Generating a feed of all but one category of posts

I think you can go even simpler in the coding. Just add the below to your functions.php file within your theme folder.

function myFeedExcluder($query) {
  if ($query->is_feed) {
      $query->set('cat','-73,-9');  // categories 73 and 9 are being excluded
      }
  return $query;
}
add_filter('pre_get_posts','myFeedExcluder');

Note particularly that multiple categories can be excluded by separating them with commas.

And here’s the codex page on customizing feeds, but it doesn’t specifically address this situation.