Downloaded from https://www.everyspec.com MIL-STD-1311D METHOD 1501A INTERMITTENT LIFE 1. Test conditions. When intermittent life-test is performed, the tubes shall be operated under the specified test conditions with the filament or heater supply interrupted periodically. There shall be 12 to 25 uniform cycles, totaling approximately 20 hours of "on" operation each 24 hours. The filament or heater supply shall be applied and removed instantaneously. Other potentials may be applied continuously, at the option of the manufacturer. The accumulation of the "on" time shall be the time considered in determining compliance with the minimum specified time value.
The filament or heater-supply impedance shall not exceed 10 percent of the hot filament-load impedance. Warm-up time, tk, when specified on the TSS as a test condition for intermittent life test, shall be adhered to at the beginning of each "on" period. This is a destructive test. 2. Tube intermittent life-test procedure. Regular life test shall be in effect initially and shall continue in effect until the eligibility criteria for the reduced-hours life test have been met. Sampling shall be as specified in MIL-PRF-1, section 4. This is a destructive test. 2.1 Regular life test. a. Regular life test shall be conducted for 500 hours. b.
Regular life-test acceptance shall be on the basis of the specified 200- and 500-hour life-test end-point limits. 2.2 Reduced-hours life test. a. Eligibility for reduced-hours life test is established when no first sample failure due to the regular 500-hour life test has occurred in the preceding 3 consecutive lots. b. Reduced hours life test shall be conducted for 200 hours and acceptance shall be based on the 500-hour life-test end- point limits. Two 200-hour life-test lot failures occurring in the last 3 consecutive lots shall result in loss of eligibility for reduced-hours life testing. c.
The life-test sample from the first lot accepted each quarter shall continue on life test for an additional 300 hours (500 hours total life-test time). Failure of this sample to meet the 500-hour life-test end-point limits shall result in loss of eligibility for reduced-hours testing. 2.3 Readings: The life-test sample shall be read at the following times: 0 hours +24 200 hours -0 +24 500 hours -0 Additional reading periods may be used at the discretion of the tube manufacturer. 2.4 Acceptance criteria.
The lot shall be considered satisfactory for acceptance if the specified allowable defects are not exceeded and the change in average of any characteristic in the life-test sample specified for life-test control of averages is not exceeded. The average percentage change shall be obtained from the determination of the individual changes for each tube in the life test sample from the zero-hour value for the specified characteristics. For purposes of computation of this average percentage change, the absolute values of the individual changes for each tube in the life test sample may be used.
Any tube found inoperable during life testing shall not be considered in the calculation of this average. METHOD 1501A 1501-1 28 March 1975