Tube failure -- what would happen in worst case?

How do you determine when a tube is to be replaced?
Can a tube ever glow bright red and blow up?
If it does, would it damage the amp itself as well as other components including the speakers?
replace when it won't hold bias or you hear a rushing sound or it becomes microphonic or the definition softens...tubes can blow and that's never a good thing for an amp....all that said, I wouldn't listen to anything but tones sound real.
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.
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.

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
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.
Thanks for all.
I did not bother to bias my tube amp, and I am selling one (Chinese made) and get an amp with auto bias feature (BAT vk60).
Any way, is this a good reference for biasing?
Any of you have other source?
I did not bother to bias my tube amp

Why not?Just curious.
I guess I was lazy and ignorant. I assumed that a new tube amp was factory biased and I should not fiddle with it, and also I did not want to damage my amp by screwing up. Now I feel like I need to know more about biasing.
I guess, this shows what would happen?

If you have to, pay someone to show you how to bias your amp. Small differences in AC voltage can change the bias. Just because the amp is new doesn't mean your house voltage is the same as the factory where the amp was built.

Another good reason to have someone show you how to bias your amp and use the proper tools is so you don't electrocute yourself.
Auto bias circuits can also fail.
You've received some good information as well as some documentation of some extreme cases.

There are products of all categories which are to be avoided.

I stayed away from tubes for some 7 years because of a bad experience - until I came to understand that there exist many reliable designs to choose from.

If you read the literature from back in the late 1980's on the ARC M-300, you'll find comments in TAS on how HP's sample was prone to blowing up. Of course, the words "blowing up" weren't used, and the amplifier was highly touted in the magazine.

I feel your pain Rrog. I too had problems with a previous generation ARC amplifier from that era - a D115-Mk II. It had a penchant for returning to Minnesota for work. In fairness, I've note not heard of issues with more recent generation ARC gear.

The 6C33B tube was mentioned in this thread. People love the sound of this tube, and yet I know of two manufacturers who will not get within the same zip code as this tube - not because they don't like the sonics, but rather because of its sample to sample consistency.

OTL's - don't criticize the breed because of the Futterman. The Atma-Spheres are bullet-proof and their failure modes are benign (warning - dealer disclaimer).

I've had solid state gear experience failure due to a self destructing electrolytic cap. You don't want a smoking cap in your house, but the fact that it was solid state had nothing to do with a random component failure.

Having said that, some failure modes are more catastrophic than others, and I don't mean to trivialize any of this.

Nothing is 100% risk-free.

Thom @ Galibier
I was playing a gig last night with my Vox AD50VT amp which has one 12AX7 preamp tube. Suddenly I lost all sound. Is that a normal failure mode for a tube? Is that uncommon? I bought this amp new almost 7 years ago and love the sound of it. I have a recording of the song when it failed and it seems like you can hear the tone breaking up and getting scratchy right before all sound was lost. Any ideas appreciated.

Did you replace the tube, Bill? Check fuses? Bad cord or problem with wireless xmitter? Rectifier problems? I don't know a lot about this stuff but,I'd check the common failure parts before taking it to the tech shop. Might be an easy fix you can do yourself. Also, it will save you some cash and embarrassment (believe me, I know about embarrassment)!!!!
I noted in another thread that I've been using and abusing various tube guitar amps since the 60's and, since 1968 anyway, no tube has blown up.
Self biasing or not, the problem comes when a cathode to anode short happens in a power tube. The least damage is blowing a fuse in the amp. But for me, the cathode resistor gets fried immedately and needs replacement. This also happened on a current source tube (CARY Rocket 88) and I had to replace that resistor too - I knew it was bad and asked mfr for the value, but they gave me the wrong resistor and I had to send it in ($60 bucks each way).

So it is a pain. But the moral is this NEVER EVER leave your house with tube equipment powered on.
Cathode to anode shorts are very very rare, and the sort of thing that should blow a fuse without damaging anything else. If not, the manufacturer has not thought things through.

I leave tube equipment on all the time. However, its tube equipment that I know is designed to not get in trouble if a tube fails. No blown cathode resistors, maybe a blown fuse. But in truth I have yet to have that happen despite using such amplifiers for over 35 years.

That's not to say that all amps are safe like that. The Fourier amplifiers mentioned earlier were prone to spectacular failures as were certain ARCs (when they were sold a bad batch of Cornell Dublier filter caps back in the 1980s).
I friend had flames coming out of the top of his power amp do to a tube failure. I'd be more worried about the house burning down than damage to a component.
When I was a kid we had a Philco color tv that my dad bought in 1968. (I loved seeing star trek in color on that tv back then.) It had lots of tubes and the repairman came to visit us often that first year it was under warranty. I remember one time we turned on the tv and we heard crackling sounds and the wall lit up behind the tv. That was exciting. The repairman came out and fixed it once again. I can remember my dad sending me down to the drug store on my bicycle with a basket of tubes from that tv to test them and buy replacements. Every drug store and hardware store had tube testers back then.
Worst case? That so much depends on the design of the circuit in which the tube is being used. In my case I'm burning tubes in extreme DIY amps and preamps. All the tubes, except the output (that's on the drawing board) have current source fed,shunt regulated B+. Should a tube fail, the shunt device would be required to shunt all the bias current. It is not heat sinked for this and will fail. I had a driver tube in my beta Venice (a 3 stage fully differential phono pre from K&K Audio) arc. The driver stage is direct coupled to the input stage which uses MOSFET based current sources as plate loads for the tube half of a tube/FET cascode. That arc caused one of the MOSFETs in the input stage to fail. I had an output power tubes arc on turn on that caused the grid to short to the plate. That smoked the grid-stopper resistor on that tube. Better the resistor than the secondary winding of an expensive IT transformer. For me, trying to design for all failures is impractical. It's much easier for me to just keep on hand the necessary parts to repair an amp in the rare instance a tube does fail.
I had twice tube failures on my Chinese MC10L.
Both times, a fuse and a register sitting next to the tube blew off, and smoke came.
First time it happened a couple of months after I changed the whole four tubes. After that, I learned how to set the bias.

Second time, it happened in an apartment that I moved to. It was a new place and I did not change the bias. It happened a month after the move.

I also had a tube failure for my BAT vk60. It happened when I tried a seemingly bad power tube. A second after powered up, the tube got bright and failed. Also, a fuse and a register blew up. I replaced the register, fuse, and used a new set of power tubes, and then it worked work OK.
I still have both amps.

After these three instances, I became more careful about using tubes and setting bias. And, I feel that it is NOT a big deal if tubes fail. Looks like the registers and fuses do their job to protect other parts of the amp.

For those who want to try tube power/integrated amps, I would suggest to try with cheap amps first before moving to more expensive models.