What are you trying to do by changing over to 6550 over EL34/6L6 as the output tubes? You cannot make a 200 watt head out of a 100 watt head if that is what you are thinking. The output transformer and power supply could support a two tube 6550 set up instead of the four tube EL34 configuration, but not four 6550 tubes. The power supply would have to be capable of supplying twice the current, thus twice the power to support four 6550 tubes. Also the output transformer would need to be able to handle the higher current without going into saturation.