> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Bill, W6WRT
> Sent: Sunday, August 30, 2009 11:29 PM
> To: amps@contesting.com
> Subject: Re: [Amps] SB-220 bias question
>
> ORIGINAL MESSAGE:
>
> On Sun, 30 Aug 2009 21:11:54 -0500, "Gary Schafer"
> <garyschafer@comcast.net>
> wrote:
>
> >
> >But the input power of the amplifier is NOT linear with respect to drive
> >power or output power. As an example at maximum power output say it has
> 60%
> >efficiency. For 1000 watts out it would have to run 1666 watts input. At
> 250
> >watts out the efficiency will be 1/2 that or 30%. So to get 250 watts out
> it
> >would take 833 watts input. We have dropped the input power in half but
> the
> >output power has dropped to 1/4. We have not touched any tuning controls.
> We
> >have only reduced drive to 1/4 of what it took to produce 1000 watts
> output.
>
> REPLY:
>
> Gary, your analysis of tank circuits in the rest of your post is very well
> done,
> but I take issue with the above paragraph. You are confusing gain
> linearity with
> efficiency linearity. The two are not related. As long as the gain is
> linear,
> the efficiency can vary all over the place and the amp will work fine.
> It's just
> how tubes work.
>
> A practical example: Watch your plate current as and power output as you
> slowly
> reduce the drive from full power to minimum. You will see the power falls
> off
> faster than the plate current does, just what you would expect. The lower
> drive
> level is less efficient. Less efficient means more DC current is required
> to
> produce the same RF output. No surprise there, just exactly as predicted.
>
> In fact look at the extreme example, a class A amplifier. The DC plate
> current
> does not vary at all from max power to minimum, yet the efficiency varies
> from
> its high point at full power to zero efficiency at minimum power. Again,
> it is
> the gain linearity that is important, not efficiency linearity.
>
> 73, Bill W6WRT
Thank you Bill.
No confusion here between gain linearity and efficiency linearity. My
example is what you see with a class B linear amplifier. Higher idling
current will of course change the efficiency curve somewhat. In operating a
class B linear amp on AM the above example is typically what you will see.
Dropping power out to 1/4 will drop the efficiency to 1/2. Orr explains this
in his chapter on AM linear amplifier operation in the Radio Handbook.
73
Gary K4FMX
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