Heathkit IM-11 VTVM

Name: Heathkit IM-11
Brand: Heathkit

Heath's "world's most popular VTVM" - a 1.5-volt-range, 11-megohm bench meter that cost 45 cents more than its 16-year-old predecessor and outsold nearly everything in the catalog.

Overview

The IM-11's big feature is its input resistance: 11 megohms on all DC ranges, with 1 megohm of that inside the probe itself. That is why service techs liked the meter. It lets you poke around high-impedance circuits without dragging the reading down, which is the whole point of a vacuum tube voltmeter. The probe is also where the IM-11 gets clever about workflow. Instead of swapping leads between functions, there's a single slim probe with a switch on the body for AC/ohms or DC, plus a little hook that clips onto component leads for hands-free testing.

The meter movement is a 4½-inch, 200 µA unit. That is sensitive enough for a minimum DC range of 1.5 volts full scale, which is unusually low for the era and one of the specific improvements Heath made over the previous generation. Seven DC ranges, seven AC ranges and seven resistance ranges cover 0-1.5, 5, 15, 50, 150, 500, and 1500 volts full scale on AC and DC, plus a peak-to-peak scale of 0.4, 14, 40, 140, 400, 1400, and 4000 volts. Resistance work runs from 10 ohms center scale across the x1 through x1 meg multipliers. The meter reads down to 1 ohm or up to 1000 megohms on the internal battery, a single 1.5-volt "C" flashlight cell. Accuracy is specified at ±3% on DC and ±5% on AC, full scale.

The circuit is a balanced bridge, push-pull, built around a twin triode, with a 12AU7 and a 6AL5 handling rectification and amplification. The voltage divider networks use 1% precision resistors. Frequency response is quoted at ±1 dB from 25 cycles to 1 megacycle, measured into a 600-ohm source. Power comes from 105-125 VAC at 50/60 cycles, drawing 10 watts. The whole thing measures 7¾ by 4-11/16 by 4½ inches and weighs 5 pounds in kit form.

Heath moved the circuit to a printed board for the IM-11, which improved stability and made assembly much easier than the point-to-point wiring of earlier kits. That board, the 1% resistors and the single-probe design are what Heath was pointing to when the catalog copy bragged about "performance that has lead the industry for nearly two decades." HiFi-Stereo (July 1967, p.71) ran a Heath advertisement in that period that leaned on the same pitch, quoting critic Julian Hirsch on the AR-15. The IM-11 was the entry-level instrument every Heathkit builder was expected to own.

The pricing is odd. Kit form was $24.95. The factory-assembled IMW-11 was $39.95. Heath liked to point out that this was only 45 cents more than the equivalent instrument from 16 years earlier. It had the same basic shape and size, but the meter movement went from 500 µA to 200 µA, the minimum voltage range dropped from 3 volts to 1.5 volts, the half-wave rectifier was swapped for a voltage doubler to get proper peak-to-peak response, and the meter was enlarged to 4½ inches for easier reading. The color styling was modernized and the shatterproof meter cover carried over.

Two factory accessories show up attached to used IM-11s and are easy to mistake for aftermarket parts. The 309-C Etched Circuit RF Probe (kit $4.50, assembled 309W-C $4.50) works with any 11-megohm VTVM and extends RF measurement to 250 megacycles. The 336 30,000-volt DC high-voltage probe (kit $4.50, assembled 336W $5.50) provides a 100x multiplication factor on the DC ranges and was marketed squarely at TV service work. There was also a combination offer, Kit EF-1-3, which bundled the IM-11 with the EF-1 Technical Applications course for $36.95. That suggests Heath was selling to students learning the trade on their own bench as well as working techs.

The IM-11 isn't collectible because it's scarce. Heath sold a lot of them. It's a usable instrument that is also a piece of mid-century industrial design. The 11-megohm input and the 1.5-volt bottom range mean it still does things a cheap modern DMM won't, particularly around tube audio gear where the high input impedance matters. The single-probe workflow is handy. And since these were kit-built, every surviving example was put together by somebody with a soldering iron.

Now the caveats. This is a tube meter, and tube meters drift. The 12AU7 and 6AL5 are common and cheap, but any IM-11 that hasn't been powered up in decades will need its electrolytics looked at before it's trusted. Calibration against a known reference is not optional if you intend to use it for real work. The 1.5-volt "C" cell for the ohms function is easy to forget until resistance readings come out nonsense. And the single probe is also the part most likely to be missing or damaged on a used unit, since the switch and hook are mechanical and the cable takes a beating. Buy one with the probe intact, or budget for a replacement.

In the Heathkit catalog the IM-11 isn't the flagship receiver, the tube amp or the scope. It's the instrument that sat on the bench while those got built, and the one that told you whether the thing you just soldered together was working. Heath kept calling it "the world's most popular VTVM" for years.