Ken,
After much helpful on and off-list discussion, I'm left with the following
corrections to my original post.
For Class A AM:
Average Input Power will remain constant at or above the level needed to
produce your target PEP.
Dissipation varies as this consistent Input Power minus the varying Power
Out.
Thanks to everyone who posted about this. I've learned some things about
Class A RF Amplifiers.
Jeff/KA5MIR
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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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