[AMRadio] 4-1000A Grounded Grid Class-C


Rob Atkinson ranchorobbo at gmail.com
Thu Oct 3 11:05:08 EDT 2013


So is a grid driven class C final different from the cathode driven
example (as pertains to feed through issues)
because grid driven takes less drive power for a given output?  I mean
do you escape the feed through power problems with grid driven because
the drive power is a lot less?

thanks

Rob
K5UJ

On Thu, Oct 3, 2013 at 9:22 AM, Steve WA1QIX <wa1qix at piesky.com> wrote:
> At 07:55 PM 10/2/2013 -0700, you wrote:
>
>> Is it possible to run a 4-1000A in grounded gridS, cathode driven, biased
>> for class-C and then plate modulate it and get 100% modulation? Has anybody
>> tried this?
>
>
> It is possible.  The only important factor is how does the unmodulated feed
> through power effect the linearity of the RF amplifier as a square law
> device.
>
> As the modulation approaches 100% negative, the feed through power becomes
> an increasing factor.  It will not behave like a classic negative peak
> limiter, which has no effect until it comes into conduction.  So, there may
> be some distortion, but how much is unknown having never actually done it.
>
> Do you know how much feed through power you actually get?  This will be an
> important number to know.  Because AM is a square law function, 1/10th the
> unmodulated plate voltage (90% negative modulation) is 1/100th the power.
> If you have, say, 5% feed through power, that is actually a lot with respect
> to 100% negative modulation.  I didn't figure it out, but it may be 20%
> modulation... just something to consider.
>
> If you do this arrangement, don't use the feed through as a negative peak
> limiter (of sorts) without also protecting the modulation transformer (if
> there is one).  Implement a protection diode/resistor/power supply across
> the transformer secondary to protect the transformer during the periods of
> the modulation cycle where there is no current.
>
> If it were me, I'd give it a try !! :-)
>
> Regards,
>
> Steve
>
>
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