Philips GM6004 Voltmeter (1951)
The Philips GM6004 is a valve voltmeter.
It has a differential amplifier circuit using two penthodes.
This is probably a /02 version, using Rimlock valves.
Philips called the GM6004 a Diode Voltmeter.
It is a valve voltmeter with built-in signal rectifiers.
Principle of operation: two-pentode bridge.
The design is clever.
This VTVM uses a bridge circuit with two pentodes.
Two EF40 pentodes 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
A bridge circuit such as this had the advantage of compensating temperature
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.
This allows for peak-to-peak AC measurements.
The ranges are:
- 3-10-30-100-300 V DC
- 3-10-30-100-300 V AC
A view from the back.
The valve line up is:
- EF40 (2x) as differential amplifier
- EA50 as signal rectifier
- EA50 in the probe as signal rectifier
- EZ40 as rectifier
I bought this meter in 2008.
It was in reasonable condition but the case had been
painted creamy white.
Originally, it has a hammered metal finish case which dates it in the 1950's.
The original version is from 1949 and used EF6 penthodes.
The /02 version dates from 1951 and has EF40 Rimlock penthodes.
This specimen seems to be from around that time.
There is something like a date code on one of the potmeters that seems
to point 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 looked well, but the power electrolytic
had been leaking.
Need to replace that one, and remove the white paint.
A closer look at the components on top of the chassis.
I will fix it later on