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Re: [Amps] IM distortion and such

To: "'Michael Tope'" <>,"'Joe Subich, W4TV'" <>, <>
Subject: Re: [Amps] IM distortion and such
From: "Gary Schafer" <>
Date: Thu, 29 Jun 2006 22:08:15 -0500
List-post: <>

> -----Original Message-----
> From: [] On
> Behalf Of Michael Tope
> Sent: Thursday, June 29, 2006 9:12 AM
> To: Joe Subich, W4TV;
> Subject: Re: [Amps] IM distortion and such
> ----- 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. 

It will also reduce efficiency.

Gary  K4FMX

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