Vacuum-Tube Voltmeters

Vacuum-tube or electronic voltmeters make use of some of the unique characteristics of electron tubes, and so are available in many types. They are available for measuring direct, rms, average or peak voltages. Of the many types available, only the simplest and possibly the most commonly used type for measuring rms values of alternating voltages will be described here.
Vacuum-Tube Voltmeters - schematic

The chief advantages of the electronic voltmeter are its high input impedance, wide frequency range, and sensitivity. Because of its high impedance it does not divert enough current to disturb the circuit to which it is connected. Fig. 28-28 shows a simplified diagram of one type of electronic voltmeter. It uses a simple evacuated triode as a detector, as described in Chapter 27. The milliammeter in the plate circuit is calibrated in terms of rms volts applied to the grid. This type of voltmeter is available with a built-in rectifier and filter, so that it may be connected to a standard 120-volt, 60-cycle supply.

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