Downloaded from https://www.everyspec.com MIL-STD-1311D METHOD 1251A AUDIO FREQUENCY NOISE AND MICROPHONICS The TUT shall have the specified operating potentials applied to all elements and shall be tested in an acoustic chamber constructed in accordance with Drawing 182-JAN, with a 7-inch or 8-inch-diameter speaker having a nominal free-air-cone- resonance of 70 Hz, or lower, and a power rating of 5 watts minimum mounted on the 11- by 18-inch closed end and facing inward, and the tube test socket mounted approximately 3 inches from the opposite open end. The top and sides of the acoustic chamber shall be free and clear of all material that might affect the acoustic characteristics.
The anode of the TUT shall be coupled through a 0.1 µ F capacitor to an audio amplifier having an input resistance of approximately 100,000 ohms and a response characteristic between 60 and 5,000 Hz, flat within ± 2.0 dB of the 400-Hz response, with a resistor load substituted for the speaker. The power amplifier shall be capable of delivering 5 watts with less than 10 percent distortion. The speaker shall be coupled to the output of the amplifier so as to present rated load to the amplifier. The output indicator shall be a VU meter. The VU type meter with its attenuator shall be bridged across a suitable tap on the output of the amplifier.
The VU meter may have the dial calibrated in electrical or arbitrary units, but the attenuator shall be designed to retain the ballistic characteristics specified for the VU meter. The calibrated points used for setting the amplifier gain shall be the rejection points and shall be determined for each test set on the basis of the power in the resistor load only. At 400 Hz and 50 mW, the resistor shall have been adjusted to the same impedance as the voice coil for which it is substituted. The amplifier gain shall be adjusted (without the tube in the test socket) to give the specified output with the specified calibration voltage at 400 Hz applied to the anode terminal of the tube-test socket.
The calibrating voltage shall be removed and the TUT inserted (see figure 1251-1). When operating under the above conditions, no objectionable noise or microphonism shall be evident either with the tube at rest or when it is tapped at least three times in each of two directions which are at right angles to each other. The tube shall be tapped with a mallet or mechanical tapping device as specified in test method 1201. Objectionable noise or microphonism shall be defined as: a. Background noise, sustained microphonics, or oscillation over 2 second in duration having greater than .5 mW output power level. b. Clicks or scratchy noises of any sort. NOTE:
The ballistic deflection as indicated by the output meter shall not exceed the maximum output specified. A - Audio amplifier Esig - 400 Hz signal source Rg - Grid resistor C1 - 4 µ F capacitor LS - Loudspeaker SW - Transfer switch (minimum) M1 - Voltmeter 1. Calibrate CN - Coupling network Ms - Output meter connected 2. Test to tap with rated T - Tapped auto impedance transformer FIGURE 1251-1. Audio frequency noise and microphonics test. METHOD 1251A 1251-1 25 March 1970