I've got a 580W 80-Plus Bronze PSU from a well-known brand where I live. It's a decent PSU
http://www.green-case.com/products/power/power.php?model=GP580A_ES
It does 42A on the 12V.
As this Power requirements list from MSI says R9 390 just needs 30A and 550W-600W PSU, I'm assuming it should be...