----- Original Message -----
From: "Bill Turner" <dezrat1242@yahoo.com>
To: "Amps" <amps@contesting.com>
Sent: Wednesday, April 03, 2013 2:35 PM
Subject: Re: [Amps] original thread: Peter Dahl transformers,remark about
capacitors for switching supplies
ORIGINAL MESSAGE:
On Wed, 03 Apr 2013 11:16:25 -0700, you wrote:
In a conventional 3.2 kV HV supply that I've built for my new amplifier, I
bypassed each
of the electrolytics with an 0.01 uf disc ceramic capacitor. My thought
was that there
will undoubtedly be RF leakage back to the power supply, and especially on
160 meters the
regular bypassing of the HV line might not be adequate to keep RF out of
the capacitors.
RF would be another source of heat that could shorten their life.
It didn't occur to me that it might be possible to 'resonate' the
capacitors at some RF
frequency and make the problem worse! I'm interested in the answer to
this, too.
REPLY:
Just a suggestion: I think it would be better to focus on keeping RF out
of
the power supply in the first place. Check with an oscilloscope to see if
there is a problem to begin with. You might be trying to solve a problem
that doesn't really exist.
Even if the RF is there, the HV line should be easy to filter.
My 2 cents.
73, Bill W6WRT
Start with the 160M lowest reactance disc cap you can scare up at the base
of the plate choke. A .0047 is adequate for a sane power level and a good RF
choke.
Then add a healthy glitch resistor and bypass the other end also.The
resistor has inductance and is self resonant somewhere in the 50-150 MHz
range for the 20-25W variety. For the 3-500 and similar I also use a 10 Ohm
20W in series and .001 bypassed as that is good for the parasitic frequency.
Some amps have an unstable layout and need all the help they can get; in a
SB-220 RFC-2 is replaced by that resistor.
Any further potentional damage to diodes and filter caps comes from
switching spikes and each complete string should be bypased with an adequate
.005/.0047 disc and another to ground at the HV output point.
Some add a 1N5408 reverse connected across each cap as a crowbar and a very
fast fault trip.
Carl
KM1H
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