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tubes, killawatt, current
tubes, killawatt, current
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Old 31st December 2017, 05:51 PM   #1
mikr is offline mikr
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Default tubes, killawatt, current

I was testing a kill-a-watt device to see how accurate vs. my meter. I had the killawatt plugged into my variac. And then I had a line splitter plugged into the kilowatt so I can monitor with my clamp meter at the same time.

When powering tube amplfiers there is difference between current readings between the killawatt and my clamp meter. However, when powering a light bulb or a heatgun, the current measurements are the same. also when powering a solid state amplifier the current measurements are the same between both devices.

What is it about the tube amps that is causing the different readings?
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Old 31st December 2017, 06:03 PM   #2
Mooly is offline Mooly  United Kingdom
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tubes, killawatt, current
Do a web search for 'Real vs Apparent Power'. Its a complex subject but the quick answer is that the power meter 'may' not accurately record 'real power' or it may be upset by harmonics caused by the load (such as a transformer saturating).


True, Reactive, and Apparent Power | Power Factor | Electronics Textbook
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Old 1st January 2018, 04:49 AM   #3
PRR is online now PRR  United States
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Aside from reactive "apparent power", most DC power supplies work by taking huge spikes of current only 10% of the time. This will read different with an averaging meter, a peak meter, or an RMS meter.

Which is "right" depends on why you want to know. If you wish to predict your Electric Bill, again you must ask how the power company meters your house. I suspect my mechanical meter (with digital data-link) averages-out sub-cycle spikes. Newer meters may read sub-cycle spikes several different ways.

If you simply wish to compare the clamp-meter to the KillAWatt (remember the saying about a man with two watches), and do not want to think about reactance or wave-forms, "simple resistors" should give the same number on both, no matter what they read. This will be incandescent lamps, heaters, toasters, etc. (Well, actually I have a heater which turns-down by dropping part-cycles; run such a thing on max heat.)
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Old 1st January 2018, 08:31 AM   #4
6A3sUMMER is offline 6A3sUMMER  United States
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If your clamp meter uses a traditional meter movement with a needle, then it almost certainly is an average-responding unit. They typically use a resistor, rectifier, and d'Arsonval movement meter.
Those current meters are calibrated using a sine wave to read out in RMS current, but it will only do so for pure sine wave current. Any other wave shape will give an incorrect reading (will not read out the correct RMS value).

As PRR stated, DC power supplies do not present a linear load to the power sine wave voltage. It distorts the sine wave. That makes the average-responding clamp meter read incorrectly.

My Kilowatt meter reads out RMS current and RMS volts. And it reads out true Watts and also reads out Volt*Amps (VA). When the phase of voltage and current is 0 degrees, then Watts = VA.
But when the phase of voltage and current is not 0 degrees, then Watts is less than VA.
W < VA.
The Kilowatt meter also reads out Power Factor (0 - 1.0). My tube amps have power factors from about 0.65 to 0.9).
For example, if the VA is 100, and the Watts is 90, then the power factor reads 0.9.
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Old 1st January 2018, 01:57 PM   #5
mikr is offline mikr
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Thanks for pointing me in the right direction....

do the readings on schematics assume true rms?

I think my clamp meter is true rms (ideal 400). So the killawatt (p3) is certainly just a rms device for current? I read conflicting information about this.

Last edited by mikr; 1st January 2018 at 02:19 PM.
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Old 1st January 2018, 02:51 PM   #6
pcan is offline pcan  Italy
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tubes, killawatt, current
From the Ideal 400AAC instruction manual, the model number 61-732 is average sensing, while only the model number 61-736 is true RMS. According to my experience, most low cost multimeters aren't RMS or have a severe bandwith limit and give wrong AC values on audio measurements. I am about to upgrade my multimeter to a real one because I am sick of the guesswork. I once had a very old analog bench multimeter and it was better on AC than the cheap brand new digital multimeter I am using now.
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Old 1st January 2018, 03:23 PM   #7
Elvee is offline Elvee  Belgium
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Originally Posted by mikr View Post
Thanks for pointing me in the right direction....

do the readings on schematics assume true rms?

I think my clamp meter is true rms (ideal 400). So the killawatt (p3) is certainly just a rms device for current? I read conflicting information about this.
Don't assume that the power measured by the product of rms voltage and rms current will be active (average) power like the one measured by your killawatt or a normal domestic meter: imagine the load is a pure capacitor, drawing 1A.
The clamp-meter will measure 1Arms, the voltmeter 120Vrms, but the active power will be zero, not 120W.
True power meters make a real-time vector product of I and V and average the result.
There is no way to achieve such a result with separate, independent instruments.
It is also the reason why overunity fans can claim positive (though false) results....
. .Circlophone your life !!!! . .
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Old 1st January 2018, 04:57 PM   #8
mikr is offline mikr
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My clamp meter is NOT true RMS and the Kill-a-watt is.

So when tinkering with tube amps and the non linear load from their dc power supply, I should probably invest in a true RMS clamp meter. Should I be concerned that my voltages in my amps are way off? With the current for example, the difference didnt appear subtle, one amp was .84 vs .99 and another was like 1.10 vs. 1.50.... or maybe that +/- is subtle!

With tube amplfiers, when is it essential to use a true rms meter?

Thanks again, lots to learn.....

Last edited by mikr; 1st January 2018 at 05:22 PM. Reason: spelling
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Old 1st January 2018, 05:35 PM   #9
zigzagflux is offline zigzagflux  United States
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Residential customers in the US have been traditionally charged for fundamental 60Hz watts, with harmonics being ignored. The old time electromechanical watthour stators were responsive to the fundamental. Electronic meters therefore extracted the fundamental 60 Hz sine wave through any number of possible analog and digital filters. Similar argument applies to reactive power, though normally only primary rate customers are penalized (or credited) for reactive conditions.

Point being, rms current is not the best way to arrive at watt consumption if the concern is $$.

If you consider the rationale for paying for power, fundamental makes perfect sense. Utility generators are strict 60 Hz positive sequence devices. They are only able to supply fundamental watts, so it is this component that the utility should be able to get reimbursed for. Harmonic quantities do not get sourced from the generator; they are absorbed by it. Harmonics are generated by the load and forced back into the positive sequence system as an undesirable circulating current which heats the supply system.
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Old 1st January 2018, 07:09 PM   #10
6A3sUMMER is offline 6A3sUMMER  United States
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Actually, the power company can be 'at fault' too. They use transformers (wow!). And some of them are pushed to the limit of saturation (not just because of customer's electronic loads that put large transient currents at and near the crest of the sine wave).

They are supposed to deliver power with 3% or less THD (the 3rd harmonic at 180Hz is the major harmonic energy contributor).
My friend has power line distortion that is sometimes at 5% or 6%.
It starts to look like it is filling in (headed just a little way toward a square wave).
The RMS voltage can be correct, but the peak voltage is low.
With the peaks of the sine wave quite badly squashed down then the Rectified DC B+ is low (with either capacitor input filters, or choke input filters).

At the initial testing of my amps I use the kilowatt to measure rms V and rms A.
When I do my schematics I measure my B+ that I get with the AC power I get from the power company.
When I check for what fuse value is best, I use the amp's operating rms A to calculate what value of slow blow fuse I use.
But I also measure the worst case inrush current at multiple cold turn-ons to set the value of the fast blow fuse I use. I can measure with a sense resistor in the neutral line, differential probing, and a scope.
Caution: you can not use a single ended probe and neutral sense resistor to measure this (Neutral is only at ground potential (0 Volts) all the way back at the power mains input at the building power delivery.
I use fast blow and slow blow fuses in series to get the best amplifier fault protection (i.e. 0.5A slow and 1.0A fast).

Last edited by 6A3sUMMER; 1st January 2018 at 07:22 PM.
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