there are many things that could have caused the original fuse to blow, and later when you replaced the fuse, to kill the output of the amp. without more info, that's kind of difficult to pin down. one place to start is by measuring the plate voltage for the outputs, which could be anywhere from 350-600V (be careful!!!!!). second, measure the voltage across the cathode resistors of the outputs (these will usually be large ceramic resistors of a few ohms, and the voltage will be anywhere from about 20mV to a volt or two depending on the resistance) a reading of zero millivolts means no current. the tubes are generaly run at 15 to 40mA depending on the tube type (calculate the expected voltage across the cathode resistor using ohm's law). also, NEVER operate a tube amp without a load.