This is the second one. I returned the first one for the same reason.

On 20m 2000w setting in PEP mode the reflected power needle oscilates and therefore the swr oscilates. The needle pulses. It also does it at 40m but not as much. It varies 0.3 swr pulsing on a real antenna and on a dummy load.

I bought it to tune my high power Heathkit SA-2040 tuner at 10 watts so I dont tune it at high power. It eorks fine for that

With the same antenna on 20m I get 3 different swr readings on 3 different scales. 1.2 on the 20w scale, 1.3 on the 200w scale and 1.4 on the 2000w scale

When my amp meter  reads 600 watts the MFJ reads 900 watts. When my amp reads 800 watts steadt cw tone the ZmFJ reads 1200 watts

The first meter was high by 200  watts

I feel like getting a refund but need a meter.

Is this garbage reading to be expected for a $110 power / SWR meter?

The reflected needle does not pulse on the average setting but the meter still reads 300 watts high compared to my amp meter which I think reads high when I multiply the plate voltage by the plate current.

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Should I just accept a crappy meter because that is what you get unless I spend 4 to 5 times as much?

What should I do?

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Is it possible the power meter on the amp is reading 300 watts low? Or maybe 150 low and the mfj 150 high.

If I multiply the plate voltage by the plate current does that give me the power output or do I have to multiply that by the efficiency of the amp?

Does anybody know if this meter can be adjusted

"M"ighty "F"ine "J"unk

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This is what I have, and compared to the MJF it's simply amazing.

Yes the meters can be calibrated to a reference source, but usually the specs are plus or minus 10% or worse.
The MFJ is a not laboratory grade power meter and this is usually what you get as far as accuracy and quality in that price range.

The analog Diawa watt meters I have owned in the past were no better either. I ended up spending more money to get something that was accurate and a better build.

In a word, yes. Multiplying the plate voltage by plate current gives you the total power consumed (minus filament current) by the amp

not how much RF is being produced. This is why "power input" was the standard many many moons ago. Most hams

had a volt meter and a current meter, but no watt meter. They could calculate power input by multiplying the two readings.

Nowadays "power output" is the standard because watt meters are cheap and plentiful.

Possibly, but what are you going to use as a standard? You could borrow a Bird or similar meter

to calibrate the MFJ, but then the MFJ may not read properly through the entire range.

The MFJ meters aren't a lab grade instrument as you know, they are designed to be an inexpensive

reference and nothing more.

To get an accurate wattmeter, you have to step up to a Bird or other "lab grade" equipment.

Confucius  says a man with one watch always knows what time it is, a man with 2 is never really sure

I would probably just adjust it to what the amp power meter says

Having the amp say 700 and the mfj say 1000 is annoying.

Over the years, I bought a number of power meters similar to the MFJ meters in the original post.
When I realized that the meter wasn't accurate, I attempted to calibrate it.
And as was posted, it was somewhat accurate at the frequency and power that I used to calibrate it and not accurate at all at other frequencies or power levels. I suspect that when MFJ (or whomever) calibrated the meter, they probably calibrated it to be as accurate as possible across the board (which isn't very accurate at all) and I should have just left it alone.

FWIW: the meters in your amplifier probably arn't any more accurate.

However, you can go down a rabbit hole with this whole meter accuracy thing. How accurate is accurate enough ? If you calibrate a meter, how do you know the standard you used to calibrate it is accurate ?  Then if you have more than one meter inline, which one is accurate ? ........................................ Hell, if it is an analog meter, where your head and eyes are in relation to the needle changes the readings which is why good analog VOM meters have a mirror on the scale behind the needle, so you can know you are looking at the needle from straight on.

So, if you really need to know how much power you are putting out, you need to buy an accurate, calibrated meter. Only you can decide how accurate it needs to be. How much error you are willing to deal with and accept. And at that point, for general ham radio purposes (not some lab test equipment) you need to then say that, that meter is the one you are going to base everything off of ignoring any other meter. If you increase your power and that meter says it's 200 watts, then that is the number you are going to use for whatever purpose you need it for.

At one time, I got into operating at very low power levels on HF. Sub-1 watt power levels. And I felt guilty telling someone that I worked them running 500 milliwatts because I didn't really know if I was running 500 milliwatts. It was one thing for me to accept the numbers my meter was giving me, but I didn't feel comfortable telling anyone else. But, I just ended up accepting the numbers my meter gave me and if nothing else, it gave me a relative power level to compare other power levels for my own personal use.

So other than spending $800 for a lab grade power meter, is there anyway to tell how much power I am putting up?

My 120v sag, sags from 2900V to 2300V plate voltage on 20m phone band with 450ma plate current the power meter on the amp shows about 725
watts.

2300 x .450 = 1025 watts, plus 70 input = 1095 watts at the plate.

Ameritron says it is 70% efficient.

so .70 x 1095 = 766 watts output?

Would this be close or is 70% efficient for an AL-80B a pipe dream under unrealistic conditions?

Surprisingly it matches the amp power meter reading very closely and essentially the 800 watts on CW that the company says is the max CW output.

Wow

Been reading about this meter.

I think one is in my future , wish I had seen this  before I bought the MFJ