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

+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..
best
k

vespacraig65-

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 !