A 1000pf has a 88 Ohm reactance at 1.8MHz, a 4700pf is 19 and a .01 is 9pf.
With a halfway decent choke one or two 4700's will be more than sufficient.
With the 110 uH in a LK-800 I opt for 2 of them as well as the
LK-450/500/500 which has a higher L but the filter voltages are very close
to maximum and they have been known to fail prematurely. Electrolytics do
not like RF. Power out improves a bit but the filament choke is a little
light in L. While that is mitigated by the input network it does waste a bit
of power. Those amps that only have a single 1000pf plate blocker get
another in parallel which cuts down on the tuning drift.
Most 160-10 amps are a compromise....in parts cost mostly.....
Carl
KM1H
----- Original Message -----
From: "Steve Bookout" <steve@nr4m.com>
To: "Carl" <km1h@jeremy.mv.com>
Cc: "Amps" <amps@contesting.com>
Sent: Wednesday, April 03, 2013 6:44 PM
Subject: Re: [Amps] original thread: Peter Dahl transformers, remark about
capacitors for switching supplies
Carl, and company,
Can you bypass the cold end of a RF choke too much?
Say, I put 6 or 8 - .0047 pf caps in parallel to carry the RF current. And,
the plate coke is 180 mh and it's on 160.
What would be the result, other than wasting caps? Just an issue of
'deminishing returns', or substantial effect on tank tuning?
73 de Steve, NR4M
On Apr 3, 2013, at 18:05, "Carl" <km1h@jeremy.mv.com> wrote:
----- 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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