RF choke for SB220 (RFC-5)

[Mark Graalman WB8JKR]

 Gentleman,

 The choke he referred to is the grid choke that
provides a DC path to ground for grid current in
one tube, 70 ma should be fine, although 100 ma
to 150 ma would be better if he does a lot
of CW or RTTY.

 Personally, if it where me I'd get rid of both chokes
and the bypass caps tie the grids to chassis ground,
and add about a 10 ohm negative feedback resistor
in the cathode.

Mark  WB8JKR

On Sun, 27 Jun 2004 06:16:55 -0400 Murray Leshner
<murrayatwork at HOTMAIL.COM> writes:
> 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.

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