[Amps] Future of RF AMPS

[Manfred Mornhinweg]

Dick,

> I have to believe that M2 knows what they are doing with these Amps. They
> offer a 12 month limited warranty on them.

I really hope so! I know nobody who owns one of those amps and uses it 
intensively, so I frankly have no idea whether or not they last. But if 
they do last, I would like to understand how and why! It is totally 
possible and even likely that the difference between my highly 
simplified math, and a really accurate calculation, would explain it, 
and show those amps to run the FETs within specs. But I have certainly 
seen many devices, specially power supplies and linear amps, built buy 
hams, that suffer failures because of totally wrong thermal design. 
That's why I wanted to bring this issue to the attention to those 
wanting to milk any high power transistors to their maximum ratings.

And then, it is well known that manufacturers are not always right in 
everything they do, even when they give warranty! Just think about all 
those many known cases of typical failures, that end up affecting most 
or all radios of some specific model. Some examples: The Kenwood TS-440 
with its glue that degrades and makes the VCOs fail; the TM-241 with its 
display going crazy because of a tiny interboard connector that develops 
excessive contact resistance; the old Yaesy FT-207 in which the 
10.245Mhz crystal cracked from excessive drive; The FT-ONE, which had a 
crystal that shifted several tens of kilohertz over twenty years; The 
Icom IC-746 series with its several thermal problems, such as with the 
pre-driver transistor, the wideband amplifier IC, and the transistor 
that regulates the display brighness - all these coming from incorrect 
thermal handling of SMDs. Do you want more? The rotting DDS chips in the 
TS-450 and TS-850, the many problems of connectors directly soldered to 
circuit boards, in many radios of most brands, so that those solder 
joints break, assorted trouble with breaking solder joints on PCBs when 
too heavy parts are supported solely from the solder joints, and so on.

Even the most professional designers at the best known companies are 
only humans, and do goof at times! So do the providers of components and 
materials.

The single most intriguing issue of misdesign, that I would like to 
fully understand, is that matter of incorrectly understood and applied 
transformers in push pull amplifiers. I have explained this in full on 
this page of my site:

http://ludens.cl/Electron/mosfetamps/amps.html

I mean the section entitled "Output architectures of conventional class 
AB push-pull amplifiers".

This is a typical case of "it's not possible that all manufacturers are 
wrong and just XQ6FOD is right". It does feel weird to me to stand up 
against all those that make their money building and selling 
transceivers containing that bug! Nevertheless I simply don't see how it 
  could be right their way. I first discovered this problem in homebuilt 
equipment 30 years ago, when I copied circuits from ARRL literature, and 
have later confirmed in in actual factory made radios from the big 
brands, and also by simulation. Recently a German ham contacted me, 
reporting that he built two test amplifiers (100W, 13.8V), which were 
identical except for the issue I describe, and that he measured a modest 
but noticeable (3dB) improvement in gain, a significant improvement in 
efficiency, and a big improvement in IMD performance, when using the 
layout I consider correct, compared to the one most present-day HF 
radios use.

Go figure.

I would really love to see either a statement by some RF guru, declaring 
that I'm right and they are wrong, and hopefully giving an explanation 
of how this mistake became so widespread, or else someone (whether he is 
a guru or not) who can explain why the method used by manufacturers in 
some radios and also propagated by Granberg et al in some papers and 
copied by the ARRL (center-tapping a single-turn winding that has
independent magnetic paths for each half winding) is correct!

Manfred

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