Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: jjasilli on January 26, 2012, 10:28:31 pm
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I just acquired this cool vintage Weston AC ammeter: http://www.ebay.com/itm/270894570907?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649#ht_1405wt_952 (http://www.ebay.com/itm/270894570907?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649#ht_1405wt_952) I want to use it to monitor AC wall current draw of an amp under test.
But I'm confused. All that's visible is a meter movement in a housing with connections on top. Is the shunt internal to the meter movement; or is an external shunt resistor required? Thanks!
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I'm pretty sure that meter is ready for use just as it is.
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Hay-wire it with a 7-watt incandescent night-light. Just tap-touch to the meter. Don't get killed!
If it PINS, it is missing a shunt.
If the meter shows 0.06 (hardly none at all), then it is already set up for 5A/10A.
The way it is packaged, I'd be 99% sure it has two shunts (or one tapped) inside the case on the hidden end of the terminals.
Or that it is one of the other types. My lawnmower's amp-meter is a compass needle on a spring, and the battery wire is clamped to the outside of the case. Yours is different but might be that simple inside. There's also thermal meters though these usually have other clues.
I _may_ have some info about that specific Weston movement in a book I just got; but too late to hunt for it.
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It is ElectroDynamic.
A coil against a permanent magnet reads DC. Give it AC and it swings both ways, vibrating around zero and tears itself up. Most modern small "AC" meters are DC meters with a rectifier, but the rectifier has issues.
Electrodynamic has a moving coil and a fixed coil both in the same circuit. It is thus insensitive to polarity. However it responds as the square of the current. If 100% is 10A, 50% is 7A, 10% is 3.2A, and smaller currents are lost. But Weston was a master of bent poles. The internal geometry weakens the magnetic interaction as the needle moves up. They could get most of the scale semi-linear, but the bottom drops right off. There's no marks below 10% of full scale.
Well bought.
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Finally got around to testing it. Works like a charm, and as predicted! Actually a little fluorescent night lite wouldn't move the meter. So I hooked it up to my light bulb limiter & Bingo! Got a reading which seems accurate. Very happy. Thanks, guys for the insights.
Meanwhile, a vintage Triplet AC volt meter arrived, which also works. It pretty much matches the ammeter in appearance.
My plan has evolved to not only run a variac > an enclosure to be built > amp under test. The enclosure will house the AC volt meter & ammeter. This will enable me to run a known, constant 120VAC into each amp under test, and monitor current draw. The enclosure will also house a Jack Darr-style live chassis tester: a power resistor is temporally placed between the chassis and earth ground, and any voltage drop measured with a DVM across the resistor. If there is voltage across the resistor, the chassis is live. Finally, I already have a light bulb limiter, and haven't decided if I should rebuild one into the new enclosure, for the sake of simplicity.