The schematic above is a Full Wave Bridge, not a doubler.
The Antek transformer has 2 HV windings. It is a 200Vac, not a 200-0-200. (Do not use a 250-0-250, or you will fry the caps or worse.) You need to parallel the Antek's HV windings to get 500ma. There will be no center tap, but you can tap off of the middle junction of the electro-caps to extract the lower voltage, like so:
Here is another example. The transformer has a secondary of 400Vac. It produces 950Vdc. The lower voltage (465Vdc) is taken off of the junction where the electro-caps meet. There is no center tap.