Thanks Carl. Most informative.
I don't think I'll ever need to bother with it
but it is of great interest nevertheless.
I'm quite happy to "suck it and see" or "do as
others have done" and hope for the best. Ha ha.
73, Alek
VK6APK
At 10:08 PM 16/07/2010, Carl wrote:
>The process has been described in a 50's QST
>article by Bill Orr, I dont have the details
>handy but a search of the ARRL archives should
>work. The article also shows a tapped L
>suppresor which I havent seen in use, its
>something Ive got to make time to test.
>
>The simplest way to start is with a calibrated
>absorbtion wavemeter which is all that was
>available in the early days. Fire up the amp at
>the lowest level necessary for it to just barely
>start indicating a parasitic but at a safe
>level. Then its a matter of calculating the
>inductor. Strays play a big part and their
>effect goes up with power especially when close
>coupled to the tube. This makes it hard when the
>tube is jammed next to a wall and a cover is in
>place but its still possible to get some indication thru the ventilation holes.
>
>Ive not found a GDO to be of much use as there
>are way too many spurious resonances that
>confuse the readings. However a GDO in the diode
>mode is a much more sensitive indicator than a
>absorbtion wavemeter. Ive used a Measurements
>Model 59 for decades in that mode.
>
>A spectrum analyzer is the tool I use now as its
>pretty foolproof and shows things the others
>cant. (When I was working I could usually bring
>a SA home on weekends. Then I also obtained a
>141T system in a rather creative way when HP was
>offering high trade ins on obsolete SA's. I
>still have that but its used in the operating
>position and a couple of newer ones are on the
>bench.) For instance the SA can show a spurious
>resonance in a multiband tank circuit building
>up before it reaches the point of blowing across
>the switch or Tune cap. This is what Measures
>keeps calling a parasitic but isnt and can be
>killed with a snubber cap across switch contacts
>as Ive mentioned for years on here and elsewhere.
>
>In a 3-500 6M amp you can see the parasitic as
>well as the 3rd harmonic which are fairly close.
>Then its a matter of tweaking the suppressor so
>that it works on both and you can place a brick
>on the key and nothing burns up. Plus the amp
>efficiency is text book. A LPF brings remaining
>harmonics into the noise. Dont bother with a
>Pi-L on 6M, they are more trouble than they are
>worth, a good LPF is a better choice.
>
>There is no magic formula that works in real
>life. I suppose some computer genius can create
>a model but IMO there are way too many variables
>to make it anything but a chore to use.
>
>The resistor has the sole purpose of just
>stopping the parasitic from starting and not
>absorbing any of its power or any of the wanted
>RF on 10 or 6M. This becomes a balancing act
>with glass tubes such as the 811/572B, 4-1000,
>250TH, etc. External anode tubes have their
>parasitic well up into the high VHF or low UHF
>region and the 4-400/3-500 somewhere in between.
>
>Its a fallacy to say use a 3T coil for
>everything. Proven designs cover from 7 to 3
>turns close wound on a 1-2W resistor, it varies
>with the tube type. With Globars being expensive
>the 2W carbon has been the R of choice. However
>they are out of production except in OEM
>quantities. Old stock may be way out of
>tolerance and ineffective. One way to use a good
>2W is to wind a 1-2 turn coil of roughly 1"
>diameter with the R centered, copper strap works
>well here and minimizes heating. This method
>reduces the coupling to the R and requires a
>more accurate setting of the L to the parasitic frequency.
>If heating in the R occurs then vary the coil
>turns spacing and see if it helps. In
>particulary bad cases with very strong
>parasitics then parallel more R and keeping it around 50 Ohms.
>
>An alternative to the carbon R is a 5W MOX. This
>has some L depending upon construction but that
>can be negated by a couple of silver mica caps.
>Ive described the construction previously.
>
>To answer the basic question....some magic is
>still required but most can be eliminated with
>proper layout, construction practices, and some common sense.
>
>Carl
>KM1H
>
>
>----- Original Message ----- From: "Alek Petkovic" <vk6apk@bigpond.com>
>To: "DF3KV" <df3kv@t-online.de>; <amps@contesting.com>
>Sent: Thursday, July 15, 2010 6:29 PM
>Subject: Re: [Amps] parasitic suppressor voodoo
>
>
>At 09:27 PM 15/07/2010, DF3KV wrote:
>>Parasitic suppressors should be designed for the circuit which needs it.
>
>
>
>
>
>OK. Just a simple question from down under.
>
>Who, among all the subscribers on this list, knows how to do that?
>
>I'm not talking about cut and try methods. I'm
>talking about mathematics and tube spec sheets
>and parasitic frequencies etc etc. I have NEVER
>seen anything, anywhere, which gives the designer
>or builder of an amplifier, the tools or formulas to work this stuff out.
>
>Time and time again, we hear this Nichrome stuff
>raised and time and time again, the two camps
>slug it out and time and time again, nobody wins.
>
>I don't wish to align with any side here. My question is genuine.
>
>How would I design a parasitic suppressor from
>scratch? What formulas would I use to calculate
>the L, R for a particular tube or a particular rf deck layout?
>
>Don't tell me stuff like "four turns and 50 ohms
>worked for me." I am talking science and that
>sort of approach is NOT scientific. Gimme calculations. Somebody must know.
>
>73 and thanks,
>Alek. VK6APK
>
>
>
>>The parasitics frequencies vary with tube selection and amp layout.
>>Most of the time I don´t even need one.
>>
>>
>>73
>>Peter
>>
>>-----Original Message-----
>>From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
>>Behalf Of Randy Pence
>>Sent: Donnerstag, 15. Juli 2010 14:10
>>To: amps@contesting.com
>>Subject: [Amps] parasitic suppressor voodoo
>>
>>Can someone explain the difference between the Parasitic Suppressors ** that
>>Rich Measures sells compared to the ones Jeff Weinberg sold.
>>
>>--
>>Randy Pence
>>N4JZY
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