Marconi Instruments TF2606 Differential Voltmeter

  • Power: selectable 115VAC or 240VAC, 50-60Hz
  •  Range: 0.001 to 1100 volts DC
  •  Valve lineup (none - transistorised)
  •  Released: mid.1960's
Marconi Instruments make a good deal of very high grade test equipment for the defense industry. As this equipment is replaced after about 20 years it is disposed of, usually cheaply at surplus auctions.

This is how I came into posession of this
particular unit. It dates from the mid-1960's (I think) and is ex-Ministry Of Defence (see the arrow sticker lower right corner dated June 1970). It is a transistorised unit driven by a selectable 240V or 115V mains supply and has a working range of 0.001V to 1100V DC. It has three sensitivity ranges of 2, 3 or 4 decimal places - the decimal point being indicated by a neon lamp according to the 'Max Input' control position.

These units are the predecessors of the now ubiquitous DMM - multi-meters with digital readouts accurate to 3 or 4 digits - and are, in many ways, still vastly superior.

But what is a 'differential voltmeter' and how does it differ from a 'regular' voltmeter? It is really a precise voltage generator coupled to a very sensitive ammeter and is used to measure DC voltages. It is connected to the circuit with the + of the generator connected to the + of the circuit (and - to -). The voltage of the generator is increased until zero current flows. The voltage is then read off the knobs of the generator. It's big advantage is that it has a virtually infinite input impedance.

Randy Guttery gives a more detailed explanation from the archives of the Usenet group

"Most voltmeters work by 'sampling' the voltage to be measured. The sampling cirucit is usually some kind of voltage divider - with either a meter, or an amplifier driving a meter. Depending on the circuit, the 'load' placed on the source of the voltage to be measured can vary greatly - and is usually expressed in ohms per volt.

Common multimeters range from cheap ones, at 1000 ohms per volt, to pretty good ones at 20,000 ohms per volt. Better meters (such as older vacuum-tube meters or the newer field-effect transistor types) may have be as high as 10Megohm per volt. But they all present some "load" to the source. One point further - the "ohms per volt refers to the "full scale voltage" - so a good multi-meter set to the 250V range will present a (250*20,000=) 5Megohms load. The same meter on 0.5 Volt range will present a 10,000 load. In most circuits -- this isn't a real problem. In a grid circuit sourced off a 200K bias resistor - this load would cause the voltage reading to be wrong - very wrong.

Enter the differential voltmeter. The differential works differently. Instead of measuring the voltage directly, it uses a form of balanced bridge so that it measures the difference (differential - get it?) between a known voltage and the voltage you are measuring. If there is no difference, then there is no current drawn from the source (i.e. infinite impedence). So all of those knobs you see form a Kelvin-Varley divider (voltage source) which you set until the meter reads null - then the input voltage is equal to (and read from) the dials. Obviously, you could use this meter for any voltage measuring purpose, but mostly where you need to ensure that the meter is not loading the source circuit in any way."

Last updated 2nd June 2008