[Amps] GS35b es 4CX1000A INPUT

[TexasRF at aol.com]

Bill and others, The comment about passive grid and neutralization brings  
up a question:
 
At VHF and UHF, running grounded grid, the need for neutralization is  
indicated when the plate current dip and/or screen current peak (when  tuning 
the plate) does not coincide with a peak in power output. Does this same  test 
apply for HF passive grid or is it a non-issue?
 
At higher frequencies, neutralization is done by removing a few fingers  
from the grid grounding fingerstock. This gives a bit of added grid 
inductance.  Does a simple circuit like this work at HF? 
 
73,
Gerald K5GW
 
 
 
 
 
 
In a message dated 10/19/2010 9:35:28 P.M. Central Daylight Time,  
dezrat1242 at yahoo.com writes:

ORIGINAL  MESSAGE:

On Tue, 19 Oct 2010 23:53:11 -0000, "Allan Greening"  <vk3pa at 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
_______________________________________________
Amps mailing  list
Amps at contesting.com
http://lists.contesting.com/mailman/listinfo/amps