The picture above is of a B&W Model 410 Distortion Meter. The instrument can be used as a distortion meter that utilizes a sensitive frequency rejection circuit plus a constant high-gain amplifier or as a voltmeter using only the constant high-gain audio amplifier plus attenuator resistors necessary to provide full-scale voltage readings.
Practical applications include:
Measuring total harmonic distortion of audio amplifiers and modulation of tansmitters.
Measuring rms voltage from 0.1 mV o 300 VAC.
Measuring decibels from -75 dB to +52 dB.
Measuring noise levels of audio amplifiers and modulation of transmitters.
An amplifier for low level AC signals. (Amplifies 1 mV AC signals approximately 200 times)
Single frequency filtering from 20 Hz to 20 kHz.
Use as a sensitive null indicator.
Checking frequency response of audio equipment.
Measuring ripple present in power supplies.
AC balancing of audio and stereo amplifiers.
Checking stability of audio amplifiers.
Measuring frequency and insertion loss of audio filters.
Measuring frequency response of telephone lines and audio characteristics of most telephone line accessories.
The instrument has 6 vacuum tubes:
1 ea 12AX7
1 ea 12AU7
2 ea 6U8
1 ea 6X4
1 ea 0A2