[Arran]

 I remember running into a website about Volksemfanger sets a while back, I even bookmarked a page about them either posted by someone in Germany or Austria. If I remember correctly they all seemed to use a three tube regenerative circuit, but I don't recall whether they all had a power transformer or whether some were series string. AF7 sounds like it would be the proper type from what I could find about most other VR301s. Unfortunately the Germans, like the British, seemed to have this silly predisposition to using 4 volt filaments in many of their tubes, which seems to lack any rhyme or reason. It looks like its an expensive tube too if fleabay BIN prices are to be believed , I wonder if the 6.3 volt EF7 is cheaper. Basically it's an RF amplifier pentode of some kind, at least with this you can work out a substitute if there is one. Maybe you can make an adapter out of it's base and use a cheap and useless series string TV tube like a 4BA6?

http://tubedata.tubes.se/sheets/030/a/AF7.pdf

Regards
Arran