Re: Don’t Pitch That Old VTVM

[Ed Stone]

I totally agree Eric – a VTVM and VOM are the most-used pieces of test equipment I have: always have been and always will be. The use of a VTVM or VOM as an output meter is a must when aligning a receiver in my mind. A DMM has its place though, as you say, for accurate measurments of resistance (and current), as well as for inductance, capacitance, frequency and temperature measurments if it has those facilities like mine has. But be careful on measuring voltages with either a VTVM or DMM – yes, the latter provides ‘digital accuracy’, but both of these instruments offer a high input resistance (several megohms/volt) – as such, you can measure very different voltages (higher) with either a VTVM or DMM than you would in the same circuit with a VOM (most noticeable in higher impedance circuits). This can cause unecessay palpitations when comparing voltage node measurments provided by a manufacturer (in the sets manual or on the schematic), which were usually taken with a lowly VOM with an input resistance as low as 1kohm/volt up to the ‘standard’ VOM input resistance of 20kohms/volt (eg AVO Model 8).

The variation in reading between a 1Kohm/volt and a 20Kohm/volt VOM can be very significant (even more so for a DVM or VTVM), depending on the circuit under test. For example, if measuring the HT line in a receiver, there will be hardly any measurable difference, as the differing current drain through the two meters (say 1mA for a 1Kohms/volt meter versus 50uA for a 20Kohm/volt meter) will easily be supplied by the receiver (low impedance) psu circuitry, however, when measuring the anode or screen voltage of an operating tube, the extra current drain through the lower resistance meter can make a considerable difference due to increasing the voltage drop across the anode or screen resistor (ohms law) – typically lowering the measured voltage by say 20% or more – thus if the voltage table indicates that the voltages were measured using a 1Kohm/volt meter and you are using a 20Kohms/volt meter, the measured higher voltages would certainly be enough to question whether the stage under test is functioning correctly (even though the more sensitive 20Kohms/volt meter or, DVM or VTVM is actually reading a much more accurate ‘real’ voltage!). The simple way to resolve this is to shunt your 20Kohm/volt (or higher) meter with a suitable resistance on each range to give approximately the required sensitivity – eg, for a 250v scale, use a 250Kohm (or 247Kohm) resistor. I made up a little unit constructed using a rotary switch and covering all the dc voltage ranges of my multimeter for this purpose (see photo).

Ed

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