[morzh]

Frequency does not affect transformer action - you still get the output equal input times the turn ratio.
However your ripple voltage after rectifying is inversely proportional to the frequency meaning that if you have Vripple=I/(2*f*C), you ripple will double when you go 25Hz vs. 50 and more than double if vs 60Hz.
Which simply means you have to use a bigger cap or your ripple will be too...ripply. Notwithstanding the fact that it will lower your rectified voltage.


As for the bucking transformer, it is simply a transformer connected sort of like the autotransformer where the secondary winding (good for the full load current) is connected in the opposite direction with the primary, thus dropping the voltage without power dissipation. This is the most accurate and clean waty of reducing the line voltage. For practical purpose you simply buy a, say, 120/12V transformer, connect the windings and end up with approximately 127-12=115V AC.

You might ask what's wrong with the regular step-down transformer 127/110V. The thing is the step-down transformer makes all winding carry the full load current (primary is almost the same as secondary) making it a big heavy expensive thing.
The bucking transformer makes only the small voltage secondary carry the full load current making it a much less power-hungry (10% of the step-down) and thus a whole lot smaller, lighter and less expensive thing.