Re: [AMRadio] power ratings

[Gary Schafer]

Donald Chester wrote:

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.


That's true except with a linear amp most of the distortion products 
fall outside the audio bandwidth. 2nd and third harmonics etc. are 
outside the audio bandwidth.




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.


Except for the distortion introduced by the modulation transformer.



Pulse-width series modulators produce perhaps the best audio.

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


Agreed.



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.


With low level modulation and a linear amp it is much easier to produce 
excellent audio than it is from high level plate modulation. Building a 
low power (driver) low distortion AM transmitter has fewer problems than 
 high power low distortion transmitters. Class A direct coupled 
modulation schemes can be accomplished much easier at low levels than at 
high levels. Use of a balanced modulator can also eliminate the problems 
associated with occasional over modulation that plagues high level 
conventional modulation.






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.


That's true. An SSB amplifier at a power output level of 1/4 its full 
power  has an efficiency level of exactly 50% of its full power out 
efficiency.
If it is 66% efficient at full output it will be 33% efficient at 1/4 
power output level. Just like it is with an AM signal as you say.




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.




With a rice box type exciter and amp for low level modulation all one 
has to do is turn the knob to insert full drive, tune both final and 
load controls for maximum output and then reduce drive to 25% of full 
output and you are good to go.


73
Gary  K4FMX

[AMRadio] Re: RFI on 75 meters

[charles free]

Howdy,
 
The fellow who made the comment about the noise being all over the eastern part 
of the US is correct.  We have been unindated for years on the 3810 AM net that 
meets on Sundays in the NC/SC area.  What is really strange about our RFI is 
that it seems to drift up and down the band, but when it finds an AM signal, it 
will stop there and hang around, many times while a station is transmitting, 
only to resume its drift after the station stops radiating.  It will zero in on 
the stronger AM signals too, marginal signals during the net it will pass by.  
Various members of the group have done the usual, call Riley, and they have 
received replies from him.  They all run along the lines of, "yes we (the FCC) 
looked into it and it is is military and don't ask again."  I have RDF'd it 
from my location in Western NC using an old Bendix RDF and get a consistant 
south (180 deg)  angle on it, which points to a myriad of military 
installations in SC and  GA.
 
Don't know if it is still in use, but years ago, there was a military emergency 
comm set that rode the carrier of WWV to enhance coverage.  You all have 
probably noticed that a signal placed just in the right spot on a strong AM 
carrier will actually ride along and sound much stronger than if transmitted in 
the clear.  Perhaps the signal down here in the south is something along this 
line.  At least it is more of a nuisance than a source of serious QRM.
 
Charlie, W4MEC in NC

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Subject: Re: [AMRadio] power ratings
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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 cu