On Wed, Aug 26, 2009 at 12:57 PM, Bill, W6WRT <dezrat1242@yahoo.com> wrote:
> ORIGINAL MESSAGE:
>
> On Wed, 26 Aug 2009 07:20:56 EDT, Gudguyham@aol.com wrote:
>
> >Interestingly enough right smack at the top of the 3-500z heading for
> >these tubes there is a link to the W8JI web site which he explains
> >parasitic's.
>
> REPLY:
>
> While I generally agree with W8JI's conclusions, he makes a serious mistake
> about halfway through the article when he says " When grid current is
> absent,
> the grid-to-cathode impedance is nearly an open circuit". In a
> grounded-grid
> linear amplifier (the kind under discussion) this is simply not true.
>
> The input impedance of a GG amplifier is always relatively low, regardless
> of
> whether the grid is drawing current or not. The reason is that the input
> signal
> is "modulating" the cathode current, regardless whether the drive is 100
> watts
> or a fraction of a watt. The cathode current represents a low impedance
> load and
> the drive signal "sees" that always. It is true that when the drive signal
> increases to the point where grid current is drawn, the impedance
> decreases, but
> the amount of decrease is relatively small.
>
> You can prove this yourself. With an SWR meter between your TX and the amp,
> turn
> the drive down low enough so no grid current is drawn. Note the SWR.
> Gradually
> increase the drive all the way up to maximum while monitoring SWR. Note the
> SWR
> changes very little. If W8JI's statement were true the SWR would be very
> high
> with low drive where there is no grid current and he claims there is an
> "open
> circuit". It isn't.
>
> The rest of his article is very well done. His idea of inserting a small
> capacitor in series with the parasitic suppressor resistor is intriguing.
> Please
> note that "small" means only in the sense of value. This capacitor will
> carry
> significant current at the higher operating frequencies and needs to be
> rated
> accordingly. Also note that this capacitor now forms a parallel resonant
> circuit
> with he suppressor inductor, although it is a low-Q circuit due to the
> presence
> of the resistor. Just something else to keep in mind.
This is the way it is done on the AL1200
>
>
> Those pesky resonances are everywhere. :-)
>
> His article is at: http://www.w8ji.com/vhf_stability.htm
>
> Comments welcome.
>
> 73, Bill W6WRT
>
>
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