[AMRadio] Swinging chokes

[Mark J. Giubardo]

It is my understanding that placing the choke in the ground side of a power
supply in a bridge or in series with the center tap in the case of a full
wave removes all of the high voltage between the choke coil and its core.
This drastically reduces the chances of failure of the insulation between
the coil and core (a common failure mode). As far as I know, the only
disadvantage is the power transformer capacitance to ground ends up shunting
the choke, and may reduce its effectiveness.

Mark W1MJG

 -----Original Message-----
From: 	amradio-admin at mailman.qth.net [mailto:amradio-admin at mailman.qth.net]
On Behalf Of Jim Candela
Sent:	Wednesday, February 20, 2002 9:16 PM
To:	amradio at mailman.qth.net
Subject:	Re: [AMRadio] Swinging chokes

Hi Guys,

    Just wanted to drop in and say that this discussion has been most
enlightening, and to add a twist to this topic.

    I have a Gonset Linear, the GSB-210 which has a choke input filter power
supply, and this was designed for all modes of operation including AM, and
SSB. I recently replaced the filter capacitors which were 5 times  100 uf @
450 volts in series, or 20 uf. I don't know about what choke is, but it is
"big". I replaced the old caps with 5 times 220 uf @ 450 volts (44 uf). This
was mostly a matter of what can I buy to fit into the old space. Modern caps
are much smaller then 30 years ago given similar ratings. Seems like by
'dumb luck' I did the right thing. I got my caps from Digi-Key in a few
days.

    The Gonset uses the choke on the negative lead. I wonder about this.
Some say the peak voltage the choke has to withstand is reduced this way.
The Gonset uses a FW bridge, and a 3K 80 watt resistor in series with the
Choke (power up & tune position) that gets shorted out in the operate mode.

    I never used negative lead filtering on my AM rigs because I normally
grounded the transformer CT, and use two 3B28's in a FW-CT rectifier
arrangement. I am afraid that a transformer designed to have the CT grounded
might arc out with negative lead filtering. Would a spark gap from the CT to
ground be advisable, or an R-C snubber across the choke to absorb the back
EMF kick when the choke field collapses?

Regards,
Jim candela
WD5JKO

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