While it is true that a lot of electronics is designed to run on either 120 or 240 volts, and can adjust accordingly, you better be damn sure that the item you are using is one of those before plugging it in with just a plug adapter.
What you need for bigger US type 120 volt electrical devices is not a "converter"- its a transformer.
and I think you are much better off buying one down here. you write down the info on the nameplate of your us device, then you march on over to Casa de Transformadores, and they will set you up. Your transformer has to be big enough to supply your needs. Which means you need to know the amps or watts your device requires.
I have no problem with things like my apple computer- just get the proper adapter, or an apple travel kit.
But on my US model sewing machine, for example, I use a locally sourced transformer. I actually brought down a really inexpensive home stereo from the USA- $150 at best buy- and run that on a transformer, too. A similar model in buenos aires is three times the price, easy. and the transformer cost maybe 20 bucks. For things like US model hair dryers, or blenders, you need a transformer.
http://www.alamtec.com.ar/servicios.htm