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Re: [Amps] IM distortion and such

To: "Joe Subich, W4TV" <>
Subject: Re: [Amps] IM distortion and such
From: R L Measures <>
Date: Wed, 28 Jun 2006 09:32:08 -0700
List-post: <>
On Jun 28, 2006, at 8:02 AM, Joe Subich, W4TV wrote:

> 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 tranfer characteristic at any instant if the slope of the  
constant current line where the load line crosses it.  If the slope  
of the 3-500Z's 3a crossing point is not the same as the slope where  
the load line crosses the 0.1a line, distortion results.
> 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.

Someone needs to tell Eimac and Amperex about this.

> If the user adjusts the drive to maintain the 400 mA
> max current in operation, the amplifier is "clean."

I don't think so.
> ...
> 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.

Good point, Joe, but not all radios have adjustable RF-out, and even  
those that do often have ALC overshoot, especially in the hands of a  
novice operator. As I see it, cathode RF-NFB is good for linearity as  
well as stability.
> 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.

In my experience, VHF instability is independent of loading since VHF  
never gets through a low pass Pi tank circuit.
> 73,
>    ... Joe, W4TV

R L MEASURES, AG6K. 805-386-3734

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