Weird Popping issue


Hey all,

I have a primaluna stereo amp and have been having some issues with it lately and was hoping to get some advice on what action to take next to try and resolve the issue. 

Each time I turn on the amp, the amp warms up, the audio signal comes through, and then maybe a minute after it has warmed up and started the audio, i hear a couple loud popping sounds from the right channel and then i lose all sound from that speaker. The audio keeps coming through the left and the sub however. The only way to remedy the situation is if you turn off the amp again, wait a little, power the amp back on and then i don't have any issues with either channel. 

pretty new to tube amps but haven't had any problems with my gear before this.

any help that you could provide would be more than appreciated. 

let me know if there is more info i can provide to help you understand my situation. 

Thanks a lot. 
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So, i waited a bit longer and turned on the amp again and this time. still with the same configuration as above. all pre tubes in the original position and the power tubes in the swapper position. this now results in there being a pop but i don't lose all of the sound on the left channel but the same pop is heard it just doesn't cut out the signal completely. im going to get some new primaluna KT88s from upscale audio. 
It does sound like a power tube; a power tube failure would reduce output, but not kill it entirely.