[Amps] original thread: Peter Dahl transformers, remark about capacitors for switching supplies

[Carl]

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 at nr4m.com>
To: "Carl" <km1h at jeremy.mv.com>
Cc: "Amps" <amps at 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 at jeremy.mv.com> wrote:

>
> ----- Original Message ----- From: "Bill Turner" <dezrat1242 at yahoo.com>
> To: "Amps" <amps at 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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