Paramount weirdness

[vetmed]

I have a pair of 300B Paramounts about 2 years of almost daily usage. Yesterday after moving the amps and other associated equipment to another room one of the amps blew a fuse on startup. Replaced the fuse and everything worked normally for several hours. Today turned on the amp that blew a fuse and was greeted by a short sharp cracking sound accompanied by a blue flash of very short duration. Immediately powered down. No obvious damage on top, and nothing abnormal looking underneath. Oddly the fuse was still intact. Got my manual out and did a resistance check, no abnormal findings so just to be on the safe side checked again and everything ohmed out OK. Now I have no way of testing tubes but I do have spare tubes so I replaced them both. Plugged it in and everything works fine, like nothing ever happened, been playing more or less continuously for the past 5 hours. Defective tube? or something else?

[Doc B.]

The big clue is that you moved the amps. My hunch is that something was disturbed under the chassis. Check the parts around the four pin socket in particular as there are some bare leads that could conceivably get pushed around and short.  The blue flash means there might be some sign of two metal parts touching and arcing. Another possibility is that a cold solder joint was functioning OK before movement, but in the move it was pushed just beyond the limit.