[Amps] parasitic suppressor voodoo

Fri Jul 16 17:25:54 PDT 2010

Thanks, Carl.  Interesting test method.

Paul, W9AC

----- Original Message ----- 
From: "Carl" <km1h at jeremy.mv.com>
To: "Paul Christensen" <w9ac at arrl.net>; <amps at contesting.com>
Sent: Friday, July 16, 2010 11:58 AM
Subject: Re: [Amps] parasitic suppressor voodoo

> Initial testing is via a small probe somewhere near the tube or plate line
> and with a healthy attenuator in line at first to be safe. That can me
> modified and moved as needed. I also like to look at feedthru isolation at
> sig gen levels before applying HV. Neutralization comes before parasitics.
>
> Once the suppressor is built the probe can be moved around looking for 
> other
> spurious.
>
> Since some LF parasitics (common in some old Class C rigs and amps) can 
> get
> thru the pi net I use a Bird -50dB take off slug and attenuators as needed
> at the antenna output. The same procedure is used for making sure the
> harmonics are adequately suppressed.
>
> Carl
> KM1H
>
>
> ----- Original Message ----- 
> From: "Paul Christensen" <w9ac at arrl.net>
> To: <amps at contesting.com>
> Sent: Friday, July 16, 2010 10:29 AM
> Subject: Re: [Amps] parasitic suppressor voodoo
>
>
> Carl,
>
> When using a SA to test for the presence of parasitics and spurious
> resonances, how are you coupling the sample, and where in the PA?  Tnx!
>
> Paul, W9AC
>
> ----- Original Message ----- 
> From: "Carl" <km1h at jeremy.mv.com>
> To: <amps at contesting.com>; "Alek Petkovic" <vk6apk at bigpond.com>
> Sent: Friday, July 16, 2010 10:08 AM
> Subject: Re: [Amps] parasitic suppressor voodoo
>
>
> 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
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>>
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>
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>
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