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Re: [Amps] IM distortion and such

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Subject: Re: [Amps] IM distortion and such
From: "Tom W8JI" <>
Date: Tue, 27 Jun 2006 23:08:26 -0400
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From: "R L Measures" <>

> As I see it, no matter what kind of modulation one is 
> trying to  amplify, at the point when an incremental 
> increase in input no longer  increases output (which call 
> the transfer function) by the same  factor as was observed 
> at lesser currents, the cathode is running out  steam and 
> the tube is beginning to lose linearity.

Not true. Even a curved line does not necessarily produce 
IM3 or IM5. (harmful IM3 is 2 * F1 - F2,  or 2*F2-F1, IM5 is 
3*F1-2*F2, 3*F2-2*F1 and so on through all odd-order 
products). There were two posts to this reflector, one from 
Steve Katz and another from Doug Coffman, indicating they 
measured a linear transfer response when the load line was 
set over 550mA during initial tuning. You might look back at 
the recent archives.

>Even though there  is a 3a line on the 3-500Z/ 8002 
>characteristic curves, does this  mean that all will be 
>well in linear service at 3 peak anode  amperes?

Comparing one set of operating parameters in one system to 
another entirely different set in another system doesn't 

> If this were the case, at 4kV, single 3-500 linear 
> amplifiers could produce 2500w PEP.

What approximation did you use?

> OTOH, Tom may be right in saying that tuning up at 550mA 
> produces  good linearity in an AL-80 or SB-1000, but from 
> the impure matter I  hear from these two amps on the air 
> that are driven with 100w, I have  doubts.

Interesting. You formed your conclusions because (over the 
years you have been listening) you "heard" two wide signals, 
one from each model. The test conditions were:

1.) unknown quality communications receiver at your end
2.) unknown measurement signal-to-noise ratio at your end
3.) unknown exciter IM quality at their end
4.) unknown operation of the offending radios and amplifiers 
at their end
5.) unknown condition of the offending radios, amplifiers, 
and antenna systems

I have a room full of newer calibrated measurement equipment 
including equipment that can automatically measure occupied 
I live in a rural area with nearly a  hundred acres of 
directional receiving antennas so the antennas can focus on 
a desired signal as clearly as possible.

With all of that at my disposal I couldn't tell for sure 
over the air if it was the exciter, operator, amplifier, or 
even something arcing or non-linear in their antenna. I 
would need to be right in the room with the offending gear 
and test equipment to tell anything about the actual cause.

I can't think of anyone else who, over the air, can tell if 
it is the amplifier, the exciter, the antenna, a feedline 
connection, or the operator with any certainty. I also don't 
know many people who consider a sample of two bad signals 
under such poor test conditions justification for going on a 
campaign against someone or something.

> I don't see how because 550mA corresponds to c. 1650mA 
> peak, which is  well into the non-linear region.

I'm not sure where your numbers come from, since you never 
really say how you get them.

73 Tom 

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