Vintage VTVM Advertising
Here we have a classic catalog advertisement for the Heathkit V7A VTVM. Everything mentioned in this ad applies to most other VTVM’s as well! As examples, they pretty well all measured peak-to-peak AC voltages, all had dB scales, all had ‘zero center’ operation, and most had 200uA meter movements. Even the ranges for AC and DC voltages were identical. So, what makes one model of VTVM superior to any other? Nothing, really. Later models used an etched printed circuit board, while earlier units were wired point-to-point. Someone somewhere will probably state the point-to-point meters work ‘better’ somehow. What you should notice is the relatively inexpensive price. Nearly every electronic experimenter owned a VTVM, which is why they are so easy to find today, and why they cannot command as much money as a vintage tube tester. There is model V7 and a model V7A, although I am not sure what the differences are. Strangely enough, I have both models(!), but haven’t had the inclination or the time to dissect each unit for comparison. What I do know is that they are both seen very regularly on eBay, which is where I purchased mine for about $15. They can be a little cheaper than others because if their ‘age’. Earlier Heathkit VTVM‘s (like the V5) can be had for about $10, while later models (like the V11) could fetch $20.
Above we see a vintage catalog advertisement for the ‘new’ Knight F-125 VTVM. If the specifications look familiar, it’s because the are. This particular model is harder to find than the ubiquitous KG-620. The ‘older’ model KG-620 is seen with increasing regularity on eBay, and typically ‘sells’ for in the $15-$20 range. Not all VTVM manufacturers were content using the ‘same old, same old’. Superior (or SICO) decided to do things a little differently, as we see below.
From the advertisement above (taken from the December, 1959 issue of Electronics World) we may notice a few things a little ‘different’ with the Superior 77 VTVM. Strangely enough, the one ‘drawback’ I see is the 400uA meter movement (the Lafayette KT202 is the only other VTVM I know of with a 400uA meter movement). This means that small amounts of current won’t budge the needle as much as with the typical 200uA meter movement employed by virtually everyone else. However, I also see Superior uses a whopping 6″ meter; perhaps this large of a meter was only available as a 400uA movement. None the less, it has other ‘quirks’ that make me avoid this meter, unless the price is really right. The 12AU7 seen in every other VTVM is also seen in the Model 77, but the 6AL5 has been replaced by two 9006 diode tubes! To be fair; the 9006 is a 7-pin high frequency diode, and many people feel the VTVM excels at measuring complex waveforms. To measure complex waveforms, and measure peak values ‘quickly’, this high speed diode is necessary. Not many tube manuals list the 9006, but they are available from people like Antique Electronic Supply for a very reasonable price. Yet, in the end, I still beg the question; “If the 6AL5 has diodes rated up to 700MHz, are 9006’s really an advantage?”
Here we see a familiar name, but on a ‘different’ piece of test equipment. The Hickok 209c is a VTVM with a few twists. First, there is the huge 9″ meter movement. Secondly, it does do a few things your Heathkit V7A cannot do. Aside from the ubiquitous resistance and voltage measurements, the 209c will measure current up to 1.5A, capacitance up to 2000uf, and inductance. Current and capacitance are read directly on a dedicated scale, whilst inductance is calculated mathematically. It makes for a handy VTVM, but the $184.50 price tag (in 1966) makes it a luxury for many aspiring technicians. Compare the price for the Hickok 209c to that of the Heathkit V7A.
Lastly, we see an advertisement for the Precision Model 78 VTVM. What is unusual about this VTVM is that it costs about twice as much as a Heathkit V7A, and operates on batteries! One of the few drawback to using a ‘regular’ VTVM is that you cannot work on a ‘hot chassis’. By using ‘battery’ tubes, and (of course) a pair of batteries inside the slightly enlarged cabinet, you can overcome this obstacle. I own one battery VTVM (a Triplette 631), and I still haven’t found a set of batteries. For those of you also suffering from a dead Triplett 631, here is an article which shows you how to convert it to use modern (freely available) batteries.
I try and avoid battery operated VTVM’s. When working on any ‘hot’ chassis with a ‘regular’ VTVM or Oscilloscope I recommend you use an isolation transformer.
The only features that compare in favor of the Precision 78 over a Heathkit V7A is the 100uA meter movement (four times more sensitive than the Precision 77), and the 13.3Meg input resistance. This makes it slightly more accurate than some, and less prone to circuit loading. I still say avoid it like a monkey plague, unless you find a stash of suitable batteries. Even then, pray that they are good batteries you find.
Sometimes, even books aimed at showing you how to get the most out of your VTVM had elaborate advertisements. Below is a sample for the Sylvania book ‘Servicing Radio and Television with a Vacuum Tube Voltmeter’.
What you may see in the advertisement above (taken from the May, 1951 issue of Radio Electronics) is the little bit toward the bottom dealing with ‘Audio Amplifier Servicing‘. We can check things such as gain, power output, distortion, noise level(!), and so on. And, we can detect gassy tubes and leaky coupling capacitors! Of course, these ‘tests’ take a considerable amount of time, and many gurus feel that time is much better spent selling you matched tubes or matched output coupling capacitors.