The content of this website and blog has been created and posted here solely for the purposes of reading interest by Rob's Radio-Active, LLC. The content is the thoughts of the author and is not intended to be an authoritative guide. There may be errors and omissions. Electronic and electrical devices pose a potential hazard to life and property if improperly handled, used, or serviced. The author cannot possibly think of, and warn of, all the ways one can get into trouble with electricity, and recommends one learn all of the technical and safety aspects of electrical/electronics before attempting to work with or use any of it. There are hazardous voltages and currents involved in almost all electrical and electronic equipment—especially vacuum tube devices or anything that plugs into mains current for operation. Furthermore, vintage equipment was never designed or built to be up the safety standards of modern times.
What? A meter dedicated to AC voltage measurements only? Yes—and one of the handiest things in many workshops and labs!
The AC millivoltmeter is kind of a one trick pony—but it does its trick very, very well. But they don't just measure millivolts, they can also measure 100V to 300V full scale, depending on the specific meter. I wouldn't tell someone to buy one of these units instead of an oscilloscope, but in addition to it. A scope can be invaluable, but down around 1 mV or less, they can be difficult to read and may not be as accurate. It depends on the scope, of course. The nicest thing about an AC millivoltmeter, in my opinion, is ease of use. It is not usually necessary to see a picture of the waveform of a signal to collect pertinent information about that signal, and especially when injecting a sine wave test signal into an amplifier or looking for a radio signal, we already know we are dealing with a sine wave. We usually need to know the voltage. AC millivoltmeters require little or no twiddling with settings knobs and little or no counting and calculation. Digital scopes can do all this for you, but most analog scopes do not.
AC voltage or current of a wide frequency voltage range, especially when the voltage gets very low or the frequency gets very high, or both, is not an easy thing to measure accurately. They require a sensitive, low noise, wideband amplifier with a very flat frequency response to boost low levels of signal to readable levels. In addition, they must maintain a high input impedance (low capacitance and high resistance).This usually requires an input amplification stage with high impedance to isolate the rest of the amplifier from the probe, a precision voltage divider for the wide voltage ranges. The following stages of amplification must have frequency compensation, such as peaking coils for example, and finally a lot of negative feedback from the output back to the early amplifier stages to stabilize it. This is how they manage to have essentially a flat response from 10 or 20Hz to 1 or several megahertz, and a full scale meter range of 300uV to 100V or 1mV to 300V. Figure 1 shows the functional block diagram of an old Hewlett Packard model 400H. This meter has a full scale voltage range of 1 mVrms to 300Vrms, and is accurate to a few percent from 10Hz to 4MHz. This meter cleverly uses a number of pentode amplifier stages that, by circuit design, behave as lower gain triodes at lower frequencies. At higher frequencies, some of the smaller capacitors re-route part of the signal to cause the tubes' screen grids to become effective and the tube becomes a higher gain pentode to compensate for otherwise falling gain of the amplifiers at higher frequencies. Finally, the feedback is taken after the meter rectification diodes, so that the amplifier chain effectively gains and gains until the diodes conduct fully—so until the meter movement is ready to move, the feedback is low. Once the meter diodes are fully conducting, the AC controls the amplifier gain to a set amount. In short, this scheme takes the diode forward voltage drop out of the circuit and makes for a very linear and accurate instrument, even at the lowest signal voltage levels. Additionally, it functions as a preamplifier that could be used for one of the less sensitive oscilloscopes of back in those days, to get a good view of a waveform on very small signals. Most service type scopes of the '50s and '60s were sensitive to only about 25mV per inch of deflection.
The HP400 series were a laboratory grade instrument that few individuals or service shops could afford to buy, nor did they really need its capabilities. Nowadays they can be had fairly inexpensively, but be prepared to do some restoration and calibration work. I have used mine for signal tracing in vintage AM radios where the highest frequency to be found is a little more than 2MHz—the radio frequency of the top of the dial plus the local oscillator frequency. Most AC millivoltmeters used by service shops and audio design laboratories had a bandwidth of somewhere around 10 to 20Hz up to 400kHz or perhaps 1MHz. Typical of these are the vintage, tube type Simpson model 715 and the Eico model 250 in the pictures below. The new REK model RV322 is also pictured below these. The REK is solid state (of course) and has two channels. Inside the cabinet of the REK there are two identical circuit boards that are about as 2/3 as large as the sides of the cabinet, with many chips and transistors, etc. I use the REK a lot for testing audio amplifier gain or signal tracing in same. The second channel is very nice to have, because the difference between the two meter pointers can be read directly in decibels (or with a simple addition operation if the range scales are set differently) very quickly. Again, little or no math required. (Figure 2) The REK also has a low range of 300 microvolts full scale (Wow!) up to 100V full scale—and is accurate from 10 or 20Hz to 1MHz. I don't know if I ever have had to measure a signal voltage higher than 100 volts RMS, and the 300uV scale is not used a lot, if ever, but it's nice to know it's there.
Should you get one? Not if you are setting aside money for a new or used oscilloscope or other basic test equipment you need, but after that, you may consider one of these to be a very handy addition to your workbench.
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Rob Tracy is the operator of Rob's Radio-Active, LLC. Parts and vintage stereo, radio, audio, and test equipment sales and restoration services.