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simpson voltmeters (very beautiful)
Simpson made thousands of different types of meters. Need better clues.
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not sure what range
Look on the lower-right of the dial. Many generic meters, even with custom scales, had the actual meter sensitivity marked. This one is 200μA "F.S." (Full Scale). There actually are no voltmeters, they are all current meters. A voltmeter will be a current meter plus a resistor, which you can sometimes swap-out to get a different volt range, or remove for current measurement.
If unmarked, use a 9V battery and 100K and 10K resistors. Find a resistor that gives a large needle reading: more than 20% but not off the top. Use Mr Ohm's Law to figure the current (assume the meter resistance is negligible, maybe 200 ohms). If 9V/100KΩ = 90μA gives a reading at 45% of full scale, then full scale must be about twice that or 180uA, and really 1/0.45 or 2.2 times higher which is 198uA which is surely 200uA rated.
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is it possible to turn a volmeter into a vu meter?
Any meter can be an audio meter.
But how much audio power do you need to drive it? Jakob (I think) had a 200 AMP meter. If we assume it drops 0.1 Volts at full current (fairly typical), then it needs 0.1V*200A= 20 WATTS of power to move it to full scale. You need a small power amp! And the 0.1V/200A= 0.000,5Ω impedance is awkward. If padded-out to a comfortable 4Ω load, it looks like 160,000 watts! You could adapt a spot-welder transformer to transform the push-pull plates of a 2x6L6 guitar amp to 0.1V 200A, but the frequency response is uncertain, probably not as good as we want.
In practice, we used to drive 100uA meters directly off 600Ω lines with "small" loading and distortion. (All suitable meters are DC, the AC-DC conversion puts an ugly load on the audio which distorts it.) "Small" was ~1dB and 1%THD. We no longer like to see THD numbers that high. So we usually want some form of buffer amp (more parts to DIY). With a dedicated buffer, meters up to 1mA are reasonable (can use 741/TL071 cheap-chips).
But the real obstacle: Dynamics. Any meter will read a steady tone. For eggample that VTVM in the picture is very accurate on steady tone.... IF you wait 5 or 10 seconds. And when the tone stops, it takes even looonger to fall back to zero. The original design has a lot of filtering on the AC ranges; I added even more for doing frequency response tests below 20Hz. If I put it on dynamic music, it hardly budges. If I put it on steady tone, some wind instrument solos, it will rise and fall so slowly that I really don't have a clue what the audio levels are. Peaks are missed, and soft passages usually hidden in the decay from a preceding loud passage.
Most basic meters can't be this bad. In fact most are "too lively": they overshoot and fall back at rates that don't really suit musical audio or eye-response. Most meters are made for steady or slow-change measurements.
SO the short answer is: yes you can make an audio meter with almost anything; but no, you probably won't be able to make music/speech audio judgements from it unless it was made to be a VU or VU-like meter.