[Boatanchors] Resonant Choke Filters (was Please review & comment on my Heath Warrior HA-10 amplifier design changes)

[Rick Poole WA1RKT]

Good afternoon, Gary.

Well, Collins engineers didn't seem too concerned about the high voltages across the choke... the tuning capacitor in the 516F-2 is rated at 1000 volts for a power supply that puts out 800+ volts under no or light load, and they have lasted for a very long time.  Note that you do have to pay attention to the AC rating of the tuning capacitor, not so much the DC rating.  I measured the voltages across the tuning capacitor on a 516F-2 under no load and got 460 volts AC and essentially zero volts DC across the cap.  Of course I wasn't able to detect any momentary high voltage spikes, nor have I tried monitoring the voltage across the cap during normal transmit-receive operation.  But as I said, Collins figured 1 KV was enough and history seems to have borne them out.

As for not using a swinging choke as a tuned choke... OK, I don't mean to appear doubtful, and I'm quite certain you know a lot more about this stuff than I do, but I'd like to see some supporting documentation on this.  It makes a lot of sense that a swinging choke would work just fine in this application, especially since as you noted, a non-swinging choke will change inductance with varying current loads anyway, just not as much.  Perhaps there is a range of current demand that is enough to pull the resonant circuit significantly out of resonance, yet not enough to meet the normal minimum load current that choke-input filter supplies demand, and within that range of currents I suppose the voltage could rise beyond acceptable limits.  Seems like that could be compensated for by reducing the value of the bleeder resistor a bit.

Anybody have any pointers to some more websites that discuss this in more detail?  Google hasn't been especially helpful...

Rick WA1RKT


----- Original Message -----
From: "Gary Schafer" <garyschafer at largeriver.net>
To: "Rick Poole WA1RKT" <wa1rkt at comcast.net>, boatanchors at mailman.qth.net
Sent: Saturday, January 3, 2015 2:21:33 PM
Subject: RE: [Boatanchors] Resonant Choke Filters (was Please review & comment on my Heath Warrior HA-10 amplifier design changes)

Be careful when fooling with a resonant choke. There are very high voltages
developed across the choke when tuned. I don't remember right now but I
think that they can be several times the high voltage applied. You need very
high voltage tune capacitors for it.

You can not use a swinging choke for a tuned choke despite what is said in
that article you supplied.

Somewhere there are some notes by Collins on the tuned choke. Maybe in the
Collins fundamentals of SSB book.

In digging around you will find that some recommend that the choke should be
tuned slightly higher or lower in frequency to prevent the high voltage
across the choke. Don't do that! If you do the choke will swing thru
resonance as current is drawn and you will really get some high voltage
across it. You want the choke to be resonant at the idle current and it will
move out of resonance when current is drawn. The Collins book explains this.

Note: a regular choke (non swinging) will change inductance as current is
drawn thru it but much less than a swinging choke.

73
Gary  K4FMX