[Tristan Lall]

Years ago, in 1999, Woburn had some trouble with (what was believed to be) metal filings at the base of the FETs. A few Victor 883s failed that way. Also, in 2003, one unexplained faulty 884 turned up in West Michigan. It may have failed for the same reason (since preventative measures weren't really taken to avoid debris).

I'd say that this is the most likely explanation for the issue. Do you typically cover the Victors before performing metal work?