[Amps] More parasitic choke questions

Carl km1h at jeremy.mv.com
Thu Jul 29 10:24:27 PDT 2010 

We used to test them in the lab with a VNA, test fixtures, etc, when 
evaluating components for 500 MHz modems.

I wouldnt trust MFJ or any handheld at 150-250 MHz.

Carl
KM1H


----- Original Message ----- 
From: "Vic Rosenthal" <vic at rakefet.com>
To: <amps at contesting.com>
Sent: Thursday, July 29, 2010 11:29 AM
Subject: Re: [Amps] More parasitic choke questions


> Concerning the metal film resistors, I just tried to measure the 
> inductance of several
> different types of 3 watt 100 ohm resistors that I had, including some 
> that I had
> successfully used in parasitic suppressors. I succeeded in proving that 
> this isn't easy!
>
> An AADE inductance/capacitance meter shows 12 uhy, which is clearly 
> impossible unless
> these are wirewound (with lots of fine wire)!
>
> Connecting them to an MFJ analyzer showed r close to 100 ohms and x = 0 up 
> to 77 or 80
> MHz.for two types. An old one-watt carbon resistor started showing 
> reactance at about 66
> MHz. So I'm guessing that this is mostly due to the lead length, 
> capacitance, etc. and
> that the metal film resistors are no worse than the carbon ones.
>
> On 7/29/2010 5:27 AM, Carl wrote:
>> 100pf 500V silver micas are about as common as illegal immigrants in the 
>> US.
>> All the major catalog outfits stock them.
>>
>> Carl
>> KM1H
>>
>>
>> ----- Original Message -----
>> From: "Martin Sole"<hs0zed at csloxinfo.com>
>> To:<amps at contesting.com>
>> Sent: Wednesday, July 28, 2010 9:12 PM
>> Subject: Re: [Amps] More parasitic choke questions
>>
>>
>>> Interesting about how they do the series capacitor arrangement though I
>>> still wonder if it is making it overly complicated. Silver Mica 
>>> capacitors
>>> are about as common as Globar resistors here though so alternatives 
>>> would
>>> be
>>> useful too.
>>>
>>> As to the power involved I got to digging around and unearthed an 
>>> article
>>> Rich Measures had published in QST of 1989, it's on his website. Is the
>>> theory/math in that article messed up? It certainly suggests there might
>>> be
>>> more than a couple of watts in the R on 10m.
>>>
>>> Martin, HS0ZED
>>>
>>>
>>>
>>> -----Original Message-----
>>> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] 
>>> On
>>> Behalf Of Carl
>>> Sent: 29 July 2010 01:33
>>> To: Jim Thomson; amps at contesting.com
>>> Subject: Re: [Amps] More parasitic choke questions
>>>
>>>
>>> ----- Original Message -----
>>> From: "Jim Thomson"<jim.thom at telus.net>
>>> To:<amps at contesting.com>
>>> Sent: Wednesday, July 28, 2010 11:33 AM
>>> Subject: [Amps] More parasitic choke questions
>>>
>>>
>>>> Date: Wed, 28 Jul 2010 15:11:22 +0200
>>>> From: "DF3KV"<df3kv at t-online.de>
>>>> Subject: Re: [Amps] More parasitic choke questions
>>>>
>>>> Why is that impossible?
>>>> On HF the resistor is almost shorted out by the inductor.
>>>> The power absorbed will be less then 0.5% of the desired HF with a
>>>> typical
>>>> amp.
>>>>
>>>> 73
>>>> Peter
>>>>
>>>> ###  .5%  of  1500w =  7.5 watts  CCS..which is a bunch.     Forget the
>>>> HF
>>>
>>>> power absorbed
>>>> by the resistor.  The poor resistor is already being cooked by the
>>>> thermal
>>>
>>>> heat from
>>>> the hot anode anyway !
>>>
>>>
>>> ** A single 3-500 is putting out 600-800W so your rating is cut in half.
>>> In
>>> addition I disagree with the .5% as its more like .1 to .2% with the
>>> typical
>>>
>>> SB-220 type suppressor otherwise they would have lasted up to 40 or so
>>> years.
>>>
>>>
>>>   I noticed the 10 x 1 meg  2 watt  CARBON HV multiplier
>>>> resistor's  on one of my old B+ supplies from the 70's  had all risen 
>>>> in
>>>> value  by
>>>> 34% !     And that 34%  rise was only after 3 yrs.    I recently 
>>>> checked
>>>> carbons that
>>>> I bought new, back in the 70's..still in their wrapper.. and most of em
>>>> are way on the
>>>> high side..and were never used.
>>>
>>>
>>> ** I recently got an old Command HF-2500 in for service. Not only were 
>>> the
>>> filter caps garbage Nichicons withoutout even a temp rating but the
>>> metering
>>>
>>> resistors for ~ 2700V were three 2W carbons! This is as built.
>>>
>>>
>>>>
>>>> ##  Ok... how does one install the cap that's used to negate the XL  of
>>>> the MOX
>>>> resistor ?    Do u wire the cap in series with the resistor.. then
>>>> parallel the entire
>>>> mess with the coil ? .   The  Drake L4B  uses wide cu strap.... no 12 
>>>> ga
>>>> wire used.
>>>
>>> Lay the coil and resistor side by each (for you Canadians) without
>>> connecting. Now at both ends install a 100pf 500V silver mica between
>>> them.
>>> This works fine with 1-2 (50-75 Ohms for one, double for a pair) of the
>>> Mouser Xicon 5W MOX. It does absolute wonders for a Clipperton L using 
>>> one
>>> resistor. 1200W key down on 10M for 30-40 seconds with no smoke or 
>>> Chinese
>>> tube anode color. I use the 5W for everything as some stuff I work on 
>>> the
>>> parasitic does get close to the operating frequency (old glass bottles)
>>> but
>>> Ive never even discolored them. Ive tried 91 and 120pf with no apparent
>>> change so its rather non critical.
>>>
>>>>
>>>> ##  I used one, real small globar in my hb  3-500z amps  from the 70's.
>>>> The GS35-B
>>>> doesn't require a suppressor on 6m.  The YC-156 doesn't require a
>>>> suppressor an any band,
>>>> and neither does the  3CX-3000A7  or  3CX-6000A7.
>>>
>>> ** I bet some could get them to take off looking at construction 
>>> practices
>>> used.
>>>
>>>
>>>>
>>>> ##  to reduce the plate load Z on the upper bands of a HF amp.. I typ
>>>> install a
>>>> .6 uh  coil [  1/4" -3/8" tubing]  between plate block caps  and input 
>>>> to
>>>> tune cap.   This
>>>> does not appear to promote instability.
>>>
>>>
>>> ** Not that Ive found even with 2X 4-1000A's at 6KV.
>>>
>>> Carl
>>> KM1H
>>>
>>>
>>>>
>>>> Jim  VE7RF
>>>>
>>>>
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] 
>>>> On
>>>> Behalf Of Bill, W6WRT
>>>>
>>>>> Of course the main L should be very low
>>>>> loss and the complete suppressor should not absorb any of the desired 
>>>>> HF
>>>>> power.
>>>>
>>>> REPLY:
>>>>
>>>> Not absorb "any" of the desired HF power?
>>>>
>>>> That is impossible and Carl should know better than to make such a
>>>> statement
>>>> Bill, W6WRT
>>>>
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>
> -- 
> Vic
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