ORIGINAL MESSAGE:
On Tue, 19 Oct 2010 23:53:11 -0000, "Allan Greening" <vk3pa@vk3pa.com>
wrote:
>
>Hi all 2 Q's. wot is the input impedance of a GS35b GNDED GRID, es also
>input impedance of a PAIR of 4CX1000 GRID DRIVEN
>I am putting tuned input into both amps..GS35b = 50 ohms? 2 x 4CX1000 =25
>ohms? Tu es Dxis vk3pa
REPLY:
The input impedance of a GS-35b is on the order of 50 ohms when run at
1500 watts or so, but the input impedance of a pair of grid driven
tetrodes like a 4CX1000 is much higher - hundreds or even thousands of
ohms. Most builders run a tetrode (or two) in passive grid mode, where
the input impedance is swamped by a low value, high wattage resistor,
typically 50 ohms.
Without the swamping resistor, a grid driven tetrode will almost
certainly require neutralization. With the resistor and careful physical
layout, you should be able to get by without it. And of course, you get
to choose the input impedance yourself. If you want 50 ohms, use a 50
ohm resistor.
With passive grid on the higher frequencies, typically above 15 MHz, you
will have to cancel the reactance of the tetrode's input capacitance to
get reasonable SWR. Below that, you can probably drive the 50 ohm
resistor directly from your transceiver without any matching network at
all.
A grounded grid triode will require a matching network with a Q of two
or three or more on any frequency unless you are driving it with a tube
type transceiver, where the transceiver's pi-net will do the job.
This is a complex issue which requires many pages to deal with in
detail, so feel free to ask questions. And study your ARRL Handbook or
equivalent thoroughly - read every word of the section on amplifiers.
73, Bill W6WRT
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