Plate Modulation vs Linear Amplifiers

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k4kyv:
Q:  Can't the best quality audio of all be gotten from low level modulation and a linear amplifier?

A:    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 solid state 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.  Control and screengrid modulation is inherently less linear than high level class-C plate modulation.  Pushpull transformer coupled class-A or class-A series modulators, with negative feedback, might be good candidates for the driver stage of a linear.  If the linear is run properly in class AB1, that would be near to 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 practical real-world 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 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 with modulation, 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.

A major advantage of plate modulation with amateur 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.

K1JJ:
Good article for the archives, Don!
We need more of this kind of stuff written up.

As a side note:

For low level driven linears and QSYing.... all it takes to tune a true
class AB2, etc linear is to peak the output of the linear for max
power. Just peak C1 and C2 and you're OK. The heavier loading
of C2 for better linearity is not really mandatory, but should be done
when time permits.  

However, the tuning hassle starts with a class C grid or screen
modulated amplifier. They ARE critical and need a scope to insure
they are loaded right.  Axe Frank/KB1AHE about the time required for him
to finally get his class C, screen modulated 4-1000A tuned and loaded
so it sounded decent.

As for cleanliness of a low level linear.... lets assume you had a
perfect driving low level signal, like something that comes out
of an NE-602 or similar balanced modulator. A normal linear
power amplifier using common zero biased triodes [designed
for linear service] like the 3-500Z, 8877, etc are down at
LEAST -30 db of 3rd order intermodulation distortion. I cannot
remember exactly, but -30db equates to something like <1%
distortion. Very clean. So, in reality, I believe that a low level
driven linear power grid triode can probably produce some of
the best audio out there -  BUT at a price. The efficiency sux.
If you run a BIG mawl, the room heats up big time.

So, as you say, Don, probably a class E rig being as clean as
it is, in additon to the 85%++ efficiency is the best all
around compromise.

Personally, I like all three: plate, class E and linear. They
all have a purpose. For fast QSYing around the various bands,
I love my FT-1000D/FT-102's and various preset/ pretuned linears.
For just sitting on one AM area and generating little heat and
sounding good overall, my class E rig can't be beat.  

In the Winter and when you feel old buzzardly, a big plate
modulated rig is FB. Mix it up baby! That's what keeps it fun.

73,
Tom, K1JJ

ve6pg:
...don..great stuff..we need more of this kind of thing to remind us and send us back to our text books to re-read alot that has been forgotten..tim...sk..

Steve - WB3HUZ:
Keep in mind that most of the concern about distortion levels at less than 2-3 percent is moot. Your receiver creates more distortion than that (some a lot more), not to mention the effects of the ionosphere. So, no matter the modulation method, if the distortion level is less than that of the receiver, they will all sound equally good.

Vortex Joe - N3IBX:
Don - That was a good meaty dissertation on modulation. I thoroughly enjoyed it!
Regards,
           Joe Cro N3IBX

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