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Re: [Amps] IM distortion and such

To: "Joe Subich, W4TV" <>, <>
Subject: Re: [Amps] IM distortion and such
From: "Michael Tope" <>
Date: Thu, 29 Jun 2006 07:12:10 -0700
List-post: <>
----- Original Message ----- 
From: "Joe Subich, W4TV" <>

> 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.  

I was looking at this earlier this evening. The load line is just a 
straight line approximation based on a couple of points. The 
actual transfer curve (Eb vs Eg) will depend on the slope and
spacing of the constant current curves as well as the slope of 
the load line (e.g. actual load impedance). Given the constant
current curves and the load impedance presented to the tube, 
it seems to me that you could actually construct the Eb vs Eg
transfer curve, but I coud be wrong. 

> 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." 

The above process should be equivalent to reducing the load 
impedance that the tank circuit presents to the tube. It will 
result in a steeper load line. It seems to me that a steeper load 
line would keep the upper left end of the load line away from 
the sharply curved ends of the constant current curves that turn
upwards at low instantaneous anode voltages, especially if the 
drive is backed-off so that anode current doesn't exceed 400 
mA. Underloading would reduce the slope of the load line 
thereby placing the upper left corner in the area where the 
constant current curves have a very high curvature. 

In any case, this AL-80B IMD horse has been beaten to death.
I wish someone would do some two-tone tests using the lightly
loaded and heavily loaded tuning procedures and then post the 
results (specA photos) on the web. 

73, Mike W4EF.....................................

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