Having tested nearly every quad-matched KT88 tube currently available on the market, none have approached the level of detail retrieval, holographic imaging, and tonal purity delivered by the Ray Tubes Select KT88 from Apos. It stands in a class of its own. Highest recommendation. https://apos.audio/products/ray-tubes-kt88-select-vacuum-tube?_pos=4&_sid=6e37ebf16&_ss=r&variant=45395661160684
KT88 Tubes
I am new to tube amps having acquired a Harmon Kardon Citation II about a year ago. A little while ago I started experiencing one of the channels dropping out and an occasional loud pop. Today I noticed one of the KT88 tubes glowing purple where the rest glow orange so I have assumed that the tube has gone bad. As I bought this amp used, I have no idea of the age of the tubes. So I have a couple of questions. First, should I replace just the two tubes on the bad channel with a matched pair, or should I replace all 4 tubes with all 4 being matched. Second, any recommendations for good quality KT88 tubes? Any other thoughts or ideas would be appreciated.