[Arran]

If you want to isolate the B+ supply from the power line you could look around for an old style razor outlet transformer, and use one of those alongside a low voltage transformer for the A+ supply, make sure you find one that can deliver at least 500 ma or more as farm battery sets typically have the tube filaments connected in parallel . Another method some people use is to use several small transformers and connect the secondaries back to back to get the B+ side, I don't really like this method as it gets bulky and complicates what should be a straight forward power supply.
Regards
Arran