Bought amp in 2007 haven't spent much time playing through it until recently. Changed out some preamp tubes, amp sounded fantastic. While playing sound just faded away. Sound came back minute or two later. I have changed all the tubes just to see if that would help. Same thing happened. Amp sounds good for about an hour and than just fades out, about a minute later it seems to work again. This amp is basically brand new, maybe 12 to 20 hours on it, any ideas?

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