[Amps] parasitic suppressor voodoo

Carl km1h at jeremy.mv.com
Fri Jul 16 07:08:26 PDT 2010 

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 at bigpond.com>
To: "DF3KV" <df3kv at t-online.de>; <amps at 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 at contesting.com [mailto:amps-bounces at contesting.com] On
>Behalf Of Randy Pence
>Sent: Donnerstag, 15. Juli 2010 14:10
>To: amps at 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
>_______________________________________________
>Amps mailing list
>Amps at contesting.com
>http://lists.contesting.com/mailman/listinfo/amps
>
>_______________________________________________
>Amps mailing list
>Amps at contesting.com
>http://lists.contesting.com/mailman/listinfo/amps
>
>
>
>
>=======
>Email scanned by PC Tools - No viruses or spyware found.
>(Email Guard: 7.0.0.18, Virus/Spyware Database: 6.15440)
>http://www.pctools.com/
>=======

 From sunny Binningup, Western Australia

http://www.qrz.com/db/vk6apk
http://www.qrz.com/db/vk6ap





=======
Email scanned by PC Tools - No viruses or spyware found.
(Email Guard: 7.0.0.18, Virus/Spyware Database: 6.15440)
http://www.pctools.com/
=======
_______________________________________________
Amps mailing list
Amps at contesting.com
http://lists.contesting.com/mailman/listinfo/amps

More information about the Amps mailing list