Grundig 80U

[Mike Watterson]

Primary across the mains, secondary  in series with it to the set

But measure voltage BEFORE connection!
If it's 260V instead of 220V you swap EITHER the 20V connections OR the 240V connections on the small transformer.

If you've got a big "continental" set, Grundig or suchlike with lots of piano key controls, its mains TX may well run hot on our mains. A buck TX is an ideal way to sort that out. To buck 20V, that's 1/12 of 240V so the transfomer rating need only be 1/12 of the power consumption of the set. In other words tiny for for most sets.

Unless they are push pull (rarer) or have a stereo decoder (very rare, really only export to USA models in Valve era) they are all about 40W to 45W max. Irrespective of how many buttons, speakers, size of cabinet or number of bands.
My "everyday" German model is about 38W as it has EL95 rather than more common EL84 and only one scale lamp.
https://www.radiomuseum.org/r/aeg_bimby_luxus_1461.html
Perhaps the big motorised one I gave away was 55W max, it had 2 x EL84 for single ended stereo from tape/Gram and push pull mono on radio.
Some German and French models do have voltage settings.