I have the Sigma 50-500 APO DG EX "Bigma" f/4-6.3 lens and use it with the Sigma APO DG EX 1.4X TC. ...
The Sigma APO TC is a 'Matched' Teleconverter. It only works with certain lenses. [See http://www.sigma-photo.co.jp/english/support/faq/condaitr.htm#05
"When the Tele Converter is attached, the zoom range will be restricted to between 100mm to 500mm."
The Sigma APO Teleconverters are very good, but it shouldn't be used with lenses that are not on that list. The lens, the TC or both could be damaged if you try.