Vacuum Tube (valve) Life Expectancy
Mil Handbook 217 (reliability prediction) has tables and
charts for predicting the average mean time before failures
of several kinds of vacuum tubes. The mean time before
failure is the time for exactly half of a large number of
devices would fail. The Mil handbook has such tables for
essentially all electrical devices including vacuum tubes.
In the following, rates are failures per million hours. To
get the overall failure rate, multiply the factors given below
together.
Per Mil 217, there are three factors affecting the life
of a tube. These are: (how to use the factors is explained below)
1. Base Failure Rate, for receiving tubes = 5, for power
rectifiers = 10, transmitting tubes below 200 kW = 75 etc.
Notice that the failure rate for power tubes is much higher.
2. Environmental factor. Our environment (office, somewhat
temperature controlled, not exposed to the elements) is
called ground benign, and the factor is 0.5.
3. Learning factor. This is the time (in years) since introduction.
The factors are 1 year:10, 2 years:2.3, 3 or more years:1.
For all NOS tubes, by definition the factor is 1. You could
make a case for other factors for recently introduced or
re-introduced models, which explains the problems associated
with these new models.
This is somewhat of a simplification of the *several* pages of
data, however, the complications are more for high power, magnetrons,
TWTs etc. There is an assumption that tube ratings, filament ratings,
power, current and voltage are operated within the manufacturers specs.
Example: 12AX7 failure rate is predicted as 5*0.5*1=2.5 failures per
million hours. (MTBF=250,000 hours). For a device operated 24 hours
per day, 365 days per year, 8760 hours are accumulated each year, so
the device can be expected to last ON THE AVERAGE for 28.5 years.
If this same device were operated for 8 hours per day, 365 days per
year, the expected lifetime is 85.6 years.
Data presented is from MIL-HDBK-217E. (Current version is F, but I
don't have that version here. The data for vacuum tubes is the same
in either version though). For those feeling the need for looking
this up, it resides in section 5.1.4.x.
You can obtain the MIL-217 handbook from...
Naval Publications and Forms Center
5801 Tabor Ave
Philadelphia, Pa. 19120