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 

84e122d0 7549 458d 8158 3f0a7afb81e6vespacraig1966
Could be a bad tube. You might try switching the tube set from the right channel to the left channel and see if the problem follows the switch. BTW, did the protections circuit activate when you heard the pop?
BTW, I did not overlook your comments about switching 'some tubes' I was recommending you switch all tubes. If it were a tube, IMHO, it would probably be a small tube, unless you were tripping the bias circuit protection when you heard the pop. Another question, is the buzz superimposed over the (sound) signal or does the right channel just buzz? Anyway, switching the entire set of tubes should show if it is a defective tube or not. If not, pack it up and send it back to PrimaLuna for warranty work. FWIW, assuming you bought this new and have the packing I wouldn't worry much about shipping - the tripple boxing and Styrofoam lining are fairly bombproof. 
Sorry to hear. Especially since Prima Luna is on my very short list of amps if I were to replace or upgrade mine. You've made all the right moves to narrow it down to something internal, like caps or transformers or wiring. The 20 minutes makes me think thermal. But if you turn it off and right back on again (no cool-down) and it still goes for 20 minutes wow, you got a mystery there. 

I've shipped enough of this stuff to no longer worry about the component. What I worry about now is the box! Cardboard gets beat up. 

Get yourself the Costco rolls of packing tape. Fold cardboard into angles and tape them to reinforce all the edges and corners. Then cut another box to fit snug and tape the you know what out of that. So a box to protect the box.

Print FRAGILE GLASS and laminate that to every side. Then call around, or ask your shipper, to stick shock sensors (liquid filled tube breaks if hit too hard) on it. 

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. 

BTW the buzzing can be drowned out by greater volume. 
I own the PL Dialogue Premium Integrated amp and experienced something similar but not the same.  It DID turn out to be a bad 12AU7. PL suggests a specific process in my manual.  When I first switched a few tubes it wasn’t in the logical process PL laid out. 

My problem was at startup, just before the protection circuit clicked in.  Low hum rising in volume until there was a pop. No damage to the speaker.  After that, no problems listening. 

Following the PL process, the problem switched to the right channel, I marked the tube bad and ordered another.   All is good now.  

Just sharing an experience. I hope your problem is as simple.  
Primaluna is an outstanding amp.   BTW, I’m running Spendor A6, not super efficient, but PL drives them just fine.  
If the problem stayed on a channel after you switched tubes and/or cables then it might be a blown cap.
I have a PrimaLuna Dialogue Premium HP that drives a pair of Magico A3s cleanly and with authority. I love this integrated amp. No issues at all. I would definitely, as suggested above, get Upscale Audio involved. Good luck and please report back as to what the resolution to the issue was. Thanks. 
could be tubes, could be caps or anything. I heard a popping noise like that on my guitar amp it was a preamp 12au7. On my hifi was an old telefunken. I have to recap the whole amp. My lesson is I gotta have a backup amp in a situation like this so I bought a used solid state. Its not the same but at least it got me by and got my amp done the right way. Now it sounds like thunder!
Good luck sir, and please keep us updated. Prima Luna products intrigue me.
Sell it now. Seems everyone else is. You see how many are for sale? It was on my short list until recently. Hope your fix is simple. 
Sell it now,seems everyone else is?..So you suggest dumping it on someone else and taking a huge loss..stupid comment at best.Prima luna makes very good products,ive had several.
"Sell it now. Seems everyone else is. You see how many are for sale?"

Listings on Agon,AA,USAudio-ZERO

Perhaps this poster is being facetious? I've used PL products for years without problems,along with many others
Hopefully, it is something simple. Doesn't look like the OP has completely eliminated a bad tube.
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 
Craig, It sounds like you have done enough to rule out a faulty tube as being the source of your problem. That said you are really going to have to have a tech diagnose and repair the problem. If you did not buy this from a dealer I doubt that Deal is going to repair it. He wouldn't work on one of my units which I bought second hand. He did, however, refer me to a tech in the LA area to whom I sent the unit and had it repaired promptly and at a fair price. These units should be easy service so if you have a tech in your area who knows tube amps you could probably have it done locally.
This might be stating the obvious, but are you sure the amp is designed to work with KT150 tubes?  Either way, it might be worth trying it with the stock EL34 and see if the problem still exists.
Craig, this isn’t hard. Just call PrimaLuna / Upscale Audio. They will help you. 
Did you flip the switch for the correct type of tube that you swapped out? Triode-Ultralinear.
Sell it now. Seems everyone else is. You see how many are for sale?
Yeah, check out the prices. If you find a good one it's only $1000-$1500 below list. Most used equipment goes for 50% or less. The Dialogue Premium series has been superseded by their new EVO series of balanced amps, I notice that many have just gone up for sale. I'd sell mine at $1000 off so I could buy the new ones. That's a good deal for me!

Have you talked to the person who sold it to you? They may be of some help or stand behind their sale and assist in it's repair.

Troll post. Why wouldn't the OP just contact the seller.
+1 Check the switch on the right side for tube type. FWIW my PL Dialogue Premium has been absolutely bullet proof for three years I’ve owned it..

Since you've switched EVERY tube, then you may have eliminated the possiblilty

Call Upscale to get a referral for a shop to repair it
The suggestion of the switch on the right side will have nothing to do the problem. The amp will work fine in either position.

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. 

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.
That pop and buzz is the interest you hear on your Credit Card bill !
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