Power Tube Failure

A little while ago a small tube (in my Phono-Preamp) died.  It was a subtle affair in which the tube turned white and the glass cracked, not necessarily in that order.  This failure was interesting, but not dangerous.

Yet, this event got me wondering - what happens when a power tube dies?

My Preamp's power tube runs quite hot, and I am concerned it may be a FIRE HAZARD when that tube fails. 

My second concern is that my Preamp may be DAMAGED when the power tube dies.

So, I am hoping to hear from anyone who has EXPERIENCED a power tube failure and can provide any insight regarding what to expect. 

notes:

a. Yes, my Preamp counts hours, but I own several power tubes and do not know how many hours of service each has provided. 

b. Testing these tubes is not plausible - I do not have a tube-tester, and there are no HiFi retailers within a reasonable driving distance.

Thanks in advance for your stories!

Best,