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Re: [Amps] Looking to purchase a good 6 meter amplifier

To: "Gary Schafer" <>, <>
Subject: Re: [Amps] Looking to purchase a good 6 meter amplifier
From: "Tom W8JI" <>
Date: Mon, 30 Jul 2007 21:24:21 -0400
List-post: <>
> It doesn't matter what the amplifier efficiency is. The 
> carrier efficiency
> for good linear AM, carrier efficiency will be exactly 1/2 
> of the PEP
> efficiency if the amp is set up properly to just handle 
> 100% modulation
> peaks. To handle greater than 100% peaks carrier 
> efficiency has to be less
> than 1/2 of PEP efficiency.
> Carrier output power will be 1/4 of PEP output power when 
> set up for 100%
> modulation peaks.


Why not measure the efficiency of a few class AB amplifiers 
when peaked at some maximum envelope power and then again 
with drive reduced to 1/4 output power.

I think you will change the word "exact" to "around".

It's certainly very close to half in a class B stage, and in 
the ideal theoretical case it is half, but it is an 
imperfect world. As a matter of fact the PA can be very 
non-linear so far as transfer characteristics and not have 
prohibitive distortion, or one with more linearity in 
transfer characteristics might have more objectionable 

It's all about the shape of the slope in this imperfect 
world, and even the class of the amplifier.

For example, I can have a class A amplifier and the input 
power won't change a bit between carrier and modulation 
peaks. A pure ideal class B will behave as you suggest. A 
class AB amp can be anywhere in between, and in the real 
world we might have to even overload the amp a bit to 
minimize distortion.

The rule efficiency doubles is generally a good guideline, 
but it isn't an *exact* rule any more than we can say any 
gain compression or operation where non-linearity occurs 
always produces objectionable distortion. Our amps are 
between class A and B, and they don't have perfect transfer 
characteristics. They also, especially on six meters with 
big tubes, can have pretty lossy tanks.

It's like saying the tank Q has to be 12.

73 Tom

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