8877 failures

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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 preceding runs. 

The main production problem with 8877's was alignment of the grid and cathode. In the late 1980's, a heat-dam problem caused the cathode to move. The tantalum barrier that serves as a support and as a gettering mechanism would shift and misalign the grid, allowing it to lay against the cathode. During one period of time nearly 90% of new tubes arriving at Ameritron would fail during 24 hour heater cycling tests, and that was without anode voltage applied! 

Another common grid problem is misalignment of grid wires in the tube. The cathode emission bands of the 8877 are deposited in layered rings or stripes. There are about 140 bands around the circumference of the short but large diameter cathode. Between each electron emitter band, Eimac positions a very thin gold plated grid wire. The function of the grid wire is to create an electrostatic field. This field controls anode current, and the placement of the grid wire outside the electron stream means very few electrons hit the grid. The result of this is very little grid current flows in normal operation, and the tube has very low distortion and very high gain. 

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!

The gold eventually re-deposits. It often collects on cool areas, where it can cause an arc path across insulation in the tube. The result is a severe HV arc from anode to grid or other elements. Sometimes the arc will clear the fault path and the tube will function again, but once the problem starts the tube will generally slowly get worse until it eventually becomes unusable.

Second less likely and less destructive failures are G/K shorts caused by poor cathode/grid alignment or caused on rarer occasions by conductive debris in the tube. This failure generally is NOT accompanied by an arc. 

To obtain maximum life from an 8877:

Do not cycle the heater excessively

Do not run more than 50 mA longer term average grid current

Do not exceed 150 mA grid current even for very brief

Do not apply HV or draw cathode current unless the heater
has been within the factory specified range for 3 full
minutes. If the heater is started very slow, the tube warm-up time should be extended

Never run the tube when heater voltage is low. Doing so
will poison the cathode

Never reduce filament voltage outside of specified allowed voltage range at the tube pins in an effort to extend emission life  

Vacuum Tubes


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