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.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of KA5MIR
> Sent: Tuesday, November 14, 2006 4:41 PM
> To: amps@contesting.com
> Subject: Re: [Amps] Class A for AM
>
> Hello Ken,
> One reason for few answers to your question might be that, like a lot of
> things, it depends. There is not "one true answer". Efficiency is a
> moving
> target with linear AM. Speech pattern is a huge variable.
>
> But, for the fun of it, lets try to get close.
> Requirements: Class A, 1500 Peak, 375 Carrier. Ok, here are two figures
> we
> can work with.
>
> Class A efficiency = Lets say 25%, an estimate.
> 1500 watt output.
> 1500 / .25 = 6000 watts input required.
>
> 6000 watts input - 1500 watts = 4500 watts dissipation at modulation peak.
>
> Efficiency is worse for the unmodulated carrier and assuming traditional
> symmetrical modulation, the carrier must be half as efficient as peak
> modulation, so...
>
> Class A CARRIER efficiency = 12.5%.
> 375 watt output.
> 375 / .125 = 3000 watts input required.
>
> 3000 watts input - 375 watts rf out = 2625 watts dissipation at carrier.
>
> The percentage of time you are closer to carrier compared to 100%
> modulation
> determines the average dissipation which will always be changing with
> speech.
>
> In this example, the dissipation varies between 2625 and 4500 watts.
>
> I bet a 3cx3000 amp would do it, but it would be a lot happier class B.
> :)
>
> 73',
> Jeff/KA5MIR
>
>
> On Tuesday 14 November 2006 13:59, kenw2dtc wrote:
> > Interesting observation. Last week I posed this question on this
> > reflector: "If one were to build a class "A" RF tube amplifier capable
> of
> > producing an AM signal of 375 watts carrier and 1500 watts PEP, what
> would
> > be the plate dissipation required by the tube? (I know how stupid and
> > inefficient this is, but I really would like an answer)"
> >
> > In my mind the question needed just a simple answer of plate dissipation
> > such as 3000 or 4000 or 5000 watts etc. Instead, I was taken off subject
> > to plate modulating an AL-1200, 4-1000 modulators, Viking Rangers, the
> use
> > of the FT-101EE etc.
> >
> > Of the 18 responders only two guys answered the question: The first guy
> > said 700-800 watts and the second guy said 3000 watts. I'm not going to
> > submit the same question to the Yahoo Amp Reflector, but based on the 18
> > responses, I'm not sure about submitting future questions to this one
> > either.
> >
> > 73,
> >
> > Ken W2DTC
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