Tube failure -- what would happen in worst case?

Tubes can have high voltages and big amps in an power amplifier output tube. Yes power tubes can very rarely 'explode' and start on fire (usually, but again, rarely other stuff starts on fire because the tube shorts out)
Some stories of spectacular tube failure in big tube amps.
NEVER leave a tube power amp turned on, unattended.
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For preamps..I would NEVER worry. A tube might fail, but it would be a small problem, and would never start a fire.
I feel safe leaving a tube preamp turned on unattended for short periods of time. (for preamps with power supply tubes, this is a bit more danger if the power supply tube fails, but that is rarer, but if a preamp HAS power supply tubes, do NOT leave it unattended.)IMO
And yes, a failing power tube in an amplifier can do damage to other things, more or less depending on design. Output transformers lessen the chance, OTL designs increase the chance. Some other designs more or less, depending..
Tube amp owners will say they are safe.
I would not buy a tube amp,(if someone wanted to give me some for free ..with piles of great replacement tubes..I would use it!)

Elizabeth, I have kept both my Audible Illusions Modulus 3 and Modulus 3A four tube preamp on continuiously since I bought them. The Modulus 3 has been on constantly for 16 years and the Modulus 3A for 10 years. The only time I have turned them off is when I changed tubes. There is no danger keeping a modern tube preamp on all the time.

Bob

There was a spectacular demonstration at a European HIFI show last summer, the whole room burned causing several 100K$ damage. This is a sign of BAD AMP DESIGN. I am old enough to remember when everything ran on tubes; a friend who is retired from AT&T recently told me that they use to put tube amps on the sea bottom for years at a time when undersea cables used them. A well designed tube amp SHOULD be able to be left on, but I wouldn't do so either. A lot of designers run their equipment right up to [or over] the edge of their capabilities. I have seen some fairly impressive failures in SS, which is what I generally use.

I have had tube equipment since 1985 and I have never seen a tube explode. I have had a number of power tubes fail. Even in the early 1990s when there was a lot of inferior Chinese tubes around. The have all gotten very bright and then just died. If this happens you turn the amp off as quickly as possible. A couple of times I have had a tube take out a resistor. Different tube equipment is made to different standards. Use good quality tubes in well made equipment. Sure it is possible for a tube to explode but it is extremely rare. I have never seen it happen or know anyone that it has happened to. Before the 1960's all amps, TVs, radios, and everything else electronic used vacuum tubes.

So don't worry, be happy. Kick back and enjoy the music!

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