What's likely to happen when an 845 tube fails?

It depends on amp's circuit design more than on type of tube being used. Most of SETs are using primitive amature DIY design, but tubes in SETs last quite a long time before anything serious can happen. There are ways to prevent bad tube to blow:
1. Turn off tube amp when you're not listening to it. It takes only 5m to get tubes to the normal operation.
2. Eyeball the tubes having a normal glow when you play the music. One tube that will glow too bright or too dim might not be normal.
3. If you hear microphony, inspect tubes.
4. Have extra set of tubes. It helps you to check if problem goes away after you swap for the fresh one.

No experience with the 845's, but generally catastrophic failures are pretty rare in tube amps. Yes, the tube may fail but the minimal number of parts in these amps means that there aren't a lot of things to go wrong. Output transformer isolates the speaker from DC so you're pretty safe there. Output transformers do NOT like open circuits when signals are present so make certain that speaker leads are hooked up properly before engaging the source. Enjoy!

Thanks for your responses. I agree that the amp's circuit is very important and that's why I'm considering the deHavilland.

I never leave tube gear on and unattended for very long and always keep plenty of spares around "just in case." I also have a Hickok tester and keep pretty good tabs on my tubes.

I was hoping for a few more responses from those who've lived with the 845 for a while, wondering how reliable they are and what I can expect if one fails.

Again, thanks to the two who responded!

I like the DeHavillands a whole lot, and think they're built pretty well, but some other amplifiers can present real issues when faced with this sort of thing.

Two specific incidents I saw with the Opera Audio/Consonance Cyber 845/211 (same amp that uses a different bias resistor to run either tube) when I handled them came from excessive current draw by the output tube.

The owner of the 211 amp was going through components in the power supply over time. The amp had previously been repaired; when it failed again, it came to us. We discovered the 211 was drawing more current than it should, made the repair, and advised him to buy a new set of 211 output tubes.

The 845, which were a more or less new pair of amplifiers, were the more hairy. Same sort of issue, but the tube was less sound than that just described. It completely barbequed the power transformer. Supposedly, the amount of smoke was incredible. The owner was truly lucky to be able to power things down as quickly as he did. Otherwise, his home might well have burned up as well.