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