Pull the 6550s. If it stays in protection, there's a protection circuit problem most likely. If it powers up green, measure the voltages on all of the power tube sockets - if they are good, you likely have a bad 6550.
The left three sockets are one side of the wave, the right three sockets are the other side. Put one tube in each side and power it up - it might go into protection if one of them is the duffer (I can't actually recall).