Tom,
What do you think about the new MFJ 868 wattmeter that has the extra large
6-1/2" meter movement? How good is their TrueActive peak readong circuit, and
how is the accuracy? Do they use another calibrated meter in line to calibrate
these, or is it done another way? I'm interested in buying one, but I'd like to
know what's under the hood, and the quality of them. I need a large meter now
and this looks like it may fit the bill.
http://www.mfjenterprises.com/products.php?prodid=MFJ-868
Thanks,
Will
*********** REPLY SEPARATOR ***********
On 7/16/06 at 9:43 PM Tom W8JI wrote:
>> My SB 200 produces 700 Watts PEP, with about 50 Watts PEP
>> drive, and my SB
>> 220 produces about 1.5 KW PEP (both amps on 20 meters)
>> with about 80 Watts
>> PEP drive.
>
>I've never seen a pair of 3-500Z's driven with 100 watts
>produce 1500W PEP ***on an accurate power meter*** with only
>3000V on the anode.
>
>Neither did B&W. Neither did Viewstar. That's why they ran
>3500V plus on the anodes. Every manufacturer has fought this
>gain problem.
>It takes about 3400 volts or more to get 1500W out with 100W
>drive.
>
>This isn't to say a pair of 3-500Z at 3000V won't show 1500W
>PEP on some wattmeters. I've just never seen that power on
>accurate meters.
>
>All of my SB220's, even with the grids grounded, only make
>around 1200-1300 with100W drive on a known accuracy meter.
>
>By the way, a typical o-scope is one of the least reliable
>common ways to read power. First, scopes have passband
>ripple. They are designed of good response to a stepped
>waveform, frequency response flatness (or lack of ripple) is
>secondary. Second, they are susceptible to common mode on
>probe leads. Third, they don't store the absolute peaks
>unless you have a storage scope and are lucky enough to
>catch the peak. Fourth, any error they do have is compounded
>by the fact power is a square of the measured voltage, so
>the error is squared. Fifth, they are load resistance
>critical.
>
>RF power meters with peak storage circuits are the best way
>by far.
>
>73 Tom
>
>
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