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Re: [Amps] IM distortion and such

To: "'R L Measures'" <>, <>
Subject: Re: [Amps] IM distortion and such
From: "Joe Subich, W4TV" <>
Date: Wed, 28 Jun 2006 14:02:02 -0400
List-post: <>
Rich writes: 

> 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.

If that is true, it is impossible to design a "clean" amplifier 
around the 3-500Z.  A simple examination of the constant current 
curves for grounded grid operation shows that the constant current 
lines are certainly not parallel ... not even between the 1 mA and 
400 mA lines.  

Loading for 550 mA at max may even improve linearity (I can't put 
one on a spectrum analyzer) if it results in moving the load line 
into an area of "better" alignment.  Anyone who looks at the curves 
can easily see that the 200 mA cure is far more non-parallel with 
the 100 mA and 400 mA curves than the 600 mA curve is with the 400 
and 100 mA curves.   

> > If the user adjusts the drive to maintain the 400 mA
> > max current in operation, the amplifier is "clean."
> I don't think so.

You don't "think" so or you have measured it so?  Put it on a 
spectrum analyzer and find out.  If the AL80 is "dirty" when 
driven to 400 mA, every other single 3-500Z is dirty at 400 
mA and every dual 3-500Z is dirty at  800 mA (or 1500 W PEP 

> > 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. 

Over loading (tuning for 550 mA) provides the same ability to handle 
the ALC overshoot and the 25 Ohms of cathode resistance.  

Any radio is capable of operating at reduced drive simply by turning 
down the mic gain. 

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

Another canard ... VHF doesn't need to get through the tank 
circuit.  The damage is almost always at the input (tune padder 
and/or 10/15 meter coil contacts) that is the point that RF voltage 
is the highest.  Remember the output tank transforms a High-Z tube 
to a low-Z antenna ... high-Z is high voltage/low current.  


   ... Joe, W4TV 

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