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Re: [Amps] IM distortion and such

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Subject: Re: [Amps] IM distortion and such
From: "Joe Subich, W4TV" <>
Date: Wed, 28 Jun 2006 11:02:36 -0400
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Rich writes: 
>     RE: A curved line:  Since the slope of the line is constantly  
> changing, it means that the transfer characteristic is too, and 
> that means it's making most all types of doo-doo.

This is a complete canard.  Amplifiers do not operate along the 
constant current curves.  They operate along a "load line" that 
starts at the zero drive condition for grid to cathode voltage 
and plate to grid voltage and runs to the maximum drive condition 
for grid to cathode voltage and plate to grid voltage.  You can 
not determine a transfer characteristic from the load line.  

The Ameritron tuning instructions allow establishing the "top 
end" of that load line at a particular point.  If one tunes for 
only 400 mA, the amplifier is underloaded.  Tuning to 550 mA 
and adjusting (decreasing) drive to operate at 400 mA single 
tone (max) simply moves the operation further up on the transfer 
curve.  If the user adjusts the drive to maintain the 400 mA 
max current in operation, the amplifier is "clean." 

This practice is very much like that followed with television 
transmitters (analog/NTSC).  The final amplifier is operated 
in the linear range between black (higher power) and white 
(minimum power).  The sync pulses are "blacker than black" and 
represent the "peak" power output ... during sync, the amplifier 
is intentionally driven into compression  - the driving signal 
is predistorted (stretched) so that the output signal has the 
proper sync ratio.  The momentary peak overcurrent from the 
cathode is not a problem because of the extremely short duration 
and the "clipping" does not cause distortion because the sync 
pulse is already properly shaped for the desired output spectrum.       

> Not true.  From a plethora of on the air observations of both  
> models.  A friend bought one, it produced feculence with 100w drive,  
> he added 25-ohms of cathode RF-NFB, tuned it up at 420mA instead of  
> 550mA, and that apparently cleaned it up.
> The exciter proved not to be the culprit since the signal cleaned up  
> with the cathode RF-NFB R, so the following straws are neutralized.

The only thing the 25 Ohms of cathode resistance did was make the 
amplifier harder to drive (or less easy to overdrive).  If you 
friend had simply reduced the drive from the transceiver he would 
have achieved the same effect in regards to a cleaner signal.  

By properly loading the amplifier, voltages (including VHF transients) 
in the output circuit would be reduced and the chance of damage to 
the bandswitch would be significantly reduced. 


   ... Joe, W4TV 

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