----- Original Message -----
From: "Bill, W6WRT" <dezrat1242@yahoo.com>
To: <amps@contesting.com>
Sent: Friday, July 30, 2010 12:49 PM
Subject: Re: [Amps] More parasitic choke questions
> ORIGINAL MESSAGE:
>
> On Thu, 29 Jul 2010 21:02:30 -0400, Roger <sub1@rogerhalstead.com>
> wrote:
>
>>
>>Radio Handbook, 20th edition (17.22), " In the process of adjusting the
>>resistor coil combination, it may be found that the resistor runs too
>>hot. The heat is usually caused by the dissipation of the fundamental
>>power int he resistor, which is an indication of too many turns in the
>>suppressor coil. Just enough turns should be used to suppress the
>>parasitic oscillation, and no more. Once the circuit is properly loaded
>>and the parasitic suppressed, no parasitic power will be present and no
>>power other than primary power will be lost in the resistor of the
>>suppressor."
>
> REPLY:
>
> Correct.
>
> Just to be clear: It is not the choke that does the suppressing, it
> is the resistor. The purpose of the choke is to provide a path for the
> lower frequency HF energy around the resistor. That is why too many
> turns cause the resistor to overheat by forcing more HF energy through
> the resistor, most noticeable on the higher frequencies like ten
> meters. Likewise, too few turns reduce the effectiveness of the
> resistor at the VHF parasitic frequency. As I said, getting the number
> of turns right is a balancing act for the designer.
>
> For some reason, Carl is unable to absorb (pun intended) this concept.
> 73, Bill W6WRT
You still dont get it yet do you Bill?
The coil must be self resonant at/near the parasitic frequency. However the
Q is way too high and the exact parasitic frequency is not rock stable and
varies with tuning, tube to tube, amp manufacturing tolerances and more. On
a spectrum analyzer it is far from a pure signal and has many sidebands and
other fuzzies popping in and out. A 3-500 amp can be anywhere in the 110-150
MHz range.
The coil is there to actually help encourage the parasitic and couple it to
the resistor which also broadens the tuned circuit bandwidth considerably so
that one design can be used in that model amp. The sole purpose of the
resistor is to stop it from happening which is why it is called a
suppressor. Ive used that description on here before as well as the term
"quench" and also "snubber" Bill, as well as on other forums.
Is it really the 10M fundamental that is heating some designs or is it the
5th harmonic or 7th on 15M, both of which fall right at/near a 3-500's
parasitic? Some of the problem amps Ive worked on indicate it may be all of
the above based on SA observations especially with hard driven and mistuned
amps. Note that a lot of the smoke is let out during tune up and some folks
have particular problems with certain tubes, isnt that right Bill?
.
The coil must also be very low loss at the desired frequency, not the
nonsense that Measures and his camp followers spout. I call him the Jim
Jones of RF.
Carl
KM1H
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