How does a preamp die from sitting unused for a few months?

Coincidence...something was on the verge.

I had an ARC SP 15 that I bought from a guy who had it in storage for years. It popped a fuse the second I turned it on. After replacement it burned up a cap that was swollen and died again. I had to re-cap the thing! Soon thereafter the switched 120v recep. on the back began heating up and failed- broken solder.

For sure the long time unused and maybe bumpity bumps in storage took it’s toll on the unit. After all that the preamp was fantastic and served me well until I bought a Ref 6 SE.

It was fun soldering and tinkering with it. That brought back fine memories of tinkering with old tube radios as a kid. Man I miss the 70s!

Depends on the storage. Anywhere unheated, or under-heated can be a problem. Things can draw moisture, which leads to corrosion.

If the rectifier tube is burned out would the rest of the tubes and power indicator light light up, or would all electricity be stopped at the first tube (here I'm assuming the rectifier is the first tube)?

@audiodwebe In a word, no.

The tubes are not lighting up because there's no power. So the fuse (which can blow due to current inrush of the filter caps in the power supply, which can be quite a lot higher if the unit has been stored for a while). The second most likely reason is its not plugged in properly.

Old caps and moisture.  Bringing the gear in from the cold, and starting it up immediately without a chance to dehumidify would do it.

Another possibility, related to moisture is salt air.