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Officially starting the 24 FET build




 
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Author Topic: Officially starting the 24 FET build  (Read 3123 times)
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N1XBM
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« on: January 05, 2018, 10:15:08 AM »

I know some people on here have heard me for years talking about how I'm going to build a 24 FET rig. My plan is to build a 160/80 rig and do PDM and I plan to use a flex 1500 which should give me my 1 watt of drive. I have a flex 1000 which will work too...anyways. Over the years I have just been collecting parts. I plan to follow the classeradio.com website.

So I am going to lay out the FQA11N90 and the IXDD614 and C2, C4 and C6, I see they are .47uf, but what should I be looking for on voltage rating? I see 12v 2a, would a minimum 16v cap be OK?

Also same thing on the the shunt cap 1500 pf I see a ATC100C with microstrip termination, Where are people getting these? I am striking out using google.

I guess this will be step one for me. When I have a completed RF deck I think there enough people in New England, hopefully I'll be able to bring it to someones house to test it.
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steve_qix
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« Reply #1 on: January 06, 2018, 11:06:14 PM »

Ok, I have made some minor, but very important modifications to the circuit since the initial schematic was published.

So please get the new schematic for the 24 FET RF amp from the class E web site.

http://classeradio.com/24_fet.htm

The differences involve putting a 300 ohm resistor in series with each driver input, and documenting the fact that a single 12 or more amp power supply may be used for the drivers, rather than an individual supply for each module.

The resistors are very important.
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ka1tdq
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« Reply #2 on: January 06, 2018, 11:52:08 PM »

I'm guessing that the reason for the 300 ohm value is that there are 6 drivers connected to one coax feed.  300 ohms / 6 modules = 50 ohms.  

So if a person were to have 2 drivers connected to a single coax feed, they'd use 100 ohms resistors instead (both in parallel with the feed and a 100 ohm resistor going in series to each driver.  Is that right?  I guess the series resistor value wouldn't matter too much since the driver is a high-impedance device.  It's just some amount of resistance to fight parasitics.

And building a 24-FET rig for your first class E rig?  Cool beans!  I built a single FET version for my first try.  It was very useful to experiment with and learn how to operate and tune these rigs.  I tend to take baby steps and am generally cautious in life... I guess that's why I decided to go with the Walmart drone version rather than the F14 fighter jet for my first transmitter.  

Anyway, best of luck with the 24-FET deck!  One of these days I'll build one too.  I mean, why not?  The parts cost like $100 or so.

Jon
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steve_qix
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« Reply #3 on: January 07, 2018, 04:40:16 PM »

More or less, yes.   For 2 drivers the resistors across the feed lines for the drivers would be 100 ohms eash, so as to terminate the junction of all of the short feed lines in 50 ohms.  But, the series resistors are still 300 ohms.
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N1XBM
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« Reply #4 on: January 07, 2018, 06:40:26 PM »


So I am going to lay out the FQA11N90 and the IXDD614 and C2, C4 and C6, I see they are .47uf, but what should I be looking for on voltage rating? I see 12v 2a, would a minimum 16v cap be OK?

Also same thing on the the shunt cap 1500 pf I see a ATC100C with microstrip termination, Where are people getting these? I am striking out using google.


OK so I have tbd schematic, but I still can't answer my original question  from my original post.
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ka1tdq
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« Reply #5 on: January 07, 2018, 07:50:31 PM »

I've had trouble locating ATC capacitors.  Instead, I've used Russian doorknob capacitors.  They're not nearly as compact, but I mount them above the FETs using #14 solid wire since it will hold them mechanically.  They're also dirt cheap and will handle about a million KW. 

My 2 cents on the .47uF's is that 16 volts or above should work fine.  Just don't go too big since space is a premium and it gets really crowded on that little copper strip really fast. 

...and just so you know, I'm not the expert on this stuff.  I've just built a few that happen to work.  That's all.

Jon
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steve_qix
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« Reply #6 on: January 07, 2018, 08:33:48 PM »

Oh, on the driver bypass caps, 16V would work.  Make sure they are high current capacitors with a low ESR.

On the shunts, I would stay away from doorknobs.  I have had numerous failures of doorknobs - the good ones - and 100% failures with the not-so-good ones.

If you can't find ATC caps (I get them directly from ATC or on Ebay), you can use 2 of the Russian silver-mica 500V capacitors.  They work quite well.  Get 2 750s or whatever you can get.  A 1000 and a 500 will work in a pinch.  I've used them on a number of class E rigs and they work.

I just found a listing for these on Ebay 750pF 500V.  Perfect.  2 per module.  Try to buy a lot of them - you get a better price.


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KD6VXI
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« Reply #7 on: January 07, 2018, 10:19:54 PM »

Not sure why ATC caps are hard to get.

RF Parts sells them directly.

Now, if you meant, hard to get at a realistic price, I feel ya.

Unelco also makes the same style.  Unelco Metal Clad Mica caps.

https://www.surplussales.com/capacitors/UnelcoMtl.html



Hope it helps.  Surplus sales has about any value you want below 600 puff.  And in damn near every case style imaginable.

Ebay metal Clad Mica for better deals.

--Shane
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N1XBM
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« Reply #8 on: January 08, 2018, 10:55:39 AM »

OK all, I think I've found what I need, for some reason .47uf caps at 16v seem to be hard to find in surplus sources. I can find 25volt, but 16v not so much.
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steve_qix
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« Reply #9 on: January 08, 2018, 05:36:42 PM »

I use 100V caps in my own rigs because that's what I had around.
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N1XBM
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« Reply #10 on: January 11, 2018, 08:08:56 AM »

OK well I've been working on the heat sink for the last couple of nights for an hour each. I've got most all of the parts needed to complete the heat sink. I'll post a picture where there is something worth looking at.
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N1XBM
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« Reply #11 on: January 15, 2018, 09:51:55 AM »

Things are coming along nicely, at least it feels/looks like it. At this point I have a big hunk of heatstink and and copper bus strips. Everything is laid out and then holes are marked with pencil and sharpie. I'm getting ready to drill and tap. I was looking at the FQA11N90s and I see they need a thermal insulator, I noticed in my searches there is a also a little washer. Should I also be drilling my holes out to accommodate this washer/insulator? Or more importantly are people using them? I can't seem to glean this information looking at the class e website or my internet searches.
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« Reply #12 on: January 15, 2018, 02:03:38 PM »

Hi Bob,

FQA11N90s require an insulator and nothing else.

I use 4-40 screws to hold down the FQA11N90s.  Some guys drill out the hole in the FQA and make it accommodate a 6-32. I don't recommend this, but it appears to work.

 
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KC2UDZ
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« Reply #13 on: January 17, 2018, 08:03:05 AM »

Try these guys http://www.passiveplus.com/
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n1ps
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« Reply #14 on: February 04, 2018, 09:02:09 PM »

Is it done yet? Shocked Shocked
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