[Boatanchors] Resonant Choke Filters (was Please review &

[k9muf at comcast.net]

Right on Rob.
I have all of those reference manuals plus a few more.

Don,  K9MUF

----- Original Message -----From: Rob Atkinson <ranchorobbo at gmail.com>To: Rick Poole WA1RKT <wa1rkt at comcast.net>Cc: Boat Anchors List <boatanchors at mailman.qth.net>Sent: Tue, 06 Jan 2015 01:13:40 -0000 (UTC)Subject: Re: [Boatanchors] Resonant Choke Filters (was Please review &

On Mon, Jan 5, 2015 at 7:37 AM, Rick Poole WA1RKT <wa1rkt at comcast.net> wrote:> Thanks, Gary. But, I'm looking for something I can look up on the web. I'm just spoiled by all this here modern day technology, I guess.>

That's your problem. You are looking for a quick answer. If you aregoing to be serious about vintage equipment, you need to build avintage reference library of quality authoritative information. Theinternet is full of garbage requiring a mental crap filter, but thatcan only be developed after reading texts. Any moron can throw junkup on a website. Go to hamfests, look for books by George Sterling,Fred Terman, old west coast Radio Handbooks edited by Orr and earlier,old RCA tube manuals, GE tube manuals, QSTs and other radioperiodicals from before 1960, ARRL handbooks pre-WW2, Radiotrondesigners handbook (the Red Bible) and if you see a copy of the IT&TReference Data for Radio Engineers, grab it. There is usually atleast one person at a hamfest with boxes of books and 95% of hamsignore it because they don't know what end of a soldering iron to pickup.

Don't overlook Electric Radio, probably the best English languagehobby radio periodical published today. Get all the back issues ofthat you can. Speaking of ER, I see in issue number 94 from February1997, an article on the design of the Collins 30K by the guy whodesigned it, Warren Bruene. Let us look at page 13 on the powersupply from which I will quote:

The high voltage DC power supply was rated for 2500 VDC and 300 ma.It employed a conventional single-phase full-wave circuit using a pairof 866A mercury vapor rectifier tubes. A two section DC filter wasused using a 12 Hy filter choke and a 2 mfd filter capacitor in each.In addition, the input choke was tuned to 120 Hz with a 0.10 mfdcapacitor. At that time it was common practice to use a swingingchoke in the first section whose inductance varied from 25 Hy, withjust bleeder current load, down to 5 Hy at full load current. Collinsengineers had been using the tuned choke idea for several yearsbecause it reduced hum better and had better transient response to avarying load such as the keyed final.

With an untuned input choke, there is a minimum "critical" inductancerequired to prevent the power supply from starting to act like acapacitor input filter. When this happens, it causes the no-loadvoltage to rise resulting in poorer regulation.

The maximum load resistance is just that of the bleeder resistance(100,000 ohms) when the final is keyed OFF. The value of criticalinductance in the single phase full-wave rectifier circuit is Lc =R/1130 = 100,000/1130 = 88 Hy.

Instead of an 88 Hy choke we tuned the 12 Hy input choke to 120 Hz toachieve the same result and with a much shorter transienttime-constant. The choke is rated for 12 Hy at full load current of300 ma. The inductance of this choke is approximately 18 Hy whenpassing just the bleeder current of 25 ma. Therefore it takes acapacitor of only 0.1 mfd to tune it to 120 Hz using 60 Hz primarypower. The best value can be found experimentally by measuring the DCoutput voltage using different values of tuning capacitance. The bestvalue produces the lowest DC output voltage.

The transient voltage across the choke may be up to double the DCoutput voltage. That is one reason why this capacitor has a DCvoltage rating of 5000 volts. The second reason is that there is ahigh value of 120 Hz current circulating in the resonant circuit.This creates a significant amount of heating. It has been ourexperience that a paper dielectric filter capacitor with a DC voltagerating of twice the DC power supply output voltage is sufficient.Plastic film capacitors available now heat less.

See what you miss only looking online? Subscribe to Electric Radio today.

73

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