Exclude categories not working

I have 2 loops on a page the first using:

query_posts("&cat=6&posts_per_page=3")
if (have_posts()) : while (have_posts()) : the_post(); ?>

the second using:

$my_query = new WP_Query("&cat=-5,-6,-7&posts_per_page=5");
while ($my_query->have_posts()) : $my_query->the_post();

The exclusion doesn’t seem to want to work though. If I change the second loop to:

$explorerhome_query = new WP_Query("&cat=7&posts_per_page=5");
while ($my_query->have_posts()) : $my_query->the_post();

Then that shows just Category 7 posts fine, but as an exclusion it doesn’t want to know.

Are there any obvious errors I am making here? I’m new to this so it’s likely!