The quad amplifier on my tube home theater system failed about a month ago. It was burning through fuses. I suspected a shorted output pentode because half of the pentodes are over 10 years old and had many hours on them.
I removed all of the tubes, but when i threw the power supply switch the fuse continued to blow. I suspected a bad capacitor but the amplifier is not old enough yet. I was surprised to find a bad rectifier diode. After replacing this and replacing all of the old pentodes for good measure i re-biased the amplifier to 25 mA for each of the 8 tubes for class B operation.
I will see how things go with the new pentodes at this bias current then increase later if i think it is necessary.
I'm happy to report that everything sounds great and is working reliably.