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 Post subject: RF plate choke
PostPosted: Sep Sat 21, 2013 11:34 pm 
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Joined: Sep Sat 06, 2008 6:17 pm
Posts: 841
Location: Central Pa, 17044
Hi Guys, I am in the process of building a linear Amp.
http://www.w7ekb.com/glowbugs/projects/JunkerAmp.pdf
Arnie,you said building chokes is easy. Ok, I have a ceramic form
it is 1/2 in. dia. 5 1/2 in long and the walls about 1/8 in.
I would like to wind a 90 to 100 uh 500ma. plate choke.
How many turns, and what wire size?
Thanks, Gary


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 Post subject: Re: RF plate choke
PostPosted: Sep Sun 22, 2013 3:06 am 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 6115
Location: Santa Rosa, CA
You will need about 230 turns on a half-inch form; assuming the total coil length is about 3.5 inches. I would use about #26 AWG.

http://www.daycounter.com/Calculators/A ... ator.phtml

Rich


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 Post subject: Re: RF plate choke
PostPosted: Sep Sun 22, 2013 4:00 pm 
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Joined: Sep Sat 06, 2008 6:17 pm
Posts: 841
Location: Central Pa, 17044
Thanks Rich, that will help :!:
Gary


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 Post subject: Re: RF plate choke
PostPosted: Sep Sun 22, 2013 4:32 pm 
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Joined: May Sat 22, 2010 4:42 pm
Posts: 2975
Amplifier plate chokes are a bit of a challenge, you want to make sure it has high impedance across the frequency range. This is not an obvious or easy thing to achieve.

One thing you can do is to look at successful plate chokes. Some are plain flat-wound on a long ceramic form, others in like 10 to 80 meter transceivers have like 3 or 4 pie-wound sections of various sizes, to better span the frequency range.


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 Post subject: Re: RF plate choke
PostPosted: Sep Sun 22, 2013 5:09 pm 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 6115
Location: Santa Rosa, CA
I don't want to over-simplify the plate choke issue. There is a bit of an art to it. You don't want to get self-resonance at any of the frequencies that you will be transmitting. That can burn out a choke (and an amplifier) pretty fast.

Too bad I sold an Ohmite Plate Choke last year. Would have been ideal.

http://www.w8ji.com/rf_plate_choke.htm

Rich


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 Post subject: Re: RF plate choke
PostPosted: Sep Sun 22, 2013 6:27 pm 
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Joined: Sep Sat 06, 2008 6:17 pm
Posts: 841
Location: Central Pa, 17044
Hummmm, That is what I was going to use as a form, A 1 ohm OMITE power
resistor..Now what?
From the pix of the prodject the plate choke looks like a single wire wound one.
But I'm certainly not an expert on chokes. I don't want to smoke anything.
Is there anyone here who might have something in the 90uh 500ma range
they would sell?
Thanks for any help, Gary


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 Post subject: Re: RF plate choke
PostPosted: Sep Sun 22, 2013 7:13 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1564
Location: Saskatoon
Rich, W3HWJ wrote:
You will need about 230 turns on a half-inch form; assuming the total coil length is about 3.5 inches. I would use about #26 AWG.

#26 wire has a diameter of 0.0159 inches. 230 turns would be more than 3.5 inches, even before you take insulation into account. Including enamel insulation, the wire diameter will be more like 0.0175 inches, so 230 turns would be 4 inches long. Given this overall wire diameter, you would need 280 close spaced turns to get 100uH, and the winding would be 4.9 inches long.

If you separate the winding into smaller sections as shown on the www.w8ji.com site, then that will reduce the inductance even more, and you will need more total turns.


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 Post subject: Re: RF plate choke
PostPosted: Sep Mon 23, 2013 12:04 am 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 6115
Location: Santa Rosa, CA
Thanks, Bob. I didn't figure out the total length, as I assumed that the 5 1/2 inch form he mentioned earlier would be enough length.

Certainly a slightly smaller wire gauge could be used.

Rich


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 Post subject: Re: RF plate choke
PostPosted: Sep Mon 23, 2013 1:11 am 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 6115
Location: Santa Rosa, CA
I still have a new (NOS) Ohmite Z-7 choke. Wasn't able to sell it last year.

viewtopic.php?f=15&t=183026&hilit=ohmite+choke

If you are willing to pay for a small USPS flat rate box (about $6.00) I will send it to you. Maybe I have a door knob HV capacitor, also. I built that same amplifier around 1973, then dis-assembled it about 10 years later.

If you do a Google search (Images) for "Ohmite Plate Choke" you can see the photo. The image was dropped by ARF when they had computer troubles, but Google still has it. The choke is about 6 inches long in green ceramic.

Rich


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 Post subject: Re: RF plate choke
PostPosted: Sep Mon 23, 2013 1:17 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1564
Location: Saskatoon
If you use #28 wire, you could wind 3 sections of 85 turns each, then have a 1/2 inch gap between each section. That would give 33uH per section for a total of at least 100uH. (It will be slightly higher than 100uH due to mutual inductance between sections.) Each section would be 1.2 inches long so the total length will be 1.2+1.2+1.2+0.5+0.5 = 4.6 inches which would fit the 5.5 inch form.

Or, 4 sections of 65 turns each (#28 AWG again) with a 1/2 inch gap between each section. This would give about 25uH per section for a total of at least 100uH, and total length of 5.1 inches.


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 Post subject: Re: RF plate choke
PostPosted: Sep Mon 23, 2013 2:19 pm 
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Joined: Sep Sat 06, 2008 6:17 pm
Posts: 841
Location: Central Pa, 17044
Rich, Please send me a PM..
Gary


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