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

[Gary Schafer]

Mr. Measure's parasitic chokes make sense if you think about what part 
is really the suppresser. The suppresser is the resistor part of it not 
the coil. The coil is there only to let enough of the high frequency 
parasitic develop across the parallel resistor so it can be absorbed. 
The coil shorts out frequencies that are lower (wanted frequencies). The 
idea of the nichrome wire is to lower the Q of the coil to broaden it 
out and also to absorb part of the parasitic.

Parasitic are a result of resonance, at the parasitic frequency, in the 
  length of the plate lead of the tube and the length of the leads in 
the tank circuit combined. The same is true for the grid circuit 
combination.

Often in audio amplifiers (such as the DX100 modulators) only resistors 
are placed in the plate and grid leads of the tubes at the tube 
connection to kill any parasites.

73
Gary  K4FMX


Richard Peterson wrote:
> Sherrill:
> 
> I am sorry I have not gotten back to you sooner. Been busy – and there 
> was LOTS of football to watch this weekend. That is always Job One for me.
> 
> Measure’s use of Nichrome wire as part of the way to cure VHF parasitics 
> in an HF amp is nonsense. First time I read it, I thought it had to be a 
> joke. But the truth is he sold kits and I suspect people installed them. 
> Too bad. It’s not science.
> 
> You have to find the frequency of the parasitic (typically by use of a 
> wavemeter) and then you have to design and tune a trap to kill it. The 
> process and the cures are covered in the ARRL Handbooks, but not in a 
> great amount of detail. It seems (to me) that the older handbooks do a 
> better job. You will find the parasitic is often around 120 megacycles, 
> seems like. But a friend of mine reports he ran into a killer parasitic 
> around 50 megacycles in a piece of commercial gear made for the 
> government. And be advised, there are ham rigs out there with the 
> standard parasitic traps such as you described that still have 
> parasitics. Just because you install a resistor and add some turns of 
> wire around it doesn’t mean you have solved any problem! But the truth 
> is, many hams operate rigs with parasitics and they sound and work fine. 
> The problem does not always cause grief – just as you can operate a 
> final that is not neutralized, and often it is satisfactory.
> 
> However, it can be hard to tune a rig that has BAD parasitics – it can 
> act really “goofy” and meters read all over the place – and that’s 
> because the amplifier has more than one frequency in it.
> 
> You will find some testing procedures described in the Handbook.
> 
> I’m glad you are interested in this. It’s good to find hams who want to 
> know, rather than you merely want to tune, talk and ship if it breaks. 
> The latter seem to lose interest in the hobby after awhile. Not always, 
> mind you, but often. The guys who tinker seem to get more out ham radio. 
> Just don't let the (seeming) complexity of it bother you -- 27 years ago 
> when I got my license, I drove myself crazy trying to understand how to 
> hook up something as simple as a VR tube.
> 
> And I still don't know much today -- but enough to get by.
> 
> Richard, WB5NEN
> 
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