What happens when a tube amp fails?

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! :-)

ARC amps utilizing 6550's are notorious for going out with a nerve shattering mini explosion and smoke! An ARC tech told me it is the nature of the tube. Typically, there is a resistor adjacent to the failing tube that burns up, sometimes damaging the circuit board. Having experienced it several times, I have become wary of these amps. Makes me nervous and thus I have switched to SS power amps. Their preamps with small tubes are great without this tendency. On the other hand, Wolcott power amps based on EL34 tubes do not do this. When a tube fails, the amp keeps playing, and a an indicator tells you which tube needs replacing. Simply swap it out.

I would like to add to Newbee's first item that the 'pop' can be very loud. When one of the output tubes in my MC2000 went, I thought at first that someone had fired a gun right outside the window behind the amp. It took out a resistor, too. The amp was idling at the time.

Bill