What happens when a tube amp fails?

I had a tube circuit go bad in an ARC CL-60.
I wasn't aware of it until I traded it in for a new VT-100 MK-I.
The store owner said a resistor burned out but, again,
it wasn't detectable sonically.
I did play at very conservative volumes so maybe this helped mask the problem.
This was one tube in a four-per-channel stereo amp.

Usually, tubes just get old and weak. The sound will lose dynamics, may sound "gummy" and less volume. That's the time for new tubes.
When a tube amp altogether fails, that's not a desireable thing...
The small tubes usually last 3,000 to 10,000 hours. Output tubes such as EL34 will last around 1,000 hours.

Any or all of what you mentioned can happen.

Chuck

A tube failure, i.e. it stops functioning, can:
1) Stop sound in the channel it is assigned to. Both channels if it works in both channels. A small tube will usually stop the sound from one channel and will usually go out quietly. A power tube can go out quietly by will more often go out with a 'pop' and/or an electric light show in the tube or a reddening of the tube plates. If a power tube goes out with a pop it will often take out a fuse or a resistor in the bias circuit.

2) A partial failure in a small tube can produce a 'snap/crackle/pop' (this could also be produced by poor contact with the poins and tube sockets).

3) Sonic degredation which is most easily determined by simply having a set of replacement tubes on hand for emergencies in any event and substituting these tube for a few days and see if you can hear any meaningful improvement.

4) All manner of gremlins associated with any audio equipment (but usually easier to fix with tube stuff).

Learn to relax and enjoy your new toy! :-)