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

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.

A friends tube power amp went up in flames a few years back. The technician that tried to repair it said the cause was a faulty new tube that the friend had installed a few days earlier.

I've been using hifi tube amps continuously since 1970, as well as guitar tube amps and of course I'm old enough to have had tubes in my life continuously, as I was born into the world of tube radios and TVs and hifi then ensured the persistence of the vacuum tube for me. The most spectacular tube failures in power amps that I've seen were 6550 implosions in the early unstable Audio Research Dual 75s of the mid-70s, and the broadcast power tubes in early Futterman OTL amps of the same period. Even in those cases, needed repairs were limited to the amps themselves.

I've been using Audion Black Shadow 845 monoblocks in one of my systems for the past seven years with no tube failures. I use the Shuguang 845B tube of which my current pair are now coming up on five years old, and still test well. I have had two 845 amp repairs in that time -- one elective and the other necessary. The elective work was a choice to recap the power supplies in the amps, for sonic improvement. In the second case, recently, breakdown of a filament supply bridge rectifier caused a massive rise of hum but otherwise the amps operated. I had the rectifiers replaced inexpensively. In a high current filament supply like that needed for the 845's thoriated tungsten, the rectifiers get hammered. They probably should be routinely replaced every 5-6 years.

845 tubes are typically (but not always) run with B+ of 1000v - 1200v. The filament supplies consume fairly high current compared to heaters in more prosaic tubes. The filament rectifiers in my amps are rated 25w, for example. If there is a filament short, some associated components will fail and in any amp this *can* but generally won't fry the filament winding in the power transformer. Usually the rectifier will fail if it's a DC supply, along with a resistor or two. Or the power supply fuse will blow, protecting the transformer. This is not unique to the 845. If the tube goes runaway or otherwise degrades, again it is possible to see the power transformer fried, but more likely some associated components in the signal path and in the power supply filtering might get taken out before a slo-blo fuse melts.

For amps of the design caliber of DeHavilland, Audion, Sophia, you are highly unlikely to see the smoke, literal glass envelope implosion and occasional flame that were occasional risks during the rebirth of tube amps after the transistor take-over, during the early years of what we now call "high end audio" in the 1970s. My Audion 845 SET monoblocks meet or exceed the reliability of any amps I've owned prior. More often than not, an 845 tube failure is a quiet non-event as the tube just dies either by defect or end-of-life dysfunction. In the latter case it starts sounding poor before that, prompting you to replace it.

Use common sense. When you buy new 845 tubes, look and listen inside, for loose metal parts or solder scraps. Quality control lets some bad tubes slip through, and shipping can shake loose some marginal internal construction. Don't install tubes with loose internal metal!

Phil

213cobra,
Thank you for that.

Phil, now that's the kind of useful info I was looking for!!! Thanks for taking the time and being so thorough.

Joe/Trelja, thanks too for your comments. You and I discussed the merits of the 845 a while back and I'm still dragging my feet. I know it's not your favorite choice for SET but the only other amps that peak my interest are the Wavelength Cardinal's. I'm fearful the 300B will fall a bit short on power and probably be a little "soft" compared to the 845. Yah, I know, many feel the 300B has a more interesting sound.

Decisions, decisions.