“Prima Luna HP”

Have you swapped tubes? Preferred from a backup, but assuming you don’t have a backup set. Switch two identical tubes. Does the tube going dead change location?

While tubes are not unreliable… they are the most likely thing to have a problem, and replacement is simple and relatively inexpensive.

I always have a backup set and swap it for there is an issue. It is usually a tube.

Yes , I have backup. That spot on the amp took out 1 tube already & now it's starting a problem with the replacement after 7/8 months.

I’ll bet there is a resistor going bad.  I would send it in for service.  It’s an expensive amp. 

Yep, sounds like off to the repair shop.

Just out of curiousity, have you checked the bias settings?