[AMPS] Problem with Plate Choke

[Phil Clements]

That particular B&W choke is used mostly by broadcast stations.
It is inside a PVC cover, and is wound over a ferrite core. Every
one I know of that has been in an amateur amp has failed on one band
or another; even on 160 meters!

(((73)))
Phil, K5PC



> None of these characteristics was exceeded (1KW output, 4KV DC under
> 800mA).
> 
> I can't understand the reason of this incident.

The plate choke was "series resonant". In that mode, the choke 
acts like back-to-back L networks. The capacitance at the center 
of the choke forms the capacitor of the L networks.

That makes the choke have extremely high voltage at the center, 
and extremely low impedance from end-to-end.

The choke can also go into higher order modes, where it acts like a 
longer series of back-to-back L networks.

If you measure the chokes impedance, while the choke is mounted 
in place, you will almost certainly find the impedance is very low at 
ten meters. One solution is to remove turns at the point where 
stray capacitance is causing the problem. You can find that point 
by running a screwdriver blade near the choke and watching the 
resonance on ten meters. When you find the hot spot, the 
resonance will suddenly shift.

Adding or removing stray capacitance in this "hot" area will have 
the largest effect on the bad spot while affecting the rest a 
minimum amount.


73, Tom W8JI
w8ji@contesting.com

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