[Amps] low pass filter fail ;)

[Carl]

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
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 2014.0.4570 / Virus Database: 3920/7407 - Release Date: 04/28/14
>