Eico 625

A line-powered vacuum tube voltmeter whose mirrored meter face still makes tube amp restorers go weak in the knees.

Overview

There's a reason the Eico 625 keeps showing up on workbenches sixty-plus years after it was sold as a kit. It's the VTVM that tube gear people actually reach for, and it has been since the 1950s. Eico — the Electronic Instrument Co., Inc. — sold this as a build-it-yourself project throughout the 1950s and 1960s, with some factory-wired units also available for those who didn't trust their soldering iron. Hi-Fi Music at Home (January 1959, p.66) listed a midrange horn H-625 with P-30 driver for sale, showing the model number's currency in the period. It became one of Eico's most popular test instruments during the golden age of tube gear, and for good reason: it's accurate, it's repairable, and that meter face is flat-out gorgeous.

The heart of the 625 is a balanced bridge circuit built around two 12AU7 twin-triode tubes. That's a classic arrangement for a VTVM — the bridge nulls out drift and gives you stable readings that a moving-coil multimeter just can't match. The unit is line-powered, meaning it plugs into the wall and runs entirely off the AC supply; there are no batteries to leak or corrode inside that steel cabinet. It draws 20 Watts at 105–125 Volts, 50/60 cycles, which is modest enough that you can leave it on all day during a long restoration session without worrying about heat.

What separates the 625 from the pack is its front panel. The meter is a 4 1/2 inch, 200 microampere movement with a 4 inch mirrored scale — the mirror sits behind the needle so you can line up your eye and eliminate parallax error when reading values. That mirrored scale was a premium feature in its day, and it's still the thing that makes collectors smile when they pull one of these out of a box. The front panel itself is etched aluminum, and the whole thing sits in a steel cabinet with a sloping front face that puts the meter at a comfortable viewing angle on the bench.

Owners report that the 625 is often recommended as a first vintage VTVM, and that's not just nostalgia talking. They're common enough on the used market that you can find one without a long hunt, and the restoration path is well-trodden. The circuit is simple, the tubes are still made, and the documentation from the kit era is thorough. It's about as close to a no-surprises vintage test instrument as you'll find.

Key Features

The Probe-Mounted Function Switch

Here's the feature that makes the 625 genuinely unusual for its era: the function switch lives at the probe tip, not on the main panel. Instead of walking back to the unit to flip from DC to AC to Ohms, you reach for the probe and twist the switch right where you're measuring. For anyone who's spent hours probing around a chassis, that's a revelation. The downside, and every owner eventually discovers this, is that the internal wafer switch in that probe is fragile. It's the single most commonly broken part on the 625, and the wafer itself has become effectively unobtainium. Many owners end up rebuilding the probe with a modern switch rather than hunting for an original part.

Balanced Bridge Circuit with 12AU7s

The two 12AU7 tubes in the 625 form a balanced bridge that gives the meter its stability. The 12AU7 was a common dual triode in audio gear of the period, which means tubes are still plentiful and inexpensive today. Owners note that the 12AU7s in a well-kept unit are usually still good — they're not run hard in this circuit. The power supply uses a 6X4 rectifier tube, and that one is more likely to fail after decades of service. Replacement is cheap and easy; the 6X4 is a common type still manufactured today. Sources differ on the exact tube count — some describe two 12AU7s total, others list a single 12AU7 as the bridge amplifier with the 6X4 separate — but either way, the tube complement is simple and entirely serviceable with modern parts.

Mirrored Meter Movement

The 4 1/2 inch, 200 microampere movement with its 4 inch mirrored scale is the aesthetic centerpiece of the 625. That mirror isn't decoration; it's a functional tool that lets you read values accurately by aligning the needle with its reflection. For a kit instrument, this was a step up. The blog documentation for this model explicitly notes that Eico made several VTVMs and that the 625 was the top-of-the-line offering, with more ranges and that mirrored scale compared to the lower-cost 232. It's the kind of detail that tells you Eico knew exactly what they were doing with this one.

Point-to-Point Wiring, Later PCB

Early production 625s were wired point-to-point, the traditional method where components are connected directly between terminal strips. Later units moved to a printed circuit board. There's no documented serial number cutoff for when the change happened, so you can't tell from the numbers alone which version you're getting. Both are serviceable, though the point-to-point units are arguably easier to work on if you're comfortable with a soldering iron and a schematic. The assembly instructions describe a calibration procedure using internal adjustment pots for AC, DC, and Ohms, so you can bring a tired unit back to spec with just a known voltage source and a bit of patience.

Historical Context

Eico sold the 625 as a kit during the 1950s and 1960s, alongside factory-wired units for buyers who wanted the instrument without the assembly project. It carved out a place as one of Eico's most popular test instruments during the tube era, and the dedicated fan sites that still exist for this model are a testament to its staying power. The 625 sat at the top of Eico's VTVM line, offering more ranges and a better meter than the company's budget offerings. The kit culture of the period — Heathkit, Knight-kit, and Eico all competed for the same hobbyist dollars — meant that a well-built 625 was as much a source of pride as a piece of test equipment.

Collectibility & Value

The 625 is well-regarded among vintage test equipment collectors and tube amp hobbyists, and the reasons are practical as much as sentimental. It's accurate, it's repairable, and that mirror-faced meter is genuinely beautiful. The used market has plenty of them, which keeps prices reasonable — though the blog documentation notes that eBay prices vary widely without pinning down specific figures.

If you're buying one, know the failure points before you commit. The probe switch is the most commonly broken part; check that the wafer switch at the probe tip actually clicks through all positions before you hand over money. The 1-megohm input resistor in the probe is often out of tolerance by now and should be replaced to maintain accuracy. The power supply electrolytic capacitors are the most likely failure point in a unit this age, so plan to reform them slowly with a variac before applying full line voltage — or just replace them outright. Calibration drift is rare, but the meter movement can be damaged if the unit is dropped, so examine the meter face for signs of impact.

Parts availability is genuinely good for the electronics: the 12AU7 and 6X4 are common tube types still made today, and the rest of the passive components are standard values. The probe is the hard part. The switch wafer is unobtainium, and many owners end up building a new probe with a modern switch rather than trying to source an original. Cleaning the range switch contacts with contact cleaner is part of routine maintenance, as that switch gets the most use. With a little work, a 625 that's been sitting in a basement for decades can be brought back to full accuracy — and that's exactly why these keep finding new homes.