That extra small tube is the phase inverter - part of the power amp. The power amp has a plain old vanilla 6vAC heaters. SO sounds like the 6v is going out. SO a pair of heavy yellow wires from the transformer is the start, it flows through a 10A fuse F4. Then through a ribbon cable to the power tube board. My first suspect is the 10A fuse. Is the clip loose? Look at the ribbon cable, are any of the pins looking burnt or discolored?