Output tube can go at any time. Tubes last approx. 2000 hours in ARC equipment. After that it's like Russian Roulette.
The first problem I always see is a customer buys a used ARC amp for a good price and then after a few months the fuses start blowing or outputs start arcing. Moral...No one sells a used ARC amp at a good price with new ARC tubes in it.
The second problem I see is a customer has an ARC amp and is unaware of the elapsed time on the output tubes. One output tube arc and takes a resistor and/or a fuse with it. Customer replaces the tube and weeks later another output tube arcs and causes more damage. Moral....When output tubes start to arc, replace ALL the output tubes. This way, you don't have to worry when the next tube will fail and it's easier to keep track of elapsed tube hours when you replace them all at the same time. I also recommend to my customers that we use ARC tubes. The reason...you don't use cheap gas in a Ferrari.