Downloaded from https://www.everyspec.com MIL-STD-1311D CATHODE INTERFACE LIFE 1. Test conditions. The tubes shall be operated for 500 hours with 110 percent of normal heater voltage applied and with other electrodes disconnected. The tubes shall then be measured for cathode interface impedance. The readings of cathode interface resistance shall not exceed the life-test end point limit(s) specified. This is a destructive test. 2. Impedance test. a. Equipment. Interface impedance may be measured by any equipment capable of meeting the calibration requirements specified below.
The equipment shall be capable of measuring the calibration network to within ± 15 percent of the Ri value given in the calibration networks of figure 1511-1. b. Calibration. Calibration shall be performed by connecting the calibration networks in series with the cathode of an interface-free tube operated at a maximum of 2,500 micromhos transconductance. Any tube type, capable of the specified transconductance, is satisfactory for this calibration. The calibration procedure need be performed using only one tube type. METHOD 1511 FIGURE 1511-1. Interface networks. c. Interface-free determination.
To determine whether the calibration tube is interface-free, use is made of the temperature sensitive characteristics of the interface resistance. With the calibration network in series with the cathode of the tube, interface resistance is measured at 20 percent lower than rated heater voltage and at 20 percent higher than rated heater voltage. If the difference between the two readings is less than 10 percent, the tube shall be accepted for use as an interface-free tube. 3. Conditions. a. The tube measured shall be operated under the conditions specified on the TSS. b. Heater voltage shall be controlled to within 1.0 percent of the value specified. c.
Preheat by applying the specified heater voltage used for the measurement for not less than 5 minutes. d. Life-tested tubes shall not be subjected to any other electrical measurements between the end of the life test and the interface test. e. When testing multisection tubes, the opposite sections may be grounded, cutoff, or floating. f. If the anode current specified on the TSS cannot be obtained on an individual tube, the interface measurement shall be made at zero bias. (Resistors shall be noninductive deposited carbon film type of 1.0 percent accuracy) METHOD 1511 1511-1 19 April 1968