Bought matched quad of these and so far they are very, very stable. Perfect bias keeping in my SIPP audio amplifier. Vg/Ia closer to 6L6 than the specs provided by JJ, though. I noticed small pieces of solder-like material in the tubes, which I think is a manufacturing issue. My observation is that this is not affecting the performance at all. Internal cage assembly riveting is poorly executed (many cracked rivets), but again this is more manufacturing problem than performance concern. Overall great sounding tube and great service from thetubestore. Will buy again.