Using an automatic antenna tuner and shunt capacitance to ground at
the cathode is an interesting idea. I have not considered it. However, if
the capacitive reactance is a fraction of the tubes input impedance it
should work. If it is 1/5 of the tube's cathode input impedance the antenna
tuner, not knowing the difference between an amplifier and an antenna,
should offer the appropriate impedance to the amplifier to provide a
conjugate match. That is the antenna tuners output resistance will equal
the input resistance of the amplifier and the tuner's output reactance will
be equal to that of the amplifier's input but inductive. Together they
become resonant with a Q of about 5. As long as the inductance (tuner) is a
short distance away ,less than 1/4 wavelength, should make no difference.
73
Bill wa4lav
At 03:17 PM 3/30/2005 +0100, Roger Parsons wrote:
>I think that Steve, G8GSQ, probably has the answer:
>
>I have now convinced myself that it is nothing to do
>with the matching network per se, or with flywheel
>effects. Many grounded grid amplifiers like/need to
>have a capacitor directly across the input. My belief
>is that this is sometimes (and only
>sometimes)necessary to ensure unconditional stability
>over all of the driving cycle, and only marginally to
>reduce harmonics.
>
>As Steve rightly points out, it is not possible to
>'remote' a capacitor over any length of transmission
>line unless the reactance at the end of that
>transmission line is optimised for each particular
>frequency.
>
>The use of a pi network at the amplifier input allows
>a capacitor to be placed directly at the input whilst
>preserving (on average) a resistive input.
>
>This could be proved with a bit more effort than I am
>prepared to make. A T network could provide exactly
>the same impedance transformation and Q as its
>equivalent pi counterpart. But it would not place a
>capacitor directly across the amplifier input, and I
>wager it would not achieve the desired effect....
>
>73 Roger
>VE3ZI
>
>
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