Just as the name implies, VTVM is the acronym for Vacuum Tube Volt Meter.Therefore, as expected, there are honest-to-God vacuum tubes inside your VTVM.On the outside, the meter looks like a typical old-fashioned analog meter. Despite the differences in outward appearance, probably 99% of VTVM’s have the identical circuit inside the cabinet.
Today’s DMM can achieve comparable accuracy, yet there are still instances where a VTVM is the only meter to have!
Next, we will see a case where the faster response to any change in the voltage being measured comes in handy.
If this high input impedance were not enough of a benefit, note that the voltage being measured is amplified by the two triode tubes.
This isolates the low-resistance meter movement from the circuit being measured.
What is further done usually is to use an isolation probe, where a 1Meg resistor is soldered into the probe casing.