[Johnson] Neutralization vs. parasitic suppression, much more informaton is needed.

[Sherrill Watkins]

Dear Richard: Thank you very much for your insightful comments. I appreciate
it very much! I vaguely remember reading the articles in QST about 15 years
ago by Mr. Measures. I remember his use of Nicrome resistance wire for the
plate lead on a high gain amplifier tube to reduce VHF parasitic oscillation.
That sounded like a interesting idea and seems to make since? However, I
wondered just how much loss is induced on the lower frequencies by this
technique? Also, on further thought, it seems that a standard parasitic
suppressor with five turns of wire around a carbon resistor may be a better
approach? The coil of wire would act as a choke to the vhf parasitic causing
them to be dissipated in the resistor but would show minimum reactance to the
lower frequencies. Resistance wire may dissipate the lower frequencies in
addition to the VHF parasitic.  Now further, are you saying the VHF Parasitic
oscillations arise from OUTSIDE (not inside) the amplifier tube? If so, I
never realized that! If this is the case, then unwanted oscillations that
arise from INSIDE the tube are corrected by neutralization of the tube?
Oscillations that arise from effects OUTSIDE the tube are called "parasitic"
type? These are corrected by "suppressors", i.e. resistors with a 4 turn coil
wrapped around them. Also, in regards to shunting a resistor from the grid to
ground, i.e. "swamping the grid", that is probably the lowest cost and
easiest way to neutralize a tube and lends itself to homebuilt type
transmitters for inexperienced builders because someone, like myself, who
does not understand what is going on does would not have to fiddle adjusting
a neutralizing capacitor. Any reduction in gain would probably not be missed
and may make the tube last longer? I really appreciate you kind efforts to
answer my questions. I will go back and try to find my old ARRL handbooks.
Let me know if I am right or wrong about parasitic oscillation or anything
else! Thanks again!  -73- Sherrill W. k4own. 

-----Original Message-----
From: Richard Peterson [mailto:zapp11 at hotmail.com] 
Sent: Friday, October 01, 2004 10:16 AM
To: Sherrill Watkins
Cc: tom at cpic.net
Subject: RE: Neutralization vs. parasitic suppression, much more informaton
is needed.


Actually, neutralization is explained pretty well in the ARRL handbooks - 
but the books have their limitations. It's a good place to start. And I am 
an amateur, not an engineer, but here goes my understanding of things. Just 
remember -- I'm not an engineer.

Let's start with a triode The problem in a triode is the feedback path that 
exists because of the capacitance between the plate and the grid - it really 
is a small capacitor, and you don't want that. Hence, you take some of the 
output signal and feed it back, out of phase with the input, and the problem 
goes away. Now, in the case of a tetrode, you have a screen grid that helps 
to isolate the plate and the control grid, and this really reduces the 
amount of interelectrode capacitance - but there is still some, and, again, 
the problem can be reduced or eliminated with neutralization.

Early-day radio receivers that used all-triodes had hellish problems with 
this feedback problem, and that's why some of them (including some in my 
collection) make some great sounding squeaks and wails when you tune them - 
there is feedback occurring inside the tubes. Later sets included 
neutralization to fix the problem. Screen grid tubes pretty-much took care 
of the problem. Good invention, those tetrodes.

Now, one of the ways a transmitting final can deal with the feedback problem 
is by swamping the input, so I've read - but that takes away some of the 
gain. Sounds dumb, to me.

Another way is to just live with it. Some ham rigs did just that. The result 
is, like I said earlier, maximum output does not occur at the dip, but it is 
usually pretty close. I just lived with reduced power output and kept the 
final dipped to reduce disipation.

Now, parasitic oscillations can occur in a rig that needs neutralization, 
but parasitics can also occur in a rig that does not require neutralization. 
If you understand my neutralization explanation, you understand that (in a 
rig needing neutralization) undesired feedback is created between input and 
output due to the interelectrode capacitance inside the tube.

However, a VHF parasitic oscillation is caused by the accidental creation of 
a VHF oscillation circuit external to the tube. It can become quite powerful 
and even burn up, for example, an RF plate choke. If you've ever had a 
quality absorption wavemeter, and brought it close to an RF final with 
parasitics, you will find the meter slam against the pin - your handy 80 
meter amplifier is also a dandy little 100 megacycle oscillator! This is 
when you have to start inserting traps and chokes and related tricks to 
remove the unwanted oscillation. It is to be hoped the engineers of a 
commercial rig have already done that -- but not always.

But, yes, you can have maximum power out at the plate dip (in other words, 
your rig is neutralized) and you can still have parasitics. They are two 
different problems, requiring very different fixes.

My experience with parasitics is limited, so you might do well to find 
another source on this one. However, there are commercial amps out there 
that have them, just as there are commercial amps out there that do not.

Warning: Be cautious of anything about parasitics that has been written by 
Richard Measures. The folks at Eimac really had some issues with his 
articles in QST.

For some decent info on parasitics, again, the ARRL handbooks are a good 
place to start.

Richard, WB5NEN

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