The AM Forum
October 09, 2024, 12:09:08 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: Filament choke  (Read 5992 times)
0 Members and 1 Guest are viewing this topic.
wd8das
Member

Offline Offline

Posts: 165


« on: April 29, 2009, 02:37:35 PM »

I'm building a 160m grounded-grid amp around the GI-7B tube
(it worked so well in my SB-230 retrofit project:
http://www.wd8das.net/SB230.html    )

Anyone know of a source for a filament choke appropriate for this tube?  The filament draws only 2 amps, so the 30-amp ones I see available online are really overkill (and awfully large).  But for 160m I'm thinking that a somewhat higher inductance than might normally be used for 80-10m amps might be in order.  Say >50 uH?.  Bifilar-wound would be best, but two separate chokes would probably work too.

Should I not find a ready-made source, then I guess I could make one if I find some ferrite rod suitable for use at 1.9 MHz.

Thanks...

Steve WD8DAS





Logged
K1JJ
Contributing
Member
*
Offline Offline

Posts: 8886


"Let's go kayaking, Tommy!" - Yaz


« Reply #1 on: April 29, 2009, 04:55:08 PM »

Steve,

Slip a stack of ferrite beads over both wires.  ie, Feed both fil wires thru the same beads.  No need for turns.

Get the appropriate permability beads for the 160-10M band.   This choke shud be at least 10X the filament input impedance for the lowest band.


If the amp works on the lowest band, then the choke is OK.

I've done this on seven linears here in the shack and works FB. 

Use standard bypass caps like any bifilar choke.

I posted a way to test the choke using a sig gen and a load in another thread, if you're interested.

T
Logged

Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
WA1GFZ
Member

Offline Offline

Posts: 11151



« Reply #2 on: April 29, 2009, 07:32:32 PM »

Type 77 /73 would be good for 160 but type 43 will be fine just need more of them. Size the pair of wires for the current then find a bead that slips over the pair.  You can test it with a low lwvel signal source. Say your input Z is 100 ohms. Take the choke and put a 100 ohm resistor (noninductive) in series. Put it across a signal source and measure the RF voltage across the resistor and then across the choke. If most of the voltage is across the resistor then the choke inductance is too low. When most of the voltage appears across the choke you have enough beads. Teflon insulated wire will work the best.
Logged
wd8das
Member

Offline Offline

Posts: 165


« Reply #3 on: April 29, 2009, 10:48:42 PM »

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
 
Logged
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 8290



WWW
« Reply #4 on: April 29, 2009, 11:23:18 PM »

Somewhere around here is a coaxial filament choke made of 1/4" copper tubing with a #10 through it, insulated. It's in the shape of a coil about 2"D by 6"L and has many taps. Sick, I tell you!! Who would create such an abomination? Might be commercial, I don't see hammy hambone making something like it. Not sure if it is going to be used or not, saved it from a box of junk going into the recycler.
Logged

Radio Candelstein
w4bfs
W4 Beans For Supper
Contributing
Member
*
Offline Offline

Posts: 1432


more inpoot often yields more outpoot


« Reply #5 on: April 30, 2009, 07:12:11 AM »

Patrick .... sounds like the input coil described by Bill Orr in his initial 3-400 or 3-500 amp .... the coil was resonated with a 3 section bc variable to both match/resonate the input and provide dc return ...gnarly ...73...john
Logged

Beefus

O would some power the gift give us
to see ourselves as others see us.
It would from many blunders free us.         Robert Burns
WA1GFZ
Member

Offline Offline

Posts: 11151



« Reply #6 on: April 30, 2009, 12:30:40 PM »

I have the choke from the GPT 10K. 3/8 over 1/4  inch copper tubing air wound teflon tube insulated.
Logged
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 8290



WWW
« Reply #7 on: April 30, 2009, 11:59:05 PM »

3/8" ? Bet that was not easy to wind. The smaller one I have also - -likely was made on a machine. I need to find that thing and shelve it, I am sure it is in a pile of junk in the garage. The Bill Orr article - have to look that one up.
Logged

Radio Candelstein
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.063 seconds with 18 queries.