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Found in the Attic: VOMs and VTVMs

Posted by Tom Vernon on Aug 25, 2016 4:11:00 PM

Vacuum Tube Volt MetersAccurate measurements of voltage, current and resistance have been an essential part of electronics troubleshooting since the beginning. Early repairmen used pocket watch meters, which were the topic of an earlier Found in the Attic. Using different meters for voltage and current, as well as for different voltage ranges, was pretty inconvenient. Soon, with the addition of rotary switches, and 'A' battery and meter multiplier resistors, the VOM, or volt-ohm milliammeter evolved. It was a giant leap forward for convenience, but left us with one of the limiting factors of pocket watch meters, namely that the internal resistance of the meter loaded the circuit under test, resulting in inaccuracies.

When General Radio introduced the first VTVM, or Vacuum Tube Volt Meter, loading problems virtually went away, since the input resistance of a VTVM is in the order of 10 megohms. Over the years, both VOMs and VTVMs improved with advances in electronics technology, although the principles of operation remained the same.

This is a genre-based Found in the Attic, looking at a collection of meters that have been brought down from the Great Upstairs to be put on display. This grouping does not include every meter in the attic. Doubtless, more will be found as additional boxes are unpacked. Pictured from left to right are: the Simpson 260, Eico 232, Triplett 801, Simpson 311, Heath MM-1, Weston model 695, and Weston model 45.

The Simpson 260 VOM is a classic. The original model was introduced in the 1930s. Although instantly recognizable as a 260, this first model had no fuses and no trimpots. All of the wiring was point to point, this was long before the days of printed circuit boards. Batteries used in this model are two 3-volt, (Burgess 422 or Eveready 750) and one 1½-volt 'C' battery. There were no battery connectors as we know them now. All batteries are soldered into place.Simpson 311 Volt Meter

As the popularity of the 260 soared, refinements were made, and the series designation came into being. It began with Series 2 the 1940s and continued through Series 9 in the late 2000s. The pictured device is a 260 Series 8P, which dates from the early 2000s. A much older 260 is also somewhere in the attic, but it is in need of intensive care.

The Eico 232 dates from the early 1950s, and recalls the days when you could save some cash and have fun at the same time by building your own test equipment. The textbook circuit is about as simple as it gets, and includes a 6AL5 as full-wave peak-to-peak rectifier, 12AU7 in the balanced bridge circuit, and a solid-state diode in the AC power supply. Eico never operated on the same scale as Heathkit, but was a popular choice among kit builders from the 1950s through the 70s.

The Triplett 801 was a milestone device. It was among the first FET VOMs, and dates from the 1970s. These devices provided the high input impedance characteristic of a VTVM, with the portability of a VOM. The early ones had issues. Some had little overload protection, and once you blew out the FET, it usually had to be returned to the factory. The 801in particular could develop problems with drift. Experienced technicians usually replaced all the capacitors and cleaned contacts if drift occurred. If that didn't solve the problem, the FET was suspect.

Two problems in particular plague 801 restorers today. First, the original 4 1/2-volt batteries are no longer available. Second, the FETs were hand-selected by Triplett. That original FET is no longer made, but one of Triplett's criteria for selection was exceptionally low drift. Unless the 801 is something you are extremely passionate about or have some emotional attachment to, it is best to think of it as a display item.

The Simpson 311 VTVM is similar to the Eico 232, although the construction of the Simpson is more robust. Same tube lineup, but the 311 also has provisions for an RF probe. The weak point on many of these meters is the AC/ohms-DC probe. Both the 232 and the 311 used the old Amphenol unbalanced microphone connectors for this probe. It is arguably the worst connector that was ever manufactured! As if that isn't enough, the switching between AC/Ohms and DC functions on the probe is by contacts that physically touch each other, that's all. The combination of corrosion on the switch or connector can make reliable measurements problematic at best. Regular maintenance of these probes is  essential to accurate measurements.

One of Heathkit's earliest entries into the test equipment kit market was the MM-1 VOM. It was a textbook 20,000 ohms/volt meter. It was offered both as a kit and fully assembled.

The most unusual meter in this display is probably the Weston 695 power level/voltmeter. This was part of a series of small 'pocket' meters introduced by Weston in the 1930s. The series included the 564 VOM and the 571 output meter. The 695 is essentially an AC voltmeter that is accurate over the audio frequency range, and the meter has both AC and dB scales. AC voltages from 1.5 volts to 150 can be measured, making it possible to check  everything from voice coils to audio output transformers. A 'series condenser' terminal is provided so the meter could block the plate voltage of tubes and measure audio output.

The dB scale ranged from -8 to +32, but here's the catch. The dB scale referenced 6 mW across 500 ohms – the old standard. It was changed many years ago to the now-familiar 600 ohms.  Doing the math with every measurement to convert from 500 to 600 ohms is simply not worth it. Like the Triplett 801, it's best to think of the 695 as a display piece, and be happy with that.

Finally, the oldest meter in this display is the Weston model 45, which dates to 1932. While the other instruments were designed for the service market, the 45 was definitely a laboratory instrument. It was used for preciInside the Simpson 311sion laboratory measurements of voltage, and as secondary standards for calibrating other meters. The mirrored scale enables the operator to avoid parallax errors. Note that this particular instrument has been standardized at the factory and was certified accurate to ±0.5% of full scale, or in this case ±0.075 volts.

These meters came from many places. Some were college surplus grabs, a few came from dumpster-diving expeditions, and a few were gifts. All were calibrated and in use at one time. Now they're retired. Batteries have been removed, and cosmetic cleanups have rendered them squeaky clean.

Although technically obsolete, there is still a great deal of interest in vintage test equipment. A large number of VOMs and VTVMs are offered for sale on eBay. Vintage electronics blogs regularly contain posts along the lines of  'How I restored a model XXX VTVM that I got for $5.00 at a yard sale.'

I am occasionally asked if analog VTVMs have any use in contract engineering. The answer is seldom, but yes. A military surplus TS-505/U portable VTVM is kept calibrated and in tip-top condition on the workbench. It gets packed for any projects that involve transmitters. Its RF and high voltage probes make it particularly useful. But what's really important is its durability. During a brief lapse of awareness, the high voltage on a 10 Kw AM transmitter was measured on the wrong side of an RF choke. The tip of the probe melted, but the TS-505 emerged unscathed. A few moments with a grinding wheel restored the probe tip.

Although they have gotten better in recent years, DVMs can still act quirky in high RF fields. Can they survive an incident like the one just described? I'd rather not find out.

Topics: Vintage Electronics