Tonight I ransacked my parts bins and found a 3/8" dia ferrite rod about 4 inches long. I covered it in two layers of quality electrical tape, and wound a bunch of pairs of turns from two spools of identical enameled magnet wire about #18 or#20. Covered about 3 inches with close turns, with a half inch open on each end. Secured the wire at the ends with superglue, then covered it all in a sleeve of heatshrink tubing.
Each winding measured about 135 uH on my LC meter, with about 0.2 ohms of DC resistance. I then put it across the high-Z output of my URM-25 signal generator and tuned the spectrum. I saw no dips anywhere, and the coil seemed to load the output down about the same amount as a 2k resistor. I also tried putting the coil in series with various resistance values, and at 1.9 MHz it appeared that the voltage division was even when the resistance was around 1.5k ohms. The calculator says that a 135 uH coil would have a reactance of 1611 ohms, so this data all hangs together. I think I've got a bifilar filament choke suitable for my untuned input 160m amp. Steve WD8DAS [email protected] http://www.wd8das.net/ --------------------------------------------------------- Radio is your best entertainment value. --------------------------------------------------------- -----Original Message----- From: [email protected] To: [email protected] Cc: [email protected] Sent: Wed, 29 Apr 2009 5:48 pm Subject: GB> Re: Filament choke Excellent ideas today, guys - thank you very much. I 've got 100 watts + of drive, so I don't believe I'll use a tuned input in this design, instead I'm going to swamp the input to the cathode with 15 1.5k 2W resistors as was successful in the SB230. So the cathode/filament/heater power circuit just needs to look open to the RF but pass the low frequency AC to light it up. Steve WD8DAS [email protected] http://www.wd8das.net/ --------------------------------------------------------- Radio is your best entertainment value. --------------------------------------------------------- ************ The Glowbugs E-mail List Website is: *************** *********** http://www.mines.uidaho.edu/~glowbugs/ ************** *** Send subscribe and unsubscribe requests to this address: **** ************** [email protected] ************** ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list Searchable Archives: http://www.mail-archive.com/[email protected]/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Post: [email protected] To unsubscribe, send an email to [email protected] with the word unsubscribe in the message body. This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
