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

[Steve Bookout]

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
> 
> 
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps