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@telus.net>
To: <amps@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@fsmail.net>
To: Bill Turner <dezrat1242@wildblue.net>, amps@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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