If drawing a conclusion from a test you must first have the proper test
setup with the proper equipment. Using an SWR bridge to determine if the
input impedance of the tube is constant is not the right equipment.
The average input impedance may be rather low but that is just what it is,
average. When the cathode is driven positive the grid to cathode becomes
negative. At some point in the RF cycle the tube is cutoff and no grid
current flows and no plate current flows. Class B, AB2 etc.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Bill, W6WRT
> Sent: Wednesday, August 26, 2009 11:58 AM
> To: amps@contesting.com
> Subject: Re: [Amps] RF parts and parasitics!!
>
> 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.
>
> 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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