convertor is also okay -- even though you'll be viewed as somewhat strange writing it :^) See the next to the last line below. Having said that, I think the usage of "convertor" would limited to a person vs. a thing; however, I'm just a first generation immigrant from non-English countries. ;^)

http://dictionary.reference.com/browse/convertor

con⋅vert⋅er  /kənˈvɜrtər/ Show Spelled Pronunciation [kuhn-vur-ter] Show IPA Use convertor in a Sentence See web results for convertor See images of convertor –noun 1. a person or thing that converts. 2. Electricity. a device that converts alternating current to direct current or vice versa. Compare inverter, synchronous converter. 3. Metallurgy. a chamber or vessel through which an oxidizing blast of air is forced, as in making steel by the Bessemer process. 4. Television. decoder (def. 5). 5. Radio and Television. an auxiliary device that permits a receiver to pick up frequencies or channels for which it was not originally designed. 6. Physics. a reactor for converting one kind of fuel into another kind. 7. a person who is engaged in converting textile fabrics, esp. cotton cloths, from the raw state into the finished product ready for the market by bleaching, dyeing, etc. 8. Also called converter lens. Photography. an additional lens attached to a lens in use on a camera to alter focal length, mounted in front of a lens to produce a wide-angle effect (wide-angle converter) or between the lens and the camera body to produce a telephoto effect (teleconverter or extender).

Also, con-ver-tor.

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Origin: 1525–35; convert + -er 1