A V.T.V.M. (Vacuum Tube Voltmeter) is a type of electronic test equipment used to measure AC and DC voltages. It was commonly used in the mid-20th century and is still used today in some applications that require high input impedance and high accuracy.
The V.T.V.M. works by using a vacuum tube amplifier circuit to amplify the input signal and a rectifier circuit to convert the AC signal into a DC signal. The output of the rectifier is then measured with a sensitive DC voltmeter, which can accurately display the voltage being measured.
One of the key advantages of the V.T.V.M. is its high input impedance, which makes it suitable for measuring high-impedance circuits, such as those found in audio equipment and electronic instruments. It also has high accuracy and sensitivity, making it useful for measuring low-level signals.
However, the V.T.V.M. has largely been replaced by digital multimeters (DMMs) and other digital test equipment in modern electronics, as they are more convenient, accurate, and versatile. Nevertheless, V.T.V.M.s are still used in some specialized applications, such as in vintage audio equipment restoration, where their unique characteristics are valued.