On Tue, 20 May 1997 10:47:26 +0000 w8jitom@postoffice.worldnet.att.net
writes:
>Actually there seems to some "confusion" about the change caused by
>nichrome. I don't intend to argue this, so this will be my single
>group post on this subject.
Seeing is believing Tom.
People should use common sense and
>think about this stuff *very* carefully.
>
>Wes Stewart's conclusion was there is no real difference at all in
>VHF performance no matter what type of material is used in the
>inductor, that even a very small change in resistance or turns could
>easily produce identical results at VHF. There was no "magic", the
>suppressors were essentially the same, or could easily be made to
>appear the same.
>
>Give the following the common sense test.
>
>1.) The Rp (the parallel impedance) of direct connection is zero
>ohms when the connection is short, direct, and lossless.
>
>2.) Rich states over and over again LOW Rp values are best for
>stability, and uses that justification now (before it was "low Q",
>now it's low Rp) as the reason his product "works".
I believe that there is simply a confusion of terminology here, and that
"most" of us are able to understand Rich's intent.
>3.) If the lowest Rp comes from a direct short, and the best
>stability comes from lowest Rp, a direct short across your
>amplifier's suppressor should produce the most stability.
>
>Traditionally, people have always added series loss resistance
>to stabilize a PA. The loss resistance can be something as simple
>as a single conventional resistor (in an audio amplifier, or in the
>screen or grid path of an audio or RF amplifier), or a parallel L/R
>circuit (the conventional suppressor we are all accustomed to
>seeing).
>
>In all cases, the lowest high frequency gain and most stability
>occurs with a HIGH value of series resistance (more turns on the
>coil- so the resistance dominates, or if it is a lumped resistance a
>larger resistance value).
>
>Ask yourself this. After years of knowing more turns (higher
>Rp) means more stability (if our PA's are unstable, we generally ADD
>turns to the suppressor - increasing Rp),
That is only an accurate statement up to a point and then it falls apart.
A VHF suppressor is a careful balancing act between suppressing ( not
eliminating it entirely) a parasitic and allowing the desired RF to pass
with as little loss as possible. You can only add so many turns before
the suppressor overheats from the desired (10M) RF.
In a typical ham size amplifier the suppressor is from 2 to 5 turns of
tinned copper wire around a 2W carbon composition resistor. In most cases
this does the job. As a practical limit, 5 turns appears to me the
maximum.
Tom seems to love bashing the Clipperton L as the worse example of
engineering and claims the L is completely unstable on 10 and 15M. That
amp uses 5 turns of wire and the proper setting of the Load control is
well documented. Go try 6 turns and see what happens Tom; if you truly
believe your statement about simply adding turns.
The problem with the Clipperton L (4 572B's for those not familiar with
it) is multi-pronged.
- Poor tank circuit layout
- Poor tank Q on most bands; resulting in high harmonic content
- Natural parasitic reasonance and PA choke reasonance very close
to each other plus 2nd harmonics of 10/15M.
It is known to eat suppressors on 10M but there are simple cures. They
are:
- Change the plate choke to a B&W 802 or an Ameritron from the
larger series of amps. The benefit is two fold; a better choke
plus much shorter lead lengths.
- Change the suppressors to ones using a material that
substantially lowers VHF Q by INCREASING the series R at VHF.
Nichrome is one possibility, but not the only.
or knowing that a resistor
>in series with the anode in an audio amplifier makes the PA more
>stable than a direct connection to the rest of the circuit, or
>knowing that HIGHER anode load resistance in a Hi-Fi amplifier hurts
>high frequency response (amplification).......
>
>why is lower Rp suddenly a better method of reducing VHF gain,
>contrary to what we all experience in actual circuits???
As I said above, it appears to be a matter of terminology and not intent
Tom. If you would be less intent in criticizing Rich and spend a bit more
time evaluating his ideas, you may learn something. At NO POINT did I
ever read that you tried ANY of Rich's suggestions AND THEN offered a
meaningful critique.
It was Ian White, I believe, who doubted the effectivness of Nichrome. In
reality, the use of a parasitic suppressor inductor with RF resistance
goes back many years. My first exposure was at National Radio in the
1960's when a vapor phase cooled amp was developed for the military. A
Globar style resistor with a two turn wrap of .5" wide, .004" thick,
strap was used. The material was similar to Mu-Metal and it worked
extremely well. I have no idea where the original idea to use this
material at RF came from, but since the tube was Eimac and the customer
the military, I am sure it received approvals along the line.
I still have a roll of this material that I kept when the contract was
finished. I use it today in suppressors, including multiple 572B's on 6M
where the "experts" say I cant use those tubes. They run extremely well
and stable, thank you.
I can FAX or mail full specs to anyone interested in evaluating this
material. Don't even bother me if you suffer from a NIH syndrome; the
stuff works and I will not waste my time arguing with anyone with a
closed mind or an overblown ego.
73.....Carl KM1H
>73, Tom W8JI
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|