On 7/30/2010 12:49 PM, Bill, W6WRT wrote:
> 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.
Hmmm...That is not what I get out of the statement. The job of the
coil/inductor is to provide enough reactance at the frequency of the
parasitic to "quench" it, yet not provide enough reactance at the
fundamental that the resistor has to carry too much current.
Rearranging the order of statements:
The key quoted sentence is:
"Just enough turns should be used to suppress the parasitic oscillation,
and no more." followed by:
"The heat is usually caused by the dissipation of the fundamental power
in the resistor, which is an indication of too many turns in the
suppressor coil"
This problem comes when the parasite is too close to the fundamental
such as in the older glass tubes with long leads. That is where the
juggling act comes in between too much and too little Xsub L and the
statement (without the previous key sentence) can be easily misconstrued.
With Tetrodes the suppression can be added to the Grid, Screen, or
Plate. Possibly a combination and of course as was mentioned earlier,
neutralization which we don't normally see in GG mode. Also it's not
unusual to find multiple parasitic oscillations. There are more reasons
than just low impedance for not using parasitic suppression at the
cathode. For one I believe it's more difficult to catch parasitics of
different frequencies without affecting "stage gain" adversely.
Actually, "degeneration" can be added to the Cathode, or Emitter to make
the particular amplifier stage more stable, but it does so at the
expense of reducing the overall gain for the stage.
It's my understanding; for a "one off" build or mod of a grid driven amp
the idea is to find the parasitic resonance(s) first, Apply signals at
said frequency or frequencies and adjust the neutralization for minimum
through put before ever applying the voltages. Even after all that and
following good construction practices such as short direct leads of
large diameter, along with isolation between the input and output there
is no guarantee the amp won't take off. There is a whole section devoted
to this in the "Radio Handbook". I tried the RSGB Radio Handbook, but
due to the conflict between English and English I never did find a
reference to parasitic chokes. Also as I believe was mentioned earlier
in this thread, that with newer tubes a list of natural parasitic
frequencies is often available.
One component I found of interest in the "Radio Handbook" was the
parasitic choke in the 4CX1500B amp. It was a 47 ohm, 2 watt
composition resistor across a *straight* 2" piece of strap (22.29).
I believe with the new Commander amps you will find the plate choke to
be a U-shaped piece of strap (0.8 uH?), shunted by a resistor.
73
Roger (K8RI)
> 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
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