Wonderful amps.
You should try to isolate the problem. Remove the Pre amp to make sure it's not that. Then cables and finally tubes. It could be a dry joint or anything. I wouldn't poke around inside though. Death awaits you if you don't know what you are doing. A lot of stored power in the power supply.
If this fails to find the problem send it back to ARC. It could probably do with a service anyway. Some new coupling caps would bring huge sonic joy I am sure after many years of hot use. One of my favourite amps of all time. Epic on a Magnepan or ML CLS
You should try to isolate the problem. Remove the Pre amp to make sure it's not that. Then cables and finally tubes. It could be a dry joint or anything. I wouldn't poke around inside though. Death awaits you if you don't know what you are doing. A lot of stored power in the power supply.
If this fails to find the problem send it back to ARC. It could probably do with a service anyway. Some new coupling caps would bring huge sonic joy I am sure after many years of hot use. One of my favourite amps of all time. Epic on a Magnepan or ML CLS