Function to exclude posts from feed by tag

I’m looking for function code to selectively exclude posts to the feeds based on tag. I found the code below in a post from a couple of years ago, but cannot make it work (php newbie). I need to exclude posts with tag ID 740, but leave all the other posts alone.

Any help would be appreciated.

C

function exclude_tags_rss($query) {
	if ( $query->is_feed) {
		if( isset($_GET['tag__not_in']) ) {
			$qv = $_GET['tag__not_in'];
			if( strpos($qv, ',') !== false) $tag = explode(',', $qv);
			else $tag[] = $qv;
		}
		$query-> set('tag__not_in', $tag);
	}
return $query;
}
add_filter('pre_get_posts','exclude_tags_rss');