Tube failure -- what would happen in worst case?

gosh, I remember when I owned Fourier Panthere OTL Monoblocks, I believe r 16 of those Russian 6c33b output tubes per side. I was having noise issues which I was trying to diagnose by swapping one by one. You had to also power down and wait several minutes before re-powering up due so to let the Caps drain out.

I had my first tube fireworks display in my system, it caused my Condo complex to lose power momentarily.

I have since stuck with tubes but went the simple 4 tube designs for a while until i found as Stanwal said, proper design in my Air Tight 6 tube per side monos.

but to answer the question, so far my output tubes have hummed, made noise like static or rushing water when they were in need of replacing. if a tube also does not bias within range, another sign of an aging or bad tube. tubes can also turn bright red before they fail....often one is not able to see the tubes when in operation due to faceplates, but I always take a peek at them after they have reached full bias point to make sure they are not glowing red

I think low signal tubes such as found in preamps and phono stages are probably okay.

Power tubes are a somewhat different story. One failure mode is where the getter material which helps maintain the vacuum inside the tube gets used up. Then, the gas that builds up inside the tube ionizes (usually at startupit arcs over) and shorts out the high voltage (B+) supply. This usually results in no more than a blown fuse in a properly-designed amp.

A more serious problem can develop if the tube goes "out-of-bias" in a manually biased power amp. Then the tube can overheat and the glass jacket will crack, again shorting out the power supply. Again, usually no damage should occur in a properly designed system.

At least two other things are important: On is that you make sure that your amps have the correctly rated fuses in them. If the rated fuses are continually blowing then get the amp checked...Don't just plug in bigger fuses as this could create damage to the amp and ultimately might produce a fire hazard.

Another thing is to make sure that the circuit that your system is on is not unnecessarily over-spec'd for the power you realistically need. This can also create a fire hazard as, say, a 40 Amp circuit (which would probably violate the local electric code) might just keep on delivering power to your system even after it has shorted out due to some failure and part of it has turned into volcanic lava.

I have had tubes short out in my Audio Research M-300s. One time a tube shorted so bad it blew pieces of a resistor all over the room and took out a 1 inch section of trace on the circuit board.

Some tubes hold up better than others. The input tube in the Quicksilver full function preamp is a 12AU7 and my Quicksilver preamp ate Golden Dragen 12AU7s for lunch. One day the system was on without music playing. From the other end of the house I heard an awful sound coming from the Martin Logan CLS. The Golden Dragon 12AU7 failed. I installed another Golden Dragon 12AU7 and it lasted 2 days before it failed. I switched to a Siemens tube and never had another problem.

When it comes to tubes never say never because you never know.

I don't want to scare anyone,but fire is always possible.A simple coupling cap shorting out can cause a tube to have thermal runaway.If a tube starts glowing orange,shut if down(the gear) as fast as possible.Solid state amps,tv's, and other electronics all have risks too.Homes burn down for various reasons,a lot are electrical,or electronic failures.

As has been said, a fire is the worst thing that can happen.

Apart from that, I've seen amps take a transformer (power or output) out. That's a repair you don't want to get into.