differential voltmeter!

[sergio]

Hi , can anyone advise me an analogic differential voltmeter?
Thanks in advance!
Sergio

[sergio]

nobody!?!

[triode3]

What do you mean? All voltage meters (digital, analog, tube, solid state) are differential... meaning they measure the voltage difference between the positive and negative probes.

e.g. if you have a circuit with -35V on one side of a resistor and +40 on the other, referenced to "ground" and you put the negative prove on the -35V side and the positive probe on the 40V side, you would measure 75V.

Remember, when dealing with electricity, it is always "what it is referenced to", so it is all differential (the difference between this point in the circuit, and that point in the circuit).

Also of note, which people forget a lot, every tube spec sheet always implies that voltages in the tube are with respect to the cathode. If they say a tube can handle 400V on the plate, they mean measured from the plate to the cathode. You can technically put 600V on the plate, as long as the cathode is at 200V (and you do not go over the heater to cathode maximum voltage rating, which is why you see that rating on a lot of spec sheets), and now, just as in the previous example, if you measure the voltage from the plate to the cathode, you will see 400V.

Does this answer your question?

[martin manning]

This is a precision instrument that measures voltage with extremely high input impedance. Why is this needed? A modern DMM has input impedance of 10M or so.

[steve_p]

are you looking for a differential probe to use with your scope?
If so, I would look at a Fluke or Textronix.... Neither are cheap though.

Steve

[sergio]

I seen on Ebay there are some old fluke , but works 120 wall voltage!

[triode3]

This is a precision instrument that measures voltage with extremely high input impedance. Why is this needed? A modern DMM has input impedance of 10M or so.

Ah, you did not say that initially... purchase a used high impedance voltmeter... Trek makes one that is 1x10^16ohms input.

Or.. build your own.

[tubelectron]

Well, you can do it vintage with a 12AU7, à la General Radio... I have some simple but efficient schems if you want it !

A+!

[sergio]

tubelectron , thanks , if you want can send me a PM. I'm curious about your schematics!
Sergio

[tubelectron]

tubelectron , thanks , if you want can send me a PM. I'm curious about your schematics!
Sergio

Here are some VTVM schematics - among hundred of others ! Most of them are all roughly based on the same principle : a differential voltmeter, which gave them their stability, precision and linearity. I think that the 1st one in that field was a "balance voltmeter" due to General Radio...

This one is from a French book (Schémas Pratiques de Radio et d'Electronique - 4ème édition - 1976) and is a very simple to read schematic, but working perfectly, as I recall about the one I have built a long time ago :

[img:800:378]http://img4.hostingpics.net/pics/814150 ... IOVTVM.jpg[/img]

This one is from Heathkit - see the similarities...

[img:800:702]http://img4.hostingpics.net/pics/734014v4HKVTVM.gif[/img]

The famous GELOSO Italian brand has for sure made VTVMs like these... Try some Google search, indeed !

A+!

[tubelectron]

but working perfectly, as I recall about the one I have built a long time ago

This said, Sergio... I wonder now if I will not rebuilt one in a nice-compact-improved version... Just for the fun !

A+!

[Aurora]

I think the OP needs to declare his intention for a differential voltmeter.
A modern handheld DMM is truly differential in the sense that it is run on batteries, and is not connevtoed to anything else.
As soon as one starts fiddling with mains driven meters, where the ground connection is truly a ground, it makes quite a different situation, as on side of the meter input is usually MAINS GROUNDED! This changes the whole game......
Do you truly need a diff. voltmeter, Sergio, or are you looking for a classic high impedance VOM... ?

[sergio]

My goal is to measure the two push-pull signals (after PI) to set the same amplitude! To do this I need an AC millivoltmeter with an analog instrument with central zero! I already used one of these, a friend of mine had homemade!

I'll be to old school, but attached to analog equipment. For me is the best way to this measurement!

[martin manning]

This is easy with a conventional DMM. Measure AC volts from one plate to the other. The two signals are equal when you have a zero reading.

[sergio]

a good DMM is expensive!!