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Author Topic: Junker Amp  (Read 2516 times)
N8CHR
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Posts: 34




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« on: July 31, 2012, 07:24:35 AM »

I need to make an RFC for the filament on an 813. Schmatic calls for # 14 ga wire, Formvar or Nylclad on ferrite core. I was going to use a 14 guage enamel but don't want to buy it. Can I use 14 ga house wire with the insulation on? Will it change the final value much?  Undecided
Thanks Tom N8CHR
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KA5N
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Posts: 4380




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« Reply #1 on: July 31, 2012, 08:10:34 AM »

Are you building "The Junker Amplifier" described by Lew McCoy in QST forty or so years ago?

If you use insulated stranded wire the inductance will be less depending on the thickness of
the insulation.  So at the very least you would have to use more turns of wire and probably
a longer rod.  Would it work anyway?  Probably.   Were I you I would go see if I couldn't
talk someone at a motor rewinding shop out of some formvar insulated wire.  Even used wire
would be OK if it wasn't kinked too much and the insulation was OK.
This is just an exercise in "real homebrewing" and you learn by doing.
Good Luck
Allen KA5N
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AD4U
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Posts: 2157




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« Reply #2 on: July 31, 2012, 08:11:49 AM »

As an amp builder from way back I have to ask why?  No offense but why try to save a dollar by not doing it right.  Most any motor rewind shop will probably give you a scrap piece of #14 enameled wire 3 feet long to make the filament choke.  

Using regular plastic insulated #14 house wire will probably reduce the inductance of the filament choke.  It is anybody's guess as to how much.  

If you must use house wire, I would recommend that you add a "few" extra turns compensate for the loss in inductance you would have had by using the right stuff.

About all anybody can suggest is to try it and see how (if) it works.

Dick  AD4U
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N8CHR
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« Reply #3 on: July 31, 2012, 08:55:49 AM »

I shall go out into the shop and chase down our maintanance man. I'm sure we have plenty of old motors lying around. Work at a sheet metal press room. Thanks for the replies. Really didn't want to use insulated wire. Thought it would look out of place. And yes, it is the old junker amp. Got almost all I need. Not sure of the power transformer. Got 750 vct but by the looks of it don't think I have thw current rating. It'd kinda small only weight 6 pounds or so./
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K1ZJH
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Posts: 949




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« Reply #4 on: July 31, 2012, 10:41:08 AM »

Critical considerations are having enough reactance at the lowest frequency, and
measuring the filament voltage at the tube pins to verify it is within specs.

Pete
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AI7RR
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Posts: 164




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« Reply #5 on: August 01, 2012, 08:49:47 AM »

Getting to know the folks at the local motor rewind shop can be very beneficial. I scored about 100 feet of #12 magnet wire, just for the asking. Of course, a dozen donuts in trade doesn't hurt either.

73 and good luck with your project.

Roger
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K4DPK
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Posts: 1077


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« Reply #6 on: August 02, 2012, 08:36:35 PM »

If you absolutely can't find any enamelled wire, you can use two equal lengths of wire, but remove the insulation from one of them.  When you wind them as bifilar, the insulated one will be in between each wrap of the bare one.  This will remove half of the insulation thickness that would otherwise reduce the total inductance, so the effect isn't so profound.

I wouldn't worry an awful lot about losing a few tenths of a microhenry.  Back when we were still winding filament chokes on broomhandles, (many of us had never seen a ferrite rod or a low-capacitance filament transformer, for that matter) some perfectly good amps were built without any filament choke at all.  Some of us made the mistake of using bypass capacitors on the filament transformer, but when those were removed, it actually did work.

Just put as much wire as you can on the rod you have, and try it. 

I congratulate you on undertaking this project.  I think you're gonna learn a lot of very useful stuff.

Phil C. Sr.
k4dpk (1955)
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