ORIGINAL MESSAGE:
On Sun, 14 Mar 2010 21:23:39 -0400, "Carl" <km1h@jeremy.mv.com> wrote:
>Which then isnt changable [grid-cathode impedance] and is out of contention
>for parasitics.
REPLY:
What makes you think the grid-cathode impedance is not changeable?
For example, in a typical legal limit GG amplifier with no input
matching circuit, the input impedance is around 50 ohms on a wide
range of frequencies. If you add a simple parallel resonant tank
circuit from cathode to ground (DC isolated of course), it is still
around 50 ohms but only on the resonant frequency of the tank and
drops down as you move above or below resonance. The higher the Q, the
faster it drops. Make the Q high enough and the VHF impedance will
drop low enough that the tube will not oscillate at VHF despite a very
high Q parasitic resonant circuit in the anode circuit.
That's just one example. You can also change the impedance using
transformers, various other kinds of L-C networks, etc.
73, Bill W6WRT
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