Ian wrote:
> It isn't quite a simple bridge neutralization network, because the
> anode-cathode capacitance is not a single capacitor. It's actually a
> T- network composed of anode-grid capacitance and grid-cathode
> capacitance in series, shunted by the grid lead inductance to
> ground... and that's before you add in the other parasitic
> inductances.
At HF, with most tubes I've seen, the tube behaves like a simple
capacitor exists between the anode and cathode WHEN the grid is
properly grounded.
Grid inductance can be ignored, and the cathode-to-grid and anode-
to-grid capacitances effectively parallel the input and output.
It's very easy to obtain a nearly-perfect feedthough null with only a
simple phase inversion transformer and variable capacitor, without
building an "image" of the tube in the opposite feedback path. Nulls
in the order of 40 dB (additional to normal leak-through loss) are
common.
It is worth mentioning that if you use that silly system of having
modest to high reactance capacitors from grids to ground you can
not neutralize the tubes effectively. The grids need to be grounded.
73, Tom W8JI
w8ji@contesting.com
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