Re: [Amps] Plate choke explodes

["Carl" <km1h@jeremy.mv.com>]

Subject: Re: [Amps] Plate choke explodes


> ORIGINAL MESSAGE (may be snipped):
> On Sat, 13 Apr 2013 15:28:13 -0600, Jim wrote:
>
> >  In fact, if the
> > SRF is near 50MHx, you should have very little RF current through the 
> > choke.
> REPLY:
> The problem with a parallel resonant plate choke is not current THROUGH 
> the
> choke, it is the "ringing" or "circulating" current WITHIN the choke. That
> current can be far in excess of the 1 amp rating of the Z-50, especially
> when the plate tank is lightly loaded or accidentally overdriven.
>
> The "Tesla Coil Effect" will suddenly make its appearance.
>
> The best plate choke will have a reactance roughly about ten times the
> reactance of the plate tuning capacitor, and no series or parallel
> resonances near the frequency of operation.   A good value for 50 MHz is
> around 7-10 uH.  This is not a critical value, but don't stray too far 
> away
> either.
>
> More inductance is NOT better, since more turns will probably lead to
> resonances closer to 50 MHz.  All chokes of this type have both series and
> parallel resonances and the trick is to use a small enough choke so the
> resonances are well above the operating frequency and yet the reactance is
> still high enough to not significantly detune the plate tank circuit.
>
> Electronic design is both art and science. That's what makes it fun.  :-)
>
> Bill, W6WRT

The Z-50 is 7uH and the more rugged ones Ive been using for 5 decades since 
I converted a NCL-2000 to 6M are 9 uH. Neither has in circuit resonances, 
good bad or indifferent below 70 MHz.
There is absolutely no reason a single band choke cant be built at home with 
even basic test equipment.
National Radio developed the first broadband choke for shunt feed in the 
40's as the R-175, and modified it as the R-175A in the early 50's when 15M 
became a ham band. This was an expensive to build multi section pie wouund 
choke.
An ARRL lab staff member developed the multi section single layer choke in 
the 60's which was copied by several including a sometimes talented tech at 
Ameritron who added sufficient inductance for 160 and added the WARC bands 
and after a few versions being shipped he got it right....or somebody else 
did and mailed it back.
Carl
KM1H 

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