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

BTW the buzzing can be drowned out by greater volume. 

Thanks everyone for your tips. It's not the tubes.  The amp plays with no issues for 33-34 minutes , then a pop then buzzing in the right channel only.  I bought it preowned .    Absolutely blown away by the way it pushes my Focal Kanta No 2's.  It worked great for about a week. Upscale Audio tech guy is going to call me back. I just wanted to thank all of you , I don't know how to respond to individual posts. 

Craig 

In response to tablejockey, regarding not switching out every tube: I have replaced all el34's with 4 brand new Tung-sol kt-150's.

I have taken each 12au7 from the left channel(which works fine) to the right problematic channel,  and the pop then buzzing continues. 

Does anyone suggest buying a 1/2 dozen brand new 12au7's?   
Please respond. Thanks Craig 

New development- John at Upscale told me to fire up the amp, not connected to the preamp. Let it pop and buzz and then wiggle the small tubes. As I was about to do this, the buzzing then came out of the LEFT channel. Prior to this is was always the RIGHT channel. This gives me hope. So I will purchase 6,   12au7 tubes ASAP and see if this fixes the issue.  I am just hoping I don't have to ship it to California from Boston. John , the tech @ Upscale said 30% of the (tube) equipment he gets to repair DOES NOT NEED REPAIR ! That fact inspires me to try more trouble shooting on my own.  Hence purchasing brand new 12au7's. 
thanks everyone for your suggestions. I'm new to the Audiogon community and very impressed with other's concern for my issue. 

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