# Test Instruments

Posted on Feb 6, 2014

Prior to the advent of FET-input digital multimeters (DMMs), the vacuum tube voltmeter (VTVM) was the primary instrument for use when high input impedance was required. For the newcomer to electrical measurements, high input impedance for the measuring instrument is needed when measuring high impedance circuits so as not to load down the circuit under test and cause an erroneous result.

For instance, suppose you are trying to measure the voltage across a 100 k © resistor that is in series with a 50 k © and your voltmeter has a 100 k © input impedance. The parallel combination of the two 100 k © resistances (resistor and voltmeter) is 50 k ©. If the supply voltage is 3 volts, the voltage across the 100 k © resistor would actually be 2 volts, but with the voltmeter across it, the reading would be 1. 5 volts. If the input impedance of the voltmeter was 10 M © instead, the parallel resistance across the 100 k © resistor would be about 99 k ©, which would only introduce a very small error. I`ll try to get the previous 5 parts of this article. In checking out the vacuum-tube voltmeter in the last two installments of Test Instruments, we discovered that one of the most important reasons for using a VTVM was "sensitivity. " In practical terms, the sensitivity of a measuring instrument determines how it affects the circuit under test. Using a low ohms/voltmeter in a high impedance circuit is like trying to gauge a person`s strength with a 10-ton weight. What you`re trying to measure crumbles under the load. One area where the VTVM comes into its own is in signal voltage measurement in hi-fi amplifiers. For not only can the VTVM measure the a. c. signal voltage without knocking it to its knees, but it will perform the measurement in the presence of d. c. at any frequency in the amplifier`s range. If there`s a large...

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