Asking many old-time tube technicians what one piece of test equipment they simply cannot be without, the unanimous answer is the VTVM. To be sure, there are many instances where a good digital multi-meter is actually preferred, but a simple, old-fashioned VTVM can help maintain your tube amplifier in ways any Fluke simply doesn't measure up to (pun intended). Many of today's tube 'experts' do not seem to own a VTVM, and I am always puzzled by this. To give you a basic understanding of what a VTVM is, and why it is so valuable, let's start with a crash course. Remember, this is just a basic primer. At the end of the 'Lesson' I will impart with a book list that can help you become a better technician in no time flat!
Just as the name implies, VTVM is the acronym for Vacuum Tube Volt Meter. Therefore, as expected, there are honest-to-God vacuum tubes inside your VTVM. On the outside, the meter looks like a typical old-fashioned analog meter. Below are a few examples from my VTVM collection.
Despite the differences in outward appearance, probably 99% of VTVM's have the identical circuit inside the cabinet. The heart of every VTVM is the balanced bridge circuit. This circuit was patented in 1916(!), and the execution of this basic principle does not seem to have changed. The concept is crudely represented below.
You should note that the input resistance is constant though out the range switch. That is, the input resistance is 10Meg, whether the range switch is operated on the 1-volt position or the 100-volt position. Accordingly, the ohms-per-volt sensitivity changes with the range selected. In this example, the sensitivity is 10Meg on the 1-volt range, and 100K on the 100-volt range. This is still significantly higher than the typical 20,000 ohms-per-volt of analog VOM's of the day. What is further done usually is to use an isolation probe, where a 1Meg resistor is soldered into the probe casing. This effectively make the input resistance 11Meg on all ranges. If this high input impedance were not enough of a benefit, note that the voltage being measured is amplified by the two triode tubes. This isolates the low-resistance meter movement from the circuit being measured. The meter movement is located in the cathode of the bridge circuit, and the differential voltage is displayed. In practice, these two triodes are a single 12AU7, which is the most common tube to be seen in every VTVM. Some very old VTVM's I have use a 6SN7, but these are few and far between. The biggest benefit here is that the meter movement cannot 'load down' the circuit you are trying to measure. Therefore, voltage measurements are far more accurate than with a regular VOM. Is this increased accuracy necessary? Probably not, but it will come in handy later, as we'll soon see.
One of the biggest advantages of a VTVM over a 'regular' VOM is accuracy. Let's study the diagram above. It assumes a standard triode tube used as a voltage amplifier. The plate supply is 100VDC, and we wish to measure the voltage at the plate itself, with a 500K (designated above as .5 Meg) plate 'load' resistor. Using a VOM with an input resistance of 100K, the voltage divider action yields a voltage measurement of 14.3VDC! Obviously, this wouldn't appear 'right' to the technician, and he would go on to use a 'better' DMM. However, for demonstration purposes, the technician decides to use a VTVM, with an isolation probe. Now the voltage divider action results in a voltage reading of 49VDC, which is pretty close to the 'expected' 50VDC. Hopefully, you can understand how much this accuracy was appreciated some sixty years ago. Today's DMM can achieve comparable accuracy, yet there are still instances where a VTVM is the only meter to have! Next, we will see a case where the faster response to any change in the voltage being measured comes in handy.
In the scenario above, we see where a VTVM performs a function with a visual panache that no DMM can even dream of doing. If you want to be serious about servicing vintage tube amplifiers, you should make a routine habit of checking any coupling capacitors for leakage. The input impedance of analog VOM's and the slow response of DMM's makes the test shown above much more difficult to perform. You can check for leakage faster and with a visual indicator using a VTVM, and it is a worthwhile test. Older paper coupling capacitors always get leaky over time, affecting the tone to the point that your Internet 'guru' espouses using only paper capacitors for 'that sound'. He has no idea what the heck is going on, and gives the 'credit' for this 'sound' that modern amplifiers cannot achieve (with 'good' capacitors) to the leaky capacitor. Now you know better. One test procedure is as follows. Assuming your amplifier is not sounding as it should, you measure for any DC voltage at pin #2 and pin #7 of every 12AX7. Now we assume a reading of +5VDC on one grid. This is definitely wrong, and we can confirm the problem thusly. First, disconnect the end of the capacitor that was connected to the 'positive' grid. Next, use your isolation probe, and set your VTVM +DC voltage scale to '150'. Connect the isolation probe to the 'open' end of the coupling capacitor, and the ground clip to the chassis. Power up the amplifier, and watch for any deflection on the meter. If you cannot see much of a meter reading, go downward through the voltage scales (usually this would be 50, 15, 5, and 1.5). Remember, even a reading of 1VDC means the capacitor has leakage, and should probably be replaced immediately. Some technicians start on the higher voltage scale as a habit, perhaps because there is always an initial 'charging kick' when a DC voltage is applied to a capacitor, or in case the capacitor has a 'dead short'. You cannot 'blow up' a VTVM meter with a voltage overload (another benefit), so many experienced technicians automatically go to the lower voltage range on their VTVM. However, you can overheat one of the precision resistors, and affect the accuracy. If you start on a lower scale, watch for the 'charging kick' and see that the voltage reading immediately 'backs off'. This would tell you how much 'dielectric absorption' the capacitor has. Ideally, you would want as little as possible, although paper capacitors always have more dielectric absorption than say Mylar or Polypropylene. But, I'm getting ahead of myself. The capacitor inside the VTVM could also be leaky, and we'll talk about that later. For now, you may want to know how much leakage the capacitor has, and if this is 'too much' leakage to be useful. Those are good questions, and here is how we find out. The 'best' way is with a variable power supply, capable of furbishing the rated voltage of our capacitor. First, we connect the suspect capacitor in series with the power supply and our VTVM and measure the actual leakage voltage. Then, we apply the following formula.
As an example, suppose we apply 500VDC to our capacitor, and see 2.5VDC at the 'output' end, using a VTVM that has 11Meg of input resistance. The math works out to 2,189 Megohms. Since leakage current is inversely proportional to leakage resistance, you may assume the capacitor has negligible leakage. However, keep this in mind. Capacitor leakage is not like wine; it will not get better with age. Does it matter? Well, the capacitor is acting like an electroplating cell. As the anode gets eaten away, metal is being deposited in the paper pores, and it's only a matter of time before the capacitor shorts out. Also, keep in mind that the amount of leakage will change with the applied voltage and signal frequency! The above example translates to almost 1.5 microampere of leakage current at this one test parameter. In a single ended output stage, using this capacitor to feed a 1 Megohm grid resistor means a 1.5 volt bias shift. This could cause the tube to run hotter than desired. Sooner or later the leakage will amount to 10 microamperes, which translates to a 10VDC bias shift. The tube will not appreciate this at all. I change anything that shows more than 1 microampere of leakage current, regardless of how 'vintage' the amplifier is.
Another repair situation where a VTVM is invaluable is in troubleshooting 'tremolo' circuits. Let us assume we have a nice Fender amplifier with a tremolo circuit that isn't working. The average 'guru' uses a technique called the 'shotgun method', where absolutely every capacitor is changed in the tremolo circuit. This works 75% of the time, but there is a way to repair the circuit, and actually learn what went wrong. The first step is to make sure the tremolo oscillator is 'on', and attach our VTVM DC-voltage probe to the plate of the oscillator tube. You should see the needle 'swing' in step with the setting of the 'Rate' or 'Speed' control. A cheesy example is seen in the 'cartoon' below.
Let us suppose the oscillator is not working, and you have no tremolo. Now, with your capacitor decade box (you do have a capacitor decade box, don't you?), you can 'bridge' the three phase-shift capacitors until you do see the needle swinging. You can always 'shotgun' the circuit, but I feel you learn more with this method. Plus, you can always try different capacitor values, and alter the 'range' of the oscillator, if the tremolo seems to go a little too fast or too slow for your liking. Pretty nifty, huh? As a side note, where this method is superior to the shotgun method is as follows. If the tremolo oscillator has a cathode bypass capacitor (the little cartoon above does not, but often they do), and that bypass capacitor opens up (a common occurrence), by replacing all three phase-shift capacitors, all you've done is waste 15 minutes of time and a few dollars worth of capacitors. By checking the oscillator with a VTVM, you can also bridge the bypass capacitor, and make sure the oscillator is non-functioning because of a lack of phase shift coupling, and not a lack of gain in the tube. For Blackface Fender amplifiers, they use an opto-coupler that often goes 'bad'. Using the shotgun approach would not find this out, but with a VTVM, you'd 'see' the oscillation at the plate, and move further towards the opto-coupler. Now you can assume the opto-coupler is bad, and repair the tremolo circuit. Granted, there are no 'medals' awarded for fixing a tremolo circuit faster than the average guru, and this method actually consumes more time. However, I still think you get a better 'education' using old-fashioned equipment and your 'noodle' then having a knee-jerk reaction and changing everything that doesn't outrun you.
Another instance where an analog meter (or a VTVM) is preferred is 'testing' potentiometers. Let's assume you have a 'scratchy' potentiometer, and cleaning it once didn't seem to help. We have three possibilities. The potentiometer may have 'DC' on it, similar to a 'Presence' control on a typical Marshall. The potentiometer may simply need a second 'cleaning', or taken apart and the 'track' touched up with a pencil eraser. I only do this with certain vintage amplifiers, seeing as replacement pots are available fairly inexpensively. Lastly, the potentiometer may indeed be beyond cleaning, and needs to be replaced completely. Here, we use an analog meter, and connect the leads to the middle 'wiper', and either remaining terminal. Let's say we have a 250K potentiometer. We would hook up our analog meter (or VTVM), and set the resistance scale to 'RX10K'. Rotating the potentiometer, observe the meter movement. The needle should smoothly progress from '0' to '25'. If you see erratic 'jumps', the pot either needs to be serviced further, or simply replaced. If the needle moves smoothly through the rotation of the potentiometer, you should have a quiet control, unless there is DC in that part of the circuit where the potentiometer is used.
A final example of where a good VTVM is preferred actually deals with biasing your amplifier! Assuming you are biasing with the 'current-draw' method, you have obstacles to overcome. How can you be sure your 1-ohm resistors are exactly 1-ohm? With your $350 Fluke DMM, you cannot ever be certain. With a $10 Heathkit IM-11 you can. Want proof? Take that Fluke, and set it to measure ohms. Short the two leads together, and note the reading. What did you get; 1.2-ohms? Tsk, tsk, tsk. My $10 Heathkit VTVM can find a 1-ohm resistor that is exactly 1-ohm. If we remember that low-wattage carbon composition resistors drift, and avoid them, we can put resistors that are 'precision matched' in our amplifier that will accurately measure what is purported to be plate current. Read 'Biased Opinions' to refresh your memory on my feelings about using 1-ohm resistors in the cathode to try and set the bias.
We will conclude our 'Lesson' with a list of recommended books. If you decide to purchase any VTVM, you need at least one owner's manual. Since the vast majority of the VTVM's out there employ the same circuit, most any schematic and manual will do. It may seem silly, but you would also do well with a few 'How To' books dealing with using a VTVM. There is, of course, the 'Bible' of VTVM books; "The VTVM, by Rhys Samuel". My copy is seen below. First published in 1955, it was actually available in paperback and hardcover versions. Surprisingly, they are not that rare to find in used book stores today, so you shouldn't have any trouble locating a copy. I have also found them to be a popular library book around the United States. Through my American sources, I am told they are available at many public and school libraries. There are eleven chapters inside, but a few are of extreme importance to me personally. The first five chapters give a very good understanding of how a VTVM works, and proper usage of your meter. There is a chapter devoted to 'Servicing Audio Amplifiers', and even the 'Miscellaneous Applications' chapter has come in handy. But the 'Troubleshooting The VTVM' chapter should be important to anyone who owns a VTVM. Magazines such as Radio-Electronics also published articles with titles like 'Fix Your Own VTVM', but these can be hard to find for most of us.
There is a second very popular VTVM book; Vacuum Tube Voltmeters, published by Rider. First printed in 1941, a second edition dating from 1951 is also known to exist. There are only a few things keeping this book from being 'the Bible'. We do get many chapters on servicing Radios and TV's, along with how to align IF's, wave traps, discriminator circuits, and the like. Sadly, there are no chapters entitled 'Servicing Audio Amplifiers With A VTVM'. On the plus side, the details on the internal workings of a typical VTVM are more involved and in-depth. Also, many commercial VTVM's are examined, and full schematics are seen along with calibrating instructions. However, most models included are quite antiquated (e.g. Barber, Clippard, Coastwise), and probably will never be seen on eBay today. There are still plenty of Ballentine, Eico, Heath, RCA, Simpson, and Sylvania schematics to help in the off-chance you do own the specific model. We do get a few chapters on low impedance measurements, and how to measure inductance and capacitance with our VTVM. However, surplus test equipment is necessary to conduct these experiments. As you can see, there are many good and bad points to this book. Many of us may be simply too lazy to go through all of this work just for a measurement. It is still excellent reading, and highly recommended for those who really want to understand their VTVM as much as possible.
There were many other books published dealing strictly with tips on using your VTVM, and here are a few I've found and what you can expect between the pages. Keep in mind there were literally hundreds of titles out there; these are just a few I have picked up over the years.
The first thing I look at is the exact title of the book. Anything along the lines of "Fix TV Troubles In 10 Minutes Flat With A VTVM" does not appeal to me for many reasons. First of all, I am not interested in collecting tube televisions, and storing a warehouse full of oddball tubes. Secondly, these books will almost invariably tell me how to check the bandpass response curve, or how to align the RF trap. Sure, there will the common thread in all books on how to check the power supply or leaky coupling/bypass capacitors. But, there will be no 'insights' that might reveal a problem unique to audio work. There is a section in "The VTVM" which explains chasing down hum in audio amplifier! The broader "Understanding Your VTVM" book is much more inviting, as it will explain the workings of my meter, and give advice on how to make the best use of it. I will investigate books aimed in repairing old radios, because that is a hobby of mine, and there are many repair parallels that can help me. I can always find a chapter on chasing down a 'motorboating' problem in an old radio, and that is more useful than knowing how to troubleshoot the horizontal sync circuit in a TV. Check out a very incomplete VTVM book guide below.
Hopefully, you get an idea how valuable a VTVM is. While it is fun to brag about how much you've spent on your fancy Fluke 'True RMS meter', you are not being a 'real' tube technician, without using a VTVM. If you decide to buy one, they are so similar that it is hard to go wrong buying any VTVM. Since Heathkit seems to be very popular, I have gone through a lot of magazine advertising in an attempt to determine when various models first appeared. Here is a quick sample of what I have 'discovered'.
That should help set up a good guide line for you. There are plenty of other Heathkit VTVM's, and even a specific audio unit (which is simply an exclusive AC-voltage unit, with a db scale)! Check around, and have a back up unit. Good brands to seek out other than Heathkit (remember; the higher the model number, the 'newer' it is) include Eico (the model 232 is the most popular), Knight (the model KG620 is very popular), and B&K (the model 177 is the most popular). RCA made an excellent line of VTVM's dubbed the 'Volt-Ohmyst'. Below left is a picture of the WV77, or the 'plain' Volt-Ohmyst. In 1961 it carried a list price of $43.95 for the WV77E. Below right is the WV98, dubbed the 'Senior Volt-Ohmyst' (list price of $79.50 in 1961 for the WV98B). It feature an enlarged meter movement, perhaps chiding the 'Senior' that he need such a large meter movement, in order to be able to read the measurements. Either make a fine VTVM for your bench. You should look at the suffix letter to your Volt-Ohmyst model number. The WV98C is a later unit than the WV98B (obviously), and would hopefully be in better condition. The circuitry should be identical, as at this time all VTVM's had a 1.5V scale, as an example. I do not know what the internal differences (if any) there would be.
The 'Cadillac' of VTVM's would have to be the Hewlett-Packard 410B. Below is a picture of one from my collection. I am lucky(?) in that I own two 410B's, and they truly are a wonderful instrument.
These are very different from any Heathkit, so have the manual handy. If you can tolerate their bulk and all of those built-in leads (one for a common ground, one for the ohms measurement, one for the DC measurement, and one for the AC measurement!), the 410B is an excellent VTVM, with very accurate readings. There is, of all things, a vacuum tube inside the AC probe, and the probe does get quite warm during use. The 410B had at least two 'versions', each with their own tube complement. The tubes were replaced completely with the 410C. Hewlett-Packard also made a '400-series' of VTVM's that strictly measured AC, and had a db scale on the meter movement. This would be used strictly for measuring audio and other AC voltages, so it's a VTVM only for the die-hard collectors out there. Heathkit made an 'audio analyzer' dubbed the AA1 (what else?), which had an oscillator (only utilizing two frequencies; 60Hz and 6kHz), a watt meter, an Inter-Modulation Distortion Analyzer(!), and an AC VTVM. They are fun to play with, but are slightly limited today, as the amplifiers we might play with are a little more powerful than the Hi-Fi amplifier this unit was designed for. Built-in load resistors of 4, 8, 16, and 600-ohms(!) had a maximum continuous power rating of only 25-watts. The IM analyzer is useful only for servicing your ST70, and not for your Deluxe Reverb. Below is a photographic example of the Heathkit AA1.
If you do not have a 410B, make sure you have the 'proper' probes for your VTVM. And I repeat this sentiment often; try to get the relevant manual, although they are also pretty similar! You can find a good selection of VTVM's for auction via eBay, and the prices seem most reasonable. While tube testers have become overpriced Ouija boards, the 'lowly' VTVM seems to command far less through online auction. Typically in the $20-$30 range, this is a very fair price to pay; providing the VTVM is in 'decent' shape, and has the proper probes included. They will invariably need to have the switches and calibration pots serviced, and many need the input coupling capacitor changed. When all is said and done, this is not that bad of a proposition. Below is a list of things to consider when you decide to own and use a VTVM.
That pretty well wraps up what little I can teach you about VTVM's. You should have at least one VTVM on your bench right now, and one of the books mentioned above, to help you understand using your meter a little more. Get a feel for using this ancient technology, and you will be able to appreciate how the real old-time tube technicians experimented with their amplifiers. While it is fun to use the latest digital test equipment, I promise you Leo Fender did not use any Fluke meter. Chances are he used a Senior Volt-Ohmyst and a vacuum-tube oscilloscope. Yet his amplifiers are considered to be the epitome of tone. Imagine how much better those amplifiers could have sounded, if he had only used nothing but a Fluke DVM and set his 6L6's to idle at 40mA of plate current each. To check out a select few samples of Vintage Advertising for the VTVM, simply CLICK HERE.