[Amps] low pass filter fail ;)

[Tom Thompson]

If you model the filter in LTSpice, you can look at all the voltages and 
currents and see if the capacitors are up to the task.

Tom   W0IVJ


On 4/28/2014 5:34 PM, Carl wrote:
> The most common cause Ive seen for filter problems is choosing the 
> wrong cap for the job such as one designed for industrial service and 
> not RF. I still see people using 60's TV doorknobs of 500pf 30-40KV in 
> a 1500W + HF amp; even in parallel they literally come apart.
>
> Jims comment about parallel caps is also valid; my 3CX1000A7 1500W 2M 
> amp uses 6 100pf  7.5KV HT-58 size caps for the split plate line and 
> has been holding up fine for 25+ years with no tuning drift.
>
> Carl
> KM1H
>
>
>
> ----- Original Message ----- From: "Jim Thomson" <jim.thom at telus.net>
> To: <amps at contesting.com>
> Sent: Monday, April 28, 2014 5:33 PM
> Subject: [Amps] low pass filter fail ;)
>
>
>> Date: Mon, 28 Apr 2014 08:32:04 +0200
>> From: peter chadwick <g8on at fsmail.net>
>> To: Bill Turner <dezrat1242 at wildblue.net>, amps at contesting.com
>> Subject: Re: [Amps] low pass filter fail ;)
>>
>>
>> With a VSWR of only 1.2:1, I wouldn't have thought it was too close 
>> to the cut off frequency. Passive LC filters (other than the 
>> absorptive types) work by having a high VSWR in the stop-band. 
>> According to the table in Geffe's book (Simplified Modern Filter 
>> Design), a VSWR of 1.22:1 represents a ripple of 0.04dB. I would 
>> suspect a lossy capacitor - we don't know the details of the filter 
>> designs and exactly what the capacitor was, nor the power level. You 
>> can have some high currents at quite low power levels.
>>
>> vy 73
>>
>> Peter G3RZP
>>
>>
>> ##  2m is  144-148 mhz.   So where is the optimum cutoff freq ?      
>> Heck, even with a 200 mhz  cutoff freq.... it will still kill the 2nd 
>> and up harmonics.
>> Most of the problems I have had with LP filters for HF   is the 
>> caps..and their current ratings.   U are  better off to use say 2 x 
>> 100 pf cap   vs a single 200 pf cap.
>>
>> ##  I have one of last built henry radio  10 kw HF  LP filters..with  
>> 7-16 dins on each end.   They used a mess of 25 +  50 pf  NPO 
>> caps...and all are  HT-57 types.
>> They used a single 50 pf at each connector ..and in the middle of the 
>> filter, between the 2 x .25” OD  tubing coils, they used 8 x 25 pf 
>> doorknobs  in parallel.
>> Cut off was at aprx 33 mhz.
>>
>> ## HT-57 caps are rated for 15 kv...and in low values..are not only 
>> NPO..they also handle one helluva  lot of current.   LP Box was  
>> aprx  5.5 inch square x 10-12 inches long.
>> Of course on the lower freqs, like  160-12m..the size of the LP box 
>> becomes huge.
>>
>> ##  Those 5 kw band pass filters  made in the uk are slick. band pass 
>> is better than  a LP.    They were band pass on all HF bands  except 
>> 160m.
>> 160m is a LP.
>>
>> ##  LP filters  for  VHF is not that big a deal.    You can see the 
>> various LP  filters  available  for  88-108 mhz FM broadcast.   Those 
>> filters  are available  with any
>> cut off freq u want..and  any power rating too...like  25 watt up to 
>> the sky is   the limit.
>>
>> Jim  VE7RF
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>
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