Analog meters

[JW34]

Is there a way to convert a analog DC Voltmeter to a DC Millivolt meter?

Example. I want to have two seperate meters measuring the bias on my output tubes. I am using the Poseidon board so the VDC per output should be 700mv if I am correct. If I'm not right, please correct me.

Anyway, is this possible, and if so, how?

[TomMcNally]

The "standard" meter movement used in most equipment
in the last 70 years had been a "0 to 1 ma" movement.
You can use it to measure current with shunt of the
appropriate value across it, in series with the load.
You can measure voltage with it by putting one side
to ground with an appropriate series resistor across
the voltage you want to read.

My 300B amp has one with a rotary switch to measure
all of the voltages and currents of interest. The voltage
scale is 750 volts ... so to read the high volatge, a 750 K
resistor in series with the 1 ma movement reads from
0 to 750 volts ... it's easy to figure out.

What type of meters do you have available ? Keep in
mind (this is important) that what is on the scale isn't
necessarily what the meter will read. Look down into
the face and there may be fine print like "0 to 1 ma movement"
or "0 to 100 microamp movement" etc. These meters
require and external series or shunt resistor, or they will
go up in smoke in milliseconds.

... tom

[JW34]

I don't have any meters at the moment. I am trying to source them out.
Having no luck though.

[msmpe]

OK Tom, I'll bite

I have a Weston #31243. The fine print says "FS=100UA" and "Model 301". I presume that means 0-100 micro-amps. The scale reads 0-600 and 0-120 and is labeled "microamperes D.C.".

If I want to read plate current (Ip) up to 100mA, what is the shunt value?

What if I wanted to determine transconductance (Gm=Ip/Eg) of a valve given some signal voltage, anything extra?

What is the difference reading AC versus DC?

[TomMcNally]

I like to measure voltage across an existing resistor (like a cathode
resistor in an amplifier) rather than having to deal with switching to
get the meter in series on a couple of tubes. You would want to use
the 0-120 scale on the meter, since it is existing, so you would need
a series resistor that would make it read correctly. Probably about
a meg would be a good place to start, then fine tune it.

Here is a good link (PDF will open) on how to do all that:

http://www.jaycar.com.au/images_uploaded/shuntmul.pdf

[msmpe]

Hi Tom,

Thanks, I'll check it out.

[mesherm]

http://cgi.ebay.com/NEW-DC-0-1V-Analog-Votage-Panel-Meter_W0QQitemZ230363212617QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item35a2b7ab49&_trksid=p4634.c0.m14.l1262

I have used these 0 to 1 volt DC meters on many of my amps and they have worked great. This guy is the only one I have found that has them.