When you rewired the power tube sockets, you get the wires from the output transformer backwards. That red wire is normally the NFB - negative feedback - line. But when the wires are reversed, that turns it into a positive feedback wire. SCREEEEEE.
You have pin 3 marked on your drawing. That is the plate of each 6L6. Swap wires going to each pin 3 to the other socket. I'll bet my lunch money that cures it.
Well, tomorrow's lunch money. At the moment, I am getting ready to go find dinner.