My Tone King Imperial Mark I blew a fuse while in standby mode. I understand this can be caused by a failed power output tube so I pulled the two 6V6EH (made in Russia) to see if anything obvious could have caused the fuse to pull enough current to open the fuse (2A250VP Slow Blow 313). The only thing that caught my eye was something I never noticed before. Both tubes had a brownish deposit on the inside wall of the socket base (see attached photo). I have looked at these tubes before and think I would have noticed something like this so I'm thinking that something either in the chassis or the tubes themselves went south. This is a class A self biasing amp. I'm going to get a replacement fuse(s) tomorrow and another set of power tubes, so any advise comments would be most welcome. Thanks!