I watched a few of Mr Carlson's Lab videos on YouTube and found them as informative as the Skunkie Designs videos. However, most relate to tube radio gear but I did find these two videos where he diagnoses and repairs some issues with a pair of Six Pac amplifiers. One thing he points out in the first video is how used double triode tubes may have differing hours of usage on each triode depending on what equipment they had been used with, for example some lab test equipment may use one side all the time and the other intermittently. Also, the way he tracks down the source of noise in the circuit is really informative.