First of all, don't plug a 220V unit into 110V (or vice versa).
Many transformers have primaries designed to be used at either 110V or 220V (depending upon how they are connected).
First of all, what amp is it? Some units can be switched quite easily. Others may require some jumpers inside the amp to be moved. It could be that the transformer in this particular amp is 220V only, which would mean that you would either have to replace the transformer with a 110V version (or have the manufacturer do it), or as has been mentioned, use a power converter of appropriate ratings.
If you have the owner's manual, it may discuss this. The service manual most certainly would. If you are not qualified in dealing with AC voltages, please find someone who is, as mistakes can be lethal.
Edited by Pars - 10/14/11 at 8:25pm