PRIMA LUNA DIALOGUE PREMIUM - 20 minutes and then loud pop and buzzing in 1 channel


Hopefully someone can help.

Amp is virtually new. The amp will operate wonderfully for 20 minutes and then a loud pop , and then buzzing from the right channel. I shut down the amp and then it repeats.  I switched some tubes and it is still there.  I switched the cables and know for sure it is only one channel, the right one. The buzzing does not increase with the volume increased. . 
It is the amp and not the Prima luna preamp.  Cables and connections are fine.   Pushing Focal Kanta No2's . 

Any help would be very much appreciated. I'd rather not send a 68 pound piece of equipment through the mail. 

Thanks lot - Craig 

128x128vespacraig1966
Perfect timing for my next piece of Audio Equipment. I've narrowed my new Audio Purchase to 2. A new PrimaLuna integrated or a new Rogue Audio integrated III.
Now, all I've got to find is a used pair of Joseph Audio Pulsars.  I'm also retired and living in a 350 sq.ft. retirement Villa in So.Cal, I got time to relax and go back to Analog. Just moved from a 3,00 sq.ft. home. Then I was listening to SS. (Krell EVO series). Turning 69 tomorrow after 50 years of Audio Hobby I think Analog will mellow me out on a Daily basis.
Along with a new pair of FOCAL Clear cans.
Just wanted to thank everyone for the help. It ended up being one of the 12au7 tubes. Replaced them all with Tung Sol , and it totally fixed the problem.

Thanks Agon,
I mean again

Sweeney