[Arran]

Ron;
  With regard to why Philco decided to use 2.5 volt tubes in the model 52 rather then 6.3 volt tubes I can speculate that economics was the reason. Philco probably had a large inventory of 2.5 volt tubes, or acquired a large quantity at bargain prices, along with the matching power transformers, and since the model 52 was a budget priced set using older style tubes was more economical then using the newer 6.3 volt ones. Why they used 6.3 volt tubes in the British model 56 was probably for similar reasons, maybe 6.3 volt replacements were easier to find for the set owners, or British Philco had a large inventory of 6.3 volt tubes, or didn't wish to have duplicate inventories of AC tubes since it was a smaller market? I know that in the post war period Philco was constantly switching tube types in the same model, such as the 46-1201, they had something like eight different code types under four production runs, the Hippos and Transitone models were similar.
Happy Christmas
Arran