[AMRadio] power ratings


Donald Chester k4kyv at hotmail.com
Wed Jul 13 12:39:09 EDT 2005


Gary K4FMX said:

>The best quality audio of all can be gotten from low level modulation and a 
>linear amplifier.

A linear amplifier has the same kind of distortion as a class-B modulator.

With tubes, the best quality audio can be had from low distortion plate 
modulators such as class-A series or Heising modulation, or pushpull plate 
modulators running class A or AB1.

Pulse-width series modulators produce perhaps the best audio.

I suspect the best quality of all comes from the new class-E rigs.

According to the tube manuals, class-B audio service has inherent distortion 
levels on the order of 3-5%.  It can be reduced with negative feedback.  My 
Gates BC1-T manual claims less than 2% distortion at 100% modulation.

The signal driving a linear amplifier has its own distortion, since the 
original signal has to be produced somehow.  Pushpull class-A audio or 
series modulation, with feedback, might be a good candidate for the driver 
stage of a linear.  If the linear is run properly in class AB1, that would 
be near the best possible audio out of a tube transmitter, even though the 
efficiency is not all that good.

Speaking of efficiency, an AM linear or grid modulated amplifier has close 
to the same overall efficiency as plate modulation, when calculated from the 
ratio of power drawn from the a.c. mains, to rf carrier output.  A linear 
amplifier running AM has exactly the same efficiency as when it runs SSB.  
It's just that the duty cycle is different.

Actually, since with the human voice, the average power is 7-8 dB lower than 
peak power (equivalent to around 30% modulation), the average efficiency of 
a SSB linear is similar to that of an AM linear because the efficiency of a 
linear is a function of the amplitude of the signal (0% at idling current, 
and a maximum of about 67% at maximum peak output just below the point of 
saturation or flat-topping).  AM linears got their reputation as "low 
efficiency" on AM because of the 100% duty cycle carrier runs about 30% 
efficiency to allow enough headroom for the positive peaks.  With an AM 
linear, you can see the glow on the plates DECREASE when you whistle into 
the mic to produce 100% tone modulation.  The DC input is the same 
regardless of modulation, but the rf output is higher, since sideband energy 
is now included.  That power has to come from somewhere, so the efficiency 
of the amplifier goes up to generate the sidebands.

The advantage of plate modulation with AM is the ease of tuning up and 
QSY'ing.  You simply dip the final and load to the desired carrier output, 
while maintaining enough grid drive to assure class-C service.  With 
low-level modulation (linear or grid modulated), the rf drive level and 
degree of antenna coupling are critical to the modulation linearity of the 
final.





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