Downloaded from https://www.everyspec.com MIL-STD-1311D METHOD 1246A AUDIO FREQUENCY NOISE The anode of the TUT shall be coupled to the input of a power amplifier, the response of which shall be within ± 5 dB of the response at 400 Hz over the frequency range of 60 to 5,000 Hz. This power amplifier shall have an overload voltage characteristic of at least 10 to 1 with the specified calibrating signal used as the reference. The value of Eb under test conditions shall be regarded as Ebb for this test. A commercial dynamic loudspeaker (4 inch min) with a rating of at least 2.5 watts, properly coupled and excited, shall be used.
The gain of the power amplifier (see figure 1246-1) shall be adjusted by removing the tube from the socket and introducing the specified calibrating voltage at 400 Hz into the anode circuit at the tube socket through a capacitor having an impedance of not more than 100 ohms at the calibrating frequency. Unless otherwise specified, the power amplifier gain shall be adjusted for 50 mW output. The signal and calibrating network shall be removed and the TUT inserted. The minimum external grid-circuit resistance for the TUT shall be 100,000 ohms and shall not be bypassed. The minimum external anode-circuit impedance shall be as specified.
When operating under the above conditions, the tube shall be 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. Tubes shall be rejected if any objectionable noise is heard in the loudspeaker that is caused by leakage, intermittent shorts, clicks, or any scratchy noise. Any electrical disturbance sufficiently large to interfere with proper operation of equipment is considered objectionable. Noise-indicating devices other than the loudspeaker and neon indicator are acceptable provided they give comparable results.
When a meter deflection is specified, this reading shall be obtained by using a VU meter. Both the meter and the speaker shall be connected to the amplifier in such a manner that the meter indicates 50 mW when the speaker is receiving 50 mW of power from the amplifier. The tube shall be tapped with a mallet or mechanical tapping device as specified in test method 1201. A - Audio amplifier Esig - 400 Hz signal source SW - Transfer switch CN - Coupling network LS - Loudspeaker 1 - Calibrate CI - 4 µ F capacitor (min) M - Voltmeter 2 - Test FIGURE 1246-1. Audio frequency noise test. METHOD 1246A 1246-1 25 March 1970