the model 444 "People's Radio" used a special tube/valve called a PenDD61, it was the power output pentode plus the detector and AVC diodes, with a pre war British style base, the rest were North American type tubes. The PENDD61 was a tube only made for the 444 as it used a 6.3 volt heater whereas similar pre war British tubes/valves had 4 volt heaters. https://antiqueradio.org/Philco444.htm
  If your set has five tubes/valves then it could be an A537, V537, or a U527, then again it looks like someone has replaced or added tube sockets/bases as Philco normally mounted them under the chassis with rivets.
Regards
Arran