Philips GM6004/02 Voltmeter (1951)

The Philips GM6004/02 is a diode voltmeter, a VTVM (valve voltmeter) with built-in signal rectifiers.

This VTVM uses a bridge circuit with two EF40 pentodes used as cathode follower. A bridge circuit has the advantage of compensating temperature drift. This circuit using two pentodes has more amplification than the ubiquitous VTVM's using a double triode, such as the Heathkit V-7A, but uses more valves. For AC voltage measurements, a probe with a built-in rectifier is used. The GM6004 has a rectifier diode in the probe and one in the VTVM itself.

The ranges are:

• 3-10-30-100-300 V DC
• 3-10-30-100-300 V AC

The valve line-up is:

• EF40 (2x) as differential amplifier
• EA50 as signal rectifier
• EA50 in the probe as signal rectifier
• EZ40 as power rectifier

I found this meter in 2008, just after I bought my first GM6004/02. It was in good condition. The case has a grey wrinkle finish case which dates it in the early 1950's. Looking inside, I saw that all components were looking well. But the probe was missing. I put the instrument on a shelf where it remained for more than a decade.

Somewhere in 2022 I picked up the GM6004 again when I discovered that I had collected two probes for it.

I checked the capacitor and power transformer. These measured ok. I switched on the instrument and connected one of the probes. I set the meter to zero. But no measurement. It turned out that both probes were defective. I had to re-solder the connections of the EA50 rectifier and repair the AC/DC switch. That fixed it. I checked all the ranges and calibrated the instrument. Now it is working again.