How Do/Can Capacitors Fail?

Can't seem to get an answer on TGP so maybe someone here has the knowledge to inform me: I need info on the failure modes of caps.
All I got there was the typical 'loose sockets' or 'replace caps' answer. Fine, I will replace the caps but I want to know if I'm hearing failing caps.

78 100 watt Twin Reverb mint original condish
Been playing the heck out of this amp for about 4 months now; probably 12 hours/week average.

Played a gig yesterday at noon; amp was great.
Last night I was practicing pretty loudly and noticed a nasty hissing crackle creeping in on certain lower notes but because of where I was sitting (in the doorway looking out on the mountains) I didn't realize the sound was coming from the amp. I thought it was a sympathetic vibration from something else in my studio. After a while it started getting worse and then I figure out it was the amp. I swapped pre tubes and checked all the tubes for snug fit and eliminated microphonics (I'm pretty sure).

In troubleshooting the problem got quickly worse. Now when I turn it on it hums then goes into LOUD crackle/spit mode. Turning down individual channel volume or master volume doesn't change the noise.

Question: Is this a typical capacitor failure mode?

Sounds like a bad output tube. Caps fail in lots of ways. A leaking coupling cap can sound like a blown speaker. Bad filter caps will cause 120hz hum or a variety of other noises.

Bad parenting?

you don't mention how long it's been since it's been serviced or tubes were replaced. I'd suspect tubes before I started replacing caps.

I mean you wouldn't go replacing pistons and valves in a poorly performing engine that hadn't been tuned in 20 years....when it just needed spark plugs.

If the electrolytic caps in the power supply in an amp are more than 10-15 years old it's a good idea to change them to new ones. Electrolytic caps will not last forever, the material inside dries out/cracks, leaking dc current after 10-20 years in most cases. This will lead to extra hum, and eventually to blown power transformers or other components. They are used in the circuit a filter caps to smooth out the ripple in the power supply, when they go bad your dc in the amp is not dc, but rather pulsing dc. Not good. When people have the attitude of "don't fix it if it ain't broke" about the electrolytic filter caps in old amps, they're playing a dangerous game. Leaky electrolytics can blow power transformers. If the filter caps haven't been changed in your 1978 Twin since it was new, they're dried out and getting noisy and/or dangerous. There may be other issues with your amp but that's a given.

Thanks all.

Brand new winged C power tubes and a mixture of interesting old and new preamp tubes. I suppose a new power tube could have failed. I can try a diff set.

And yes, the sound is a ripping/tearing sound; a harsh, high pitched buzzy crackle. At first I had to induce it with my guitar but not anymore. Turn it on and there it is; scary.

New tubes don't necessarily mean good tubes. Do this - pull the inside set of output tubes, and see if the noise is still there. If it is, replace them and pull the outboard set. This will tell you if one of the two pairs is bad.

Caps - The enemy of electrolytics is heat...either from an external source, or from internal heating due to AC current. Most of these type capacitors are rated in "degree/hours" so that factory power equipment can have predictable periodic service. In that light, and seeing that tubes amps get pretty hot......periodic replacement is a good thing.

I've never seen one explode, unless it was hooked up backwards, and their typical failure mode is leaking (as mentioned). When their equivalent series resistance starts to drop, they become less of a cap, and more of a heater.

"New tubes don't necessarily mean good tubes."
BUT THESE ARE GT6L6GE's!!! *SHOCK* *DISMAY* ;)

Thanks, EMan; I'm going to give that a try after everybody leaves this morning.

Woohoo, EMan makes the right call.
I know I said I had winged c tubes but I forgot I replaced them with GT GE copies. One of those is bad.