> -----Original Message-----
> From: Tom W8JI [mailto:w8ji@w8ji.com]
> Sent: Tuesday, November 14, 2006 8:06 PM
> To: Gary Schafer; 'KA5MIR'; amps@contesting.com
> Subject: Re: [Amps] Class A for AM
>
> > Except in class A the input power never changes. If it is
> > set up for 6000
> > watts input in order to provide 1500 watts output @25%
> > efficiency then the
> > input power will always be a constant 6000 watts with or
> > without modulation
> > or carrier.
> >
> > When it is putting out 375 watts carrier the input power
> > will still be 6000
> > watts.
> >
> > So the dissipation would be 6000 watts minus power out for
> > any given output
> > power.
>
>
> That's the way I see it also.
>
> Since it should make around 50% on peaks it should make
> around 25% on carrier, and 3000 watts plus headroom of
> standing power should do it.
> It will just be useful output power minus standing zero
> signal dissipation.
>
> 73 Tom
>
Hi Tom,
I was just reading some info on class A amplifiers.
It seems that a typical triode class A1 amplifier (no grid current) the
efficiency is in the range of 20% to 35%.
A class A2 (some grid current) triode amp the efficiency would run from 38%
to 45%.
A class A1 tetrode or pentode would have an efficiency of from 30% to 40%.
So I guess it depends on how the class A amp is configured as to the maximum
efficiency it would produce. I have always used the figure in the area of
33% as a ball park value but obviously that can be different.
Using a class A amp as an AM linear I don't think the 50% efficiency change
rule applies as it does with a class B linear amplifier. With a class B
amplifier the efficiency difference between maximum peak envelope power
output and carrier output is right at 50% difference.
In other words if the efficiency of the class B amp is 70% at maximum PEP
then it will be 35% efficient at carrier power level of the AM signal.
(carrier at 1/4 power of PEP)
With class A amplifiers I think it is stated that the efficiency must not
exceed 50% of full output efficiency but it can be less, and by looking at
the fact that plate input power does not change at all from zero power out
to maximum power out on a class A amp the efficiency would not be a 50%
change but something greater.
In the original example of 25% efficiency at full output (maximum PEP) then
carrier output efficiency would be 6.25% with the class A amplifier. It
would also seem that that % would change with a class A2 amplifier slightly
because grid current would only flow on modulation peaks.
73
Gary K4FMX
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