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Old 29th December 2010, 08:41 AM   #1
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Default Measuring milliamp with Fluke 771 Clamp Meter Question?

I am considering the Fluke 771 Milliamp Clamp Meter to measure the plate current of tubes. It seems like the meter would be very convenient considering you don't have to open the circuit. I need to be able to measure in milliamp range. I currently own a Fluke 87V.

The problem is: this meter only measures up to 100 mA.
I would like to get a meter that can measure mA and up to 5A.

Do you guys know of any other meter that can do this or any other solution.
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Old 29th December 2010, 12:17 PM   #2
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At up to 5A you can use something like a LEM LA25NP with pretty good accuracy and a bandwidth well beyond audio - so good for isolated connection to an oscilloscope, or to a simple meter. The 771 looks great for rapid idle DC current level, but simple datasheet doesn't indicate AC performance, and no good for scope interface. Can't say I've looked at what's commercially available nowadays, but TEK used to have a great 20A wand for oscilloscope use.
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Old 29th December 2010, 05:34 PM   #3
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Thank for the reply.

I looked up this unit and I don't know how it works.

Do you have to connect the unit to the circuit that you are measuring?

Where is the display?
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Old 29th December 2010, 07:24 PM   #4
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I just learned that the Fluke 771 will not work in this application. The voltage rating of the clamp is only 300V. Most plate voltages are much higher than that.

Does anyone know how I can measure the plate current? Preferably without opening the circuit.
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Old 29th December 2010, 09:14 PM   #5
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If you put the Fluke clamp over a section of insulated wire then you should achieve a suitable insulation level (the wire insulation would need to be rated suitably). But for general use on any amp you may have a problem, although their is insulation on the wires going to many output transformers.

Sorry about the LEM suggestion - which requires you to provide a +/-15V supply and a load resistance (the current signal is measured across the load resistance as a voltage). The measured anode circuit is connected to the LEM input windings (which are isolated from the load resistor side), and the LEM windings need to be set up for the 5A current range - so it is not a 'clamp' type device and requires the opening of the circuit.

Ciao, Tim
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Old 30th December 2010, 02:08 AM   #6
tomchr is offline tomchr  United States
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One solution, that does require breaking the circuit, is to install a 10 ohm resistor in series with the OPT primary. Then measure the voltage across the resistor. Even a $5 meter can measure voltage with enough accuracy to get the tube bias dialed in. 10 mV across the resistor will equal 1 mA of plate current.

You can leave the resistor in the circuit after you're done. That way you only have to break the circuit once.

~Tom
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Old 30th December 2010, 02:39 AM   #7
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Thank you for the sugestion.
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Old 30th December 2010, 03:00 PM   #8
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Default alternative measurement technique

For general purpose current measurement without breaking the ciruit you will need to follow one of the previous suggestions. But if what you want is specifically the DC, no signal, plate current of an output tube loaded by a transformer, then you may want to try the following technique I learned from an old radio repair manual.

Just use an ordinary current meter connect across the transformer primary. One lead to the B+ terminal of the transformer, the other to the plate connection. Since the meter resistance is milliohms and the transformer winding resistance is tens or hundreds of ohms, the meter is shorting out the transformer and drawing all the plate current. Yes, the total resistance in the plate circuit is now lower, but the net effect on the current is quite small.

Obviously this is no good for ac measurements, but it is perfectly good for setting bias etc.

As already pointed out by someone, be aware of the meter voltage rating. Also be careful when poking around the B+ with meter probes and alligator clips etc in what may be cramped area of the chassis. You may want to set it all up with the power off first.
The safest way is with permanent test points on the outside of the chassis, assuming it is a diy amp, or one you can mod.
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Last edited by Robert McLean; 30th December 2010 at 03:02 PM. Reason: spelling
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Old 30th December 2010, 06:28 PM   #9
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Thank you.

Yeah I want to avoid poking the B+. My 845 amp has 1098VDC. Permanent test points and bias adjustment on the outside of the amp is a good idea.
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Old 31st December 2010, 05:51 AM   #10
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Are you happy to go for cathode bias current level (voltage across a sense resistor) as a ground level external test point?
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