While many or most tube failures are caused by age, there have also been bad tube manufacturing runs over periods of time. More by far with 8877's than any other tube. While the 8877 is a good tube with excellent IM characteristics and very high gain, it is probably the more problem plagued tube Eimac manufactures.
The failures and problems are actually why MRI equipment in the 80's and early 90's quickly moved away from 8877's. Field failures in 8877's are a major cause of ETO losing a contract manufacturing medical amplifiers.
While one west coast amateur blames all failures on amplifier design issues (and oddly enough sells what he says is the only cure for problems), the truth is power grid tubes have multiple causes of failure, virtually none of which have anything to so with parasitics. Like it or not, tubes are very difficult to manufacture. No surprise, they are a very common point of failure even with conservative operation. The fortunate thing is when you get a good tube, as long as you watch grid current, the tube last a very long time. One thing you can do to hurt the tube in a very short time is to run excessive grid current. Excessive grid current is bad news.
Newer 8877 tubes from San Carlos have had very few problems compared to older
tubes from Salt Lake City. Bad runs of 8877's included almost the entire year of
1987 and sporadic smaller batches since then. I have put brand new 8877's in my
amplifiers, had the tube fail within a year, and without making any other
changes except the tube had a tube from another production run last several
years of daily use. Some runs, like it or not, are not as good as subsequent or
If the grid or even if a few wires in the grid are misaligned, or if the grid
is allowed to go significantly positive during operation (high grid current),
high velocity electrons will strike the grid. High velocity electrons have
a large amount of kinetic energy. When these electrons strike a grid wire,
minute amounts of the gold grid coating can be released or "evaporated".
This slow deterioration can occur from kinetic energy of electrons striking the
gold even if the grid does not become excessively hot. Even a few thousands of
an inch alignment error compromises long term life of the tube!
since December 15 2004