After having given a very brief run-through of my VTVM, I realized there’s a bit of explanation missing.
Why would I want to have a fifty year old piece of test equipment sitting on my bench? Why not a nice new digital multimeter?
Well, there are a few simple reasons, and the’re mostly based upon what a digital meter does.
I do have a digital multimeter, and it’s a very nice, versatile tool. It provides me with easy to read, auto-ranging Volts, Amps, Ohms, capacitance, frequency, diode testing but to name a few functions.
That sounds wonderful, and in all essence it is. However, a trip down memory lane is in order. In days gone by, if you wished to measure a voltage, you’d place a calibrated moving-coil meter across the points to measure, and the deflection of the meter needle against a scale would give you a reading. My VTVM has such a moving-coil meter attached to its faceplate.
However, in order for the meter to move, it must pass a current. My VTVM’s meter is “200 micro-Amps to full deflection”, which doesn’t sound like a lot (and in the grand scheme of things it isn’t really) but that current draw is too much to measure sensitive circuits where 2-3 micro-amps is all that’s being dealt with. Connecting a regular meter across the circuit would present it with an effective dead short, killing what you were trying to measure. Or, at best, it’d make the reading highly inaccurate*.
Examples of such circuits are radio-frequency tuned circuits, where the signals being tuned into are miniscule. What is needed is a method of connecting a meter with a very high resistance- so high that the circuit to be measured hardly “notices” it. But, a very very high resistance meter will pass next to no current, and wouldn’t move. Enter, the vacuum tube amplifier.
(For those of us young enough, vacuum tubes are.. er, a bit like glass transistors. Er, yeah. That’s a poor analogy, but it’ll do for now.)
Vacuum tube amplifiers do what the name suggests. They take a small signal and make it larger. In some cases, they need to pass virtually no current in order to operate. See where we’re going with this? Sure. So, if we connect the vacuum tube amplifier up to the meter, a tiny little signal can be made to make the meter move.
That’s basically what is done. A very high resistance is placed between the testing probe and the amplifier (in this meter, that’s 11 Mega-Ohms, or 11,000,000 Ohms) which is an improvement over the resistance of the meter’s coil (about 200 kilo-Ohms, or 200,000 Ohms).
This will lead on to a little very basic vacuum tube concepts and amplifier theory. But, for now, that’s basically why it’s used. Also, all the old manuals I have are marked out to be measured using one of these devices. If I connect my digital meter.. first, it can only read up to 2.0 Mega-Ohms before it goes off-scale. My VTVM can read up to 1,500 Mega-Ohms. In fact, my VTVM can measure the resistance across my digital meter (about 8 Mega-Ohms).
Hence why it’s not in the trash any more. It’s still very useful.
* Side note: Yeah, Volt-meters actually aren’t. They measure current, through a fixed resistance. Sorry.