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Little Boy -- A Water Cooled LDMOSFET Amplifier

from W6KAN on March 22, 2019
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Little Boy W.C.
A Water Cooled LDMOSFET Amplifier

by W6KAN

Little Boy in an old Heathkit Chassis

Since I built my first solid state MOSFET HF amplifier in 2006 (I named it UNO). It was featured in the June issue of QST that year. The UNO was designed following a Motorola data sheet authored by one of their engineers – Helge Granberg, now a SK.

UNO – 4 MRF 150s (MOSFETS) with analog 120Vac Power Supply

Over the past 12 years the technology has changed dramatically with the advent of LDMOSFET’s (lateral double diffused metal oxide semiconductor field-effect transistors). The new devices have amazing properties and reflect a dramatic advancement in technology. They are the catalyst for the new commercial amplifiers, most running legal limit, that are in the market. The advantages of these transistors are:

• Tremendous gain – upwards of 25-30db.
• A very small foot print – hard to imagine such a small device could output so much power.
• Some of the newer ones can run into an open load without damage.
• Most circuity does not require feedback.
• They can handle a lot of heat operating near 100C 24/7.
• They are “Gemini” designs – two transistors inside one package – perfect for push-pull circuits.
• They operate up into microwave frequencies.

LDMOSFETs have an Achilles heel - they will not tolerate excess drive on the gates. The gain is so incredible that four to five watts will drive them to full output. Many transceivers do not allow for very low power output – my Kenwood will reduce output to five watts minimum. Accordingly, most commercial designs have attenuation pads ranging from -7db and up to prevent transistor failure. The attenuation is also needed to meet FCC rules.

One persistent problem with solid state amplifiers is heat – especially with long transmissions. Muffin fans and heat sinks can only do so much in lowering temperatures. They do work okay if the ambient temperature in the shack is on the cool side. If temperatures rise it becomes difficult to cool them – often with numerous and noisy fans.

I decided to build an amp that would be water cooled. I also wanted my design to use a diplexer filter. These advanced versions of low-pass devices are significantly more efficient. The main difference is their separate circuits for grounding third order harmonic energy that would otherwise kick back to the transistor. This feature also improves distortion and lowers transistor heat.

I needed a plan for the various components so a roughed up a block diagram for the project.

My research revealed a Russian company ( that had ready-made modules for my project. Their water-cooled amp unit was perfect. The back side of the small device had tubing soldered to the copper spreader. Included was a copper clamp that fits snugly over the transistor. It helps significantly to spread out the junction heat. The clamp is displayed in the picture. I used a special thermal transfer paste from a German company in mounting the transistor. While the compound works okay in the future, I plan on using carbon nanotube pads. This technology results in up to six times better heat transfer and is used in computer CPU mountings.

Most of the newer circuits like the Russian one’s are conventional. The layout has not changed that much since Granberg’s work in the 1980s – bias voltage control, a high to low impedance input transformer, and low to high output transformer. The latter have been computer designed to reduce reactance at higher frequencies significantly improving efficiency. Most circuits do not have feedback from the drains to the gates – a must for MOSFET transistors.

Water cooled amplifier module (schematic available at http;//

The back side with the tubing soldered to the copper spreader

Because the LDMOSFET’s can nominally handle no-load conditions I did not include projection for high SWR. I programed an Arduino Uno to monitor the temperature of the copper. It is programmed to open a relay and remove the bias from the transistor if the copper exceeds 70C. In order to deal with reducing the input drive I installed a -7db pad in front of the “gates”

Attenuation -7db pad.

The table shown below is an approximate estimate where the drive needs to be to output one KW (the Ampleon BLF 188XR will go up to 1,200 watts).

Kenwood Drive Watts	-7db' Output Watts	Estimated Gain	Input Power Watts 	Output Power 70% Watts
5	1	26db	398	278.6
7	1.4	26db	557.2	390.04
9	1.8	26db	716.4	501.48
11	2.2	26db	875.6	612.92
13	2.6	26db	1034.8	724.36
15	3	26db	1194	835.8
16	3.2	26db	1273.6	891.52
17	3.4	26db	1353.2	947.24

The data sheet for the BLF188XR indicates higher gain in the HF range. A +26db level was used as a compromise. Efficiency worked out to about 70%. From this chart I decided to use 17 watts (3.4 watts at -7db) of drive from my Kenwood to output about 1KW - a level which limits distortion products.

The Russian diplexer filter module fits my design. The diplex design is superior to conventional low pass filters. They have separate circuits to discharge third order harmonic energy rejected by the main filter to ground – not back to the transistor.

The diplexer low pass filter – 160,80,40,20, 17/15 and 10 meters (relay activation).

I tested the filter with an antenna analyzer as the frequency generator and looked at the output with a scope. All of the bands checked out. A ham friend (K6XQ – Roger Leone) confirmed my analysis. A band switch on the front panel activates the relays for each band.

Following the diplexer filter I inserted a tandem match module to output power and SWR readings to a dual meter on the front panel.

The tandem module

The next challenge was to design and build a water cooling system. I knew the “gamers” used small radiators and related equipment to keep their CPUs cool. I found the perfect pieces on the internet – a radiator, a small reservoir, and a 12Vdc pump.

I mounted two muffin fans on the radiator and assembled it on a wooden board. Water held in the reservoir feeds the pump and on to the amp returning to the radiator. The amp temperature has never exceeded 40C.

The radiator with the muffin fans, the reservoir, and the 12vdc pump.

I added an Arduino Uno with a temperature probe programmed to constantly display the heat level of the copper. It turns on a digital pins to open a relay (at 70C) which will shut off the bias. When it is first activated it displays “CHECK THE DRIVE” – a reminder to make sure the Kenwood output is set okay.

The amp built into a Heathkit discarded chassis is pictured below. The Arduino is on the lower center – front panel. (I will email the “sketch” to interested parties). The transistor module is in the upper right corner with the attenuation pad shown lower right. The filter board is lower left with the tandem match on the vertical panel, and the upper left shows the T/R relay and 12Vdc transformer for the bias and other circuits.

I used a new HP computer power unit (HP 226579 ESP 120 - $60 including shipping) for the power supply. It is rated to provide 50+Vdc at over 50 amps - a beast. It looks like the “slider” boxes used in bank vaults. It requires 240Vac for its power.

I could not find a schematic or any information as to how the HP “switcher” could be setup. A fellow ham directed me to an article that shows the “pin” connections needed to activate the device. Notice the three pins marked in red in the photo. They need to be shorted together for the supply to work.

The bottom view of the “slider” showing the three pins to be shorted, the 240VAC input connections on the right, and the output contacts on the left.

I soldered the 240VAC three wire feed to the right side with the common lead to the center and tied them, so they would not pull lose. I applied a generous portion of liquid insulation to all three leads. On the left side I put together a cable with different colored wire for the plus and minus DC output using number 10 wire to handle the current. Male and female plugs were added to the cables to disconnect the DC supply from the amp.

In order to keep the 240VAC voltage removed I assembled a remote relay box next to the circuit breaker. The relays are activated from the front of the amp with 12Vdc via a push switch. Once they engage the high voltage is sent to the “switcher” and the DC comes up (50Vdc) in a matter of seconds. The water-cooling system has to be running before the power supply can be activated.

This was not a “paint by the numbers” project. It has taken me about a year from start to finish. I did a lot of the work in my spare time, so it crept along slowly. During the construction I took time to test each phase. Once it was ready to go, I set up the bias. In my experience this is the ultimate test to verify the condition of the transistor. Initially I was worried since the bias voltage would not exceed 1.9Vdc – possibly a design flaw. Fortunately, the Ampleon BLF188XR came to life at 1.8Vdc and started drawing about ½ amp.

Finally, Little Boy came to life with an initial five watts drive from the Kenwood – the output matched the data from the chart noted earlier. Running with 17 watts (3.4 watts after the -7db pad) from my Kenwood 590SG seems to be the “sweet spot” for clean operation. The copper mounting has never exceeded 40C.

Starting up a new project is always stressful. Some call it the “broomstick” phase – a joke about needing a wooden pole to turn the switch on. Accept the fact that most “home brew” undertakings don’t initially work. One of my ham friends (now a SK) used to say, “if projects work right away it is a bad deal.” His comment was followed by “you can’t learn unless something fails”. Cognitively working through the issues allows one to make the necessary changes to get it “right”. As time goes on it becomes a moving target for improvements and it evolves to a better end.

I like to name my projects. I recently watched the movie Fat Man and Little Boy, a 1989 great movie about the Manhattan project and the development of the atomic bomb. The film centers around the collaborative effort to affect the end result with a lot of very independent minds and ideologies. “Little Boy” is definitely the result of new technology and the efforts of many bright minds around the world that are way beyond my “paygrade!”

The new commercial amplifiers, some outputting legal limit, take advantage of these new transistors, and can be lifelong additions to the ham shack. The older MOSFET amps are still the basis for some excellent commercial amplifiers. My older one, UNO, running on four MRF150s, continues to do yeoman’s duty and easily outputs 600-700 watts12 years in daily use! It will outlast me!

Member Comments:
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Little Boy -- A Water Cooled LDMOSFET Amplifier  
by KJ4DGE on March 22, 2019 Mail this to a friend!
Outstanding! I am sure even with the time spent and attention to detail like anything else the results speak for themselves. Truly a very good job and great pics to show the details. Curious, what Heathkit did the case come from?
RE: Little Boy -- A Water Cooled LDMOSFET Amplifier  
by K6AER on March 22, 2019 Mail this to a friend!
What a incredible project and wonderful article. Thank you for sharing the project and the detail and photos in your construction.

Do yo have an over drive cut-out in the amplifier. For instance if the amplifier is driven with lets say 6 watts it goes into bypass.
Little Boy -- A Water Cooled LDMOSFET Amplifier  
by NY7Q on March 22, 2019 Mail this to a friend!
It is heartening to read such a concise building story. Reminds me of the OLE days when hams had direction and need to be better for themselves and others.
Thank you for sharing.
RE: Little Boy -- A Water Cooled LDMOSFET Amplifier  
by W6KAN on March 22, 2019 Mail this to a friend!
Yes...their is a circuit to cut off the bias if it us out of range set by a potentiometer. I usually leave the Kenwood's output at 17-18 watts as it is always used with the amp. Thanks for your comments. Tom
RE: Little Boy -- A Water Cooled LDMOSFET Amplifier  
by W6KAN on March 22, 2019 Mail this to a friend!
thanks for your was an old SB104 HK...Tom
RE: Little Boy -- A Water Cooled LDMOSFET Amplifier  
by W6KAN on March 22, 2019 Mail this to a friend!
thanks it was fun but demanding...I think it will be my last amp project (:>)...Tom
RE: Little Boy -- A Water Cooled LDMOSFET Amplifier  
by KJ4DGE on March 23, 2019 Mail this to a friend!
While nowhere near the knowledge or skill of the author the article again shows we should not be afraid to build something. The experience of others is a teacher. Today many us the radio community are stuck in a rut so to speak due to the easy way we can buy cheap knock-offs or think we don't have the time to take what we have perhaps in a box in the basement and repurpose it towards something really useful to our hobby.

Make a personal project. Envision what you want to have and if it takes a trip to the hardware store or drawing it out on paper then just do it and build it. Even if it does not work the way it should the first time, try again or troublsshoot it until it does, then look back on the effort as something YOU created with your own two hands and it works! That feeling is worth more than you can imagine.
RE: Little Boy -- A Water Cooled LDMOSFET Amplifier  
by K4EJQ on March 25, 2019 Mail this to a friend!
BOY, OH BOY....A GREAT piece of work OM. There's nothing like seeing a homebrewer's handiwork featured here. Keep that iron hot!!! 73, Bunky, K4EJQ
Little Boy -- A Water Cooled LDMOSFET Amplifier  
by AJ4SN on March 25, 2019 Mail this to a friend!
Excellent work! I've been waiting for just such a high-power, solid-state linear project. Thanks for blazing the trail!


RE: Little Boy -- A Water Cooled LDMOSFET Amplifier  
by K6AER on March 25, 2019 Mail this to a friend!
If the amp is water cooled and named Little Boy, maybe the amplifier should be named LITTLE WATER BOY.
Little Boy -- A Water Cooled LDMOSFET Amplifier  
by KI3R on March 25, 2019 Mail this to a friend!
Excellent article !!! I have been greeted by the "magical smoke" on many occasions but always have learned something. It is sad that more ops do not get out the iron and build something ... anything as I feel that they are missing out on the most rewarding part of amateur radio. The cost of parts from China these days are a mere pittance as compared to what I paid 60 years ago. There are those who will say that the quality is not there. You do test the components before installation you? The cheep-o testers are themselves cheep. I have 3 and they keep getting better. Keep up the spirit of ham radio .... build and learn.

God Bless All Tom KI3R Belle Vernon PA.

Little Boy -- A Water Cooled LDMOSFET Amplifier  
by SWL377 on March 27, 2019 Mail this to a friend!
Ham radio at its best. Thanks so much for sharing this adventure.

Little Boy -- A Water Cooled LDMOSFET Amplifier  
by VA2PBJ on April 7, 2019 Mail this to a friend!
Why not just use glycol (car anti-freeze) instead of water? I would think it would be less hassle in the long corrosion.
RE: Little Boy -- A Water Cooled LDMOSFET Amplifier  
by W6KAN on May 4, 2019 Mail this to a friend!
Sorry...I should have replied earlier. There is a cut off circuit that removes the bias if the drive exceeds a certain level. I set it at 24 watts as this would be the outside limit required. It takes more drive in the upper bands probably due to the increased reactance of the output transformer. If running barefoot the power out would likely be 100 watts. This would fry the LDMOSFET but will never get through due to the protection circuit.

Thanks for asking.

RE: Little Boy -- A Water Cooled LDMOSFET Amplifier  
by W6KAN on May 4, 2019 Mail this to a friend!
Not a bad idea...(:>) Tom
RE: Little Boy -- A Water Cooled LDMOSFET Amplifier  
by W6KAN on May 4, 2019 Mail this to a friend!
Everything I have read indicates water is the best medium for cooling. I use distilled water and so far there is not evidence of any contamination. Maybe later I will go to an anti-freeze solution.
RE: Little Boy -- A Water Cooled LDMOSFET Amplifier  
by W6KAN on May 4, 2019 Mail this to a friend!
Yes, I test each component as I work through the construction. I had issues with the 20M portion of the dyplex filter using a antenna analyzer for sweeping the individual bands and a scope to look at the output. I finally asked a friend to look at my work and he reported that the 20M section was okay. It has played out that I was partially correct as the output on 20M is somewhat low. When I get time I will look at all of the components and see if they are within specs.


PS By testing and going slow there were very few issues with the startup. So far the amp has worked flawlessly and stays very cool. The copper spreader has never exceed 30C - it will be interesting to see the range this summer when the ambient temperature in the shack goes up.
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