Many transformers have a pair of primary windings. You use them in parallel for 120V, in series for 220/240. A lot of consumer gear allows this to be selected with jumpers.
voltage transformer and tube gear
I have tube pre and considering buying a tube amp. I will be posted overseas to a 220v/50hz destination soon and will bring along my toys. Could anyone tell me whether using a step up transformer is all I need or is there more to it than that. I know in the case of turntable we have to be concerned about the difference in hz (50hzvs60hz)but what about amp and pre.