I am trying to find out what you are trying to do.
"I redid the neutralizing circuit and instead of a hard wired feed off the
anode through a blocking cap to the neutralizing cap, I setup a single
adjustable disc looking at an anode much like a gimmick wire would do and
fed that to the grids. I'm questioning exactly where to apply this, though.
At present it's going to the input side of the grid blocking cap."
If you are directly connecting the neutralizing capacitor to the grid,
the plate-grid capacitance is increased, and there will be impossible to get a
"null" of the feed-through.
"I put the plate load resistor in place and with the Anritsu looking into
the output I tuned the deck and was able to attain a 60db return loss and
an absolute flat SWR."
I take this as that is the plate tank circuit matching, and that a perfect
match can be
made at one frequency.
"Then I hooked up the service monitor and using the tracking generator and
spectrum analyzer, I was able to get a deep null at the op freq. It all
looked perfect."
Is this meant to measure the input-output isolation with the tracking generator
connected to the pi-network output
and the SA connected to the grid circuit?
Generally, it can be quite difficult to "calm down" several parallel connected
high gain beam tetrodes.
They often generate parasitic resonances which have to be damped out with
various resistive loading schemes.
Workable schemes can be the use of ferrite beads on the grid leads and on the
screen voltage connections together with stopper resistor to each screen bypass
capacitor.
Should a low-frequency parasitic circuit be caused in the screen bypasses, the
bypass capacitors might be destroyed. Precautions like these kept the 4
paralleled 4CX250's in the SRT ST1200/1400/1600 docile in linear service.
In linear service at 50 MHz or lower, the electron transit time loading of the
input circuit is often not sufficient to limit the power gain to manageable
amounts even with perfect neutralization.
It is "good engineering practice" to utlize so much extra loading of the input
circuit so that power gain is limited to 100 or 20 dB.
I have learned this "the hard way" albeit with 4CX350A:s on 22 MHz.
A 50 MHz amplifier using 3 parallel connected 4CX250's was described in the
ARRL VHF manuals for ages.
This circuit uses a center-tapped grid circuit, but the bridge neutralization
scheme will probably work as well.
73/
Karl-Arne
SM0AOM
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