Converting a 220V input power supply to 110V

What amp do you have specifically?  Does it use a switching power supply?  If so you can switch it over manually or some of them switch by themselves.  Is the power input labeled for both 220v and 110v?  If so you can plug it into either.  If it exclusively says 220v then you will need a 220v to 110v Step Down Converter.  They cost anywhere from $20 - $200.

If you do end up purchasing one of these be sure to error on the side of higher wattage to ensure it does not melt under load which can be a fire hazard.  The only below is rated for 300W which is pretty nice if you just use it for you amp and nothing else.

http://www.amazon.com/VCT-Voltage-Transformer-Converts-Worldwide/dp/B000J65PJQ

I also am in the market for a power converter.  In my case for a Proxxon 220 volt, 300 watt drill press.  Does anyone have experience with brands of power converters they could recommend?

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.