I experienced strange thing with my 12k build. After turning it on and during playing the amp, the V6 and V7 suddenly started glowing like crazy. There was also some (not very noticable) crackling noise along with this happening. I hit the standby switch and tried to turn it on again after a while. The tubes were behaving fine since then. Although I lost some of their power - there is approx. 8mA bias difference from the other pair which is not good for my (originally closely matched) quad of NOS Tesla EL34s.
I've been playing this amp for like two months with no serious issues and this is the first time I experienced this. I'd like to avoid it in the future so my question is - what might be the cause of this and how can I make sure not to happen again? Thanks!