If one power tube fails, do I have to replace all four?

Not necessarily. If one power tube fails in one of our stereo amplifiers, and the tubes have not seen excessive use, you may only need to replace the two tubes from the side of the amp that experienced the tube failure. There are two independent internal bias controls in our stereo amplifiers. Please refer to a qualified technician to perform this procedure as it is common for a screen grid resistor on the tube socket to burn open to take the faulty tube out of the circuit to protect the output transformer. Also, there are possibly lethal voltages present inside of the amplifier, even if the amplifier is turned off and unplugged.