RF choke for SB220 (RFC-5)

[Murray Leshner]

I agree with BC -

Not having a schematic nearby, I can think it can only be in one of 2 places
- if it really IS the main series plate choke, clearly it will not handle
the plate current.

If it is actually the shunt choke on the output of the amplifier, it is
supposed to present a short circuit in case the DC blocking capacitor
shorts. Such a failure would allow B+ to be fed to the feedline. The shunt
choke is supposed to blow the breaker by shorting the B+ present due to the
DC blocking cap fault. I don't know of a way to calculate how robust it has
to be other than having at least the current rating of the original.

Under normal conditions, the output shunt choke "wants" to have as high an
impedance as possible because it's just standing off the RF, and you want to
'give up' as little of your output signal as possible, as well as not
wanting the choke to 'cook'.

2.5 mH looks like a 55k ohm reactance at 3.5 MHz and 471k ohm at 30 MHz, so
80 M is our worst case.

800 W into 50 ohms = 200 V; 200 V/55kohms = 3.6 mA of RF current. There is
never any DC thru the output 'safety' choke unless the DC blocker fails
shorted.

As far as substituting a different winding arrangement, I don't know what
the self-resonance of the original is like, but a sectioned winding helps
avoid problems. I don't think it's as critical as the series plate choke,
because I've never heard of a problem with self-resonance on a safety shunt
choke. It's a major problem with series plate chokes. If you do have
self-resonance in a ham band, that 55k-471 k impedance goes out the window
and it's going to cook.





>From: Brian Carling <bcarling at CFL.RR.COM>
>Reply-To: bcarling at cfl.rr.com
>To: HEATH at LISTSERV.TEMPE.GOV
>Subject: Re: RF choke for SB220  (RFC-5)
>Date: Sat, 26 Jun 2004 18:44:02 -0400
>
>Hi Joe,
>
>If I were you I would NOT try a plate choke that is only rated at 70 mA in
>a Heath
>SB-220. It will open up in no time with that amplifier
>
>If you are using it at some low level stage like a grid circuit or the osc.
>plate, then
>it may do OK.
>
>On 25 Jun 2004 at 10:46, joe wrote:
>
> > Found a RFChoke in Mouser catalog and going to try it
> > and see how it works out.  its part nr 542-70F103, 1000 uf,
> > max ohms 24.0 and max current is 70ma ...
> >
> > Thanks to all!
> >
> > 73 de Joe w0tut
> >
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