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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