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A simple power divider for driving a linear amplifier with a 50 watt AM rig




 
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Author Topic: A simple power divider for driving a linear amplifier with a 50 watt AM rig  (Read 4612 times)
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AB2EZ
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"Season's Greetings" looks okay to me...


« on: January 07, 2006, 12:42:45 PM »

Hi!

There are lots of ways to reduce the power output of a 50 watt AM rig to the ~12.5 watt level needed to drive a 3-500Z grounded grid linear amplifier.

WA1HLR (Tim) has provided a nice way that works great on a Ranger... but which does require some minor modifications of the transmitter.

On my Web site:

http://mysite.verizon.net/sdp2/id11.html

I have placed a diagram and a description of a simple power divider that has the following characteristics

a. It requires only a single ferrite core of the type used to build Class E rigs, and a standard 50 ohm dummy load capable of handling at least 50 watts of continuous power (plus some SO-239 connectors and a small Bud box if you build it the way I built mine)

b. All three ports (input from the AM transmitter, output to the dummy load, and output to the amplifier are referenced to the same, common ground.

c. I presents a 62.5 ohm resistive load to the AM transmitter... which should be no problem for most AM transmitters to drive.

It works great for me using my 1-FET 160 meter Class E transmitter (~50 watts output at carrier) to drive my grounded grid 3-500Z linear amplifier.

Best regards
Stu
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ka1tdq
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« Reply #1 on: December 25, 2017, 11:46:42 AM »

Iím interested in this, and Iíve seen it before but the link is no longer good. I might possibly want to drive a linear with my 40 meter QRP E rig.

I know, I know... efficiency, Prius on steroids, 1956 Chevy, blah blah blah.

I really like my 40 meter rig and Iíve never been able to build a stable, high power E rig for the band. When all else fails, throw a Sherman tank after it.

Jon
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AB2EZ
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"Season's Greetings" looks okay to me...


« Reply #2 on: December 25, 2017, 12:26:25 PM »

Jon

Here it is.

Happy Holidays!

Stu


* 1 to 4 Power Splitter.jpg (63.01 KB, 960x720 - viewed 220 times.)
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KA2DZT
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« Reply #3 on: December 25, 2017, 01:08:07 PM »

Jon

Here it is.

Happy Holidays!

Stu

Stu,

Where is the link??

Fred
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ka1tdq
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« Reply #4 on: December 25, 2017, 01:11:30 PM »

Hey Stu,

Merry Christmas to you too!  Yeah, I don't see the link or the attachment either.

Anyway, I've rearranged the shack to put the new Big Bertha in the corner (see attached).  It's hard to tell from the picture, but sitting on top of the cabinet is a piece of 1/4" steel that will be raised 9" from the top with plexiglass all around.  This will allow viewing of the RF deck.  Inside will be the power supply, meters and blower.

Jon


* IMG_3027.JPG (1757.36 KB, 4032x3024 - viewed 142 times.)
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w8khk
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« Reply #5 on: December 25, 2017, 01:25:59 PM »

Jon

Here it is.

Happy Holidays!

Stu

I saw the attached schematic after Stu posted it, but it's not there now.  Strange!
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W2PFY
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« Reply #6 on: December 25, 2017, 03:57:13 PM »

Nothing to add here except that I had to post a comment so I can tag it to give me a holler when someone like Stu updates link. HHTA ie, HAPPY HOLIDAYS TO ALL Grin Cheesy Cool
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W2NBC
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« Reply #7 on: December 25, 2017, 04:54:20 PM »

This was from Stu's page via the "Wayback" machine.
Merry Christmas!



"Above is a drawing of a power splitter that I built to enable my 1-FET class E rig to drive a grounded grid linear amplifier... without dropping the voltage on the FET or mistuning the FET output circuit.

 

The 1:2 ferrite transformer converts the amplifier input impedance (as seen from the primary of the transformer) to 12.5 ohms.

 

The 50 ohm dummy load adds to the 12.5 ohm amplifier load, in series... producing a 62.5 ohm load on the 1-FET FET rig.

 

Meanwile, the input to the splitter, the output to the dummy load, and the output to the amp are all referenced to ground.

 

I built this in a small "Bud box" with three S0-239 connectors mounted on it... and the transformer inside. I used #18 insulated wire for the primary and the secondary windings on the 1:2 transformer. 

 

If the primary (input-to-dummy load) is 1 turn, and the secondary is 2 turns... then 80% of the power goes to the dummy load, and 20% of the power goes to the amplifier.

If the primary (input-to-dummy load) is 2 turns, and the secondary is 3 turns... then 69% of the power goes to the dummy load, and 31% of the power goes to the amplifier."


* powersplitter.jpg.w560h420.jpg (18.7 KB, 560x420 - viewed 92 times.)
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AB2EZ
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"Season's Greetings" looks okay to me...


« Reply #8 on: December 25, 2017, 05:49:26 PM »

Sorry about the delay. The schematic is now attached to my post, above. Also, thanks to Jeff for posting it.

Stu
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ka1tdq
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« Reply #9 on: December 25, 2017, 06:10:16 PM »

Thanks for the posts.  I could go this route or build an attenuator pad, but it's easier to build this than to find really big correct value resistors.

I moved things around in the shack and wanted to show my wife my new project.  I'm sure she was just on pins and needles waiting in anticipation (that's sarcasm).  Anyway, when I showed her what I wanted to do, her first words were, "Is it going to cost much?"  I said no, and quoted her the Alpha linear amplifier price for a commercial model.  Basically, I'm saving $5,000.

Jon

https://www.alpharfsystems.com/?p=1467
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KB2WIG
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« Reply #10 on: December 25, 2017, 09:31:01 PM »




......... you say she has a rolling pin?


klc
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K1JJ
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« Reply #11 on: December 26, 2017, 12:53:23 AM »

Ah yes, the $5K Alphas...

I've got this thing against cheap knobs. Most radio marketing people have no idea how much hams love to look at, fondle and twist knobs. Cheap knobs can make or break a great radio.

Example:  Look at the cheap "Radio Shack" junk knobs used on that $5K Alpha. Disgraceful.

Now look at a class act, Art Collin's  30S-1 amplifier.  He knew how to make a rig look manly.

The Japanese did a great job with the Kenwood and Icom stuff too. They made their plastic panels look almost metal US military. Magicians.

And, check out the knobs on Dr. Love.

T






* Cheap Knobs Alpha.jpg (40.35 KB, 653x372 - viewed 104 times.)

* 30S1 -Real Man's Knobs.jpg (82.33 KB, 640x480 - viewed 108 times.)

* 4X1 Rig 501.jpg (317.87 KB, 960x1280 - viewed 127 times.)
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ka1tdq
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« Reply #12 on: December 26, 2017, 11:49:03 AM »

I bid 1 cent (plus $39 shipping) on a Russian GU-40b tube on eBay and won. It looks like thatís going to be the tube. Itís a tetrode so Iíll configure it similarly to a 4-1000 grounded grid amp. The socket is unobtanium so Iíll have to make something.

Knobs are just as critical as any other component. The last amp I build used solid aluminum. I like the weighted feel.

Jon
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ka1tdq
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« Reply #13 on: December 29, 2017, 06:31:04 PM »

Actually, the linear is going to be cost prohibitive. The Russian tube isnít sexy and Iíd want to go with 3-500ís again. Iíd also need to run 240 into the shack.

So, I guess itís another class E attempt since I already have all the parts, aside from a few Home Depot hardware runs. I was able to get a single-ended rig to work on 40 meters, so this time Iíll parallel 4 FETs in single ended mode to see what happens. It should be good for 200 watts carrier (good enough for 40 meters).

Jon


* DB4566FD-8F65-4F2D-B3A7-B147A372B552.jpeg (2298.73 KB, 4032x3024 - viewed 83 times.)
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KD6VXI
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« Reply #14 on: December 29, 2017, 06:47:49 PM »

Just buy a gs35b and get it over with

A hundred bucks.

No socket

Uses a radio shack (oh well) filament xformer.

Has more dissipation than a pair of 500Zs.  The way the CBers run them, it's more like 2500 watts PDiss.

Oh, and a single tube costs about the same as a single 500z.  Without sockets.

And if you ever want a spare, the spare is cheaper since the cooler is removable.

Stop trying to reinvent the wheel.    😀


--Shane
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ka1tdq
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« Reply #15 on: December 30, 2017, 04:34:41 AM »

It comes down to economics mostly.  I literally have every component required to build another E rig, down to the resistors.  For an amplifier project, just running 240 volts to the shack (properly) would be expensive in my situation.  Back in the day I just paralleled the kitchen stove, but I don't want to do that again.

Also, since I'll be looking at this everyday, I don't want some boring ceramic tube.  I liked the look of my previous 3-tube 3-500 amp, but a plexiglass enclosure would've been awesome!  All being said, the project would cost about $2000 to do it the way I want to. 

This E rig will cost me about $50 with just a few hardware trips to Home Depot.

(with the added benefit of not having a furnace in the shack during the summer)


Jon
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« Reply #16 on: December 30, 2017, 01:59:18 PM »

What is the number and source for the ferrite core used in this  ferrite transformer?

I have no experience working with any solid state transmitters and related parts so therefore, the need of procurement of above.

Thanks Terry
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ka1tdq
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« Reply #17 on: December 30, 2017, 02:35:02 PM »

Similar to Steveís transformer published for the 400 watt RF deck, but using 3 stacked cores instead of 4 (less power so less metal) and still using 4 rows. 1:1 turns ratio. But instead of two phases 180 degrees apart, it will just be one phase. Iím guessing it should work. Close enough, all that stuff.

Jon
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ka1tdq
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« Reply #18 on: December 30, 2017, 02:53:55 PM »

...eh, I guess a schematic is easier to explain.

This is what I'm thinking:

4 FETs in parallel, each with its own driver.  The output is transformer coupled with a 1:1 transformer using a total of 12 FB43-1020 cores.  

The output network uses typical values.

...and I forgot to draw in the shunt capacitor.

Jon


* IMG_0005.jpg (1881.89 KB, 4032x3024 - viewed 64 times.)
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AB2EZ
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"Season's Greetings" looks okay to me...


« Reply #19 on: December 30, 2017, 04:23:20 PM »

Terry

If you are asking about the ferrite core that is used in the power divider:

It is not very critical. There is no DC current in either winding... and you are not concerned about a little bit of loss. However, you still have to be careful not to saturate/heat the core with too much H-field on RF peaks.

I use type 43 (material) ferrite cores that are approximately 1.125 inches long, 1 inch in outer diameter. and which have a hole through the center that is 0.5 inches in diameter. Those are the cores that are typically used to build transformers for Class E transmitters (following Steve's designs).

You can find the 1 inch long cores here:
http://www.amidoncorp.com/fb-43-1020/

When using the power divider, always check (very carefully) the temperature of the ferrite core after a minute or so of key-down operation (about 50 watts going into the power divider)... obviously with the transmitter off during the test... to make sure it is not getting hot (warm is okay) due to saturation of the core material.

If the core is getting hot, then you need to double the number of turns on each winding (i.e. 2 turns on the primary and 4 turns on the secondary... for a 4:1 power divider). Alternatively, if the core is getting hot, you can use two (2) cores... stacked one on top of the other (to create the equivalent of a 2.25 inch long core).

Stu




  
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« Reply #20 on: January 07, 2018, 04:10:10 PM »

Thanks Stu and all, This will work great driving the crystal socket on my BC-610D and for the Desk KW.

I hope spring comes early this year? With all that Global Warming going on, I may have to wait an extra two weeks for the ice to melt at the camp in the Adirondacks, where my ham shack is located??
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