Philips GM6004 Voltmeter (1951)

Philips called the GM6004 a Diode Voltmeter. It is a valve voltmeter with a signal rectifier built in the probe.

This VTVM uses a bridge circuit with two EF40 pentodes. These are used as a cathode follower. The use of pentodes should result in a better linearity than with a VTVM using triodes, such as the Heathkit V-7A. A bridge circuit such as this had the advantage of compensating temperature drift. For AC voltage measurements, a probe with a built-in rectifier is used. There is a second rectifier in the VTVM itself for temperature compensation.

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 in the VTVM.
• EA50 in the probe as signal rectifier.
• EZ40 as rectifier.

I bought this meter in 2008. It was in reasonable condition. The case had been painted over creamy white by a previous owner. The GM6004 model was released in 1949 and originally used EF6 pentodes. The /02 version dates from 1951 and uses more modern EF40 Rimlock pentodes. This specimen seems to be from around that time. There is something like a date code on one of the potmeters that hints at 1950. Someone has scribbled a note on the chassis that says the instrument was ok (callibrated?) in 1953. Looking inside, I saw the chassis was looking well, but the power electrolytic had been leaking.

A month later I acquired a second, better-looking GM6004/02 specimen and decided to trade this one.