I'd try contact cleaner in the valve sockets first.
[b]Switch off amp and let the valves cool down[/b]; remove valve; spray cleaner (NOT WD40); replace valve with a slight circular wiggle motion to scrape the contacts together.
Then, if this doesn't work, try swapping the pre-amp valves (the small ones) around and see if that makes a difference. If it does, you'll need to swap out a pre-amp valve.
If any this makes no difference... take it to a tech.