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Issue #9- QRO with the Mercury LUX Legal Limit Amplifier

By Dave Jensen, W7DGJ


A few weeks ago, I wrote about my love for QRP and how a small signal can go a great distance. It was well received, and I had fun discussing the topic in the QRZ forum that week. It’s amazing how people can be on one side or the other of QRP. In my opinion, the fun thing about ham radio is that you can squeak out 500mw in the morning and then jump into a full legal-limit QSO later that afternoon. I enjoy both scenarios and don’t restrict myself.


In 2020, a ham buddy and I decided to sell our Ameritron AL-80B’s (a workhorse tube amp with a large following) to pick up the KM3KM Electronics amplifier kit, the Mercury IIIS. I was thrilled to have it up and running just a few days after receipt and I later reviewed my build for CQ Magazine. My soldering skills weren’t that rusty after all! A bit of time with that amp and it appeared I had found another “workhorse,” but in this case, an amp that I could have on the air an instant after hitting the power button. I loved the bandswitching and the Mercury’s great protection circuits. I got “beeped at” more than a few times due to those protections, but with other amps I could be replacing transistors.


When I decided to buy another Mercury IIIS for a second station, I found the KM3KM.com list of existing orders had expanded and the wait was now 8 or 9 months. However, an announcement of a brand-new amplifier as an option – the Mercury LUX – improved the timing for those who would agree to try an untested, new model. I’d have enjoyed building another kit, but the LUX looked too cool to pass up. I jumped on the email and have now been working with one of the first LUX amplifiers to hit the market. (Note: This company has since discontinued kits from their lineup. They now sell only FCC-certified, 3rd party tested and fully assembled units.)


The Mercury LUX is a legal limit amplifier with a lot of features that the Mercury IIIS does not have, some intended for remote operations. See the forum for a roster of facts on this amplifier.


The Mercury LUX


There are inherent issues to be concerned about with linear amplifiers of this size, and the biggest of these is how the amp deals with heat. Most manufacturers simply point a big, obtrusive fan at the problem and it goes away. I remember trying to hold a QSO over the sound of a National NCL-2000 amp . . . the noise would sometimes be far beyond my ability to hear my receiver. If done right, a solid-state amp manufacturer needs to balance fan noise along with the required cooling protection for sensitive transistors. Clearly, the designer of the LUX has figured this out, as I can’t seem to get the amp to heat up . . . it has twice the LDMOS of the Mercury IIIS, with a full legal limit output, but manages to stay cool with only a minimum of noise. The Mercury LUX uses something called KoolPlusTM to make this happen. I understand this is a suite of technologies that includes a huge heat sink with multiple, smaller fans that never become obtrusive. The RF board is mounted on copper as well. The amp rarely goes into the full-speed fan mode unless you are contesting, running in digital modes, or sending high-speed CW (semi break-in compatible). You can hear it, but it’s not going to disturb your QSO. My guess is that with two LDMOS, full power at 1500 Watts uses only 65-70% of the two transistor’s total capacity; this by itself would also make them easier to cool.


Greg (N7GR) has used many amps in his 50 years as an amateur radio operator. He tells me about his LUX experience running in the recent RTTY Roundup, “I managed about 400 contacts with the amp running at a consistent 400 watts. It stayed cool the entire time. Just amazing to me. That wouldn't have been true with any other amp I have owned.”


The bandswitching feature I liked on the Mercury IIIS has been improved in the LUX as well; it allows the user to either automatically switch via RF detection or to be commanded directly by your radio (Icom, Yaesu, Elecraft, Kenwood, ANAN, Flex and so on). Cables for this are available for purchase from KM3KM. I bought the ICOM cable and I’m very pleased at how much faster it is to move from band-to-band than via RF sensing.


The beautiful 7” touchscreen front panel of the Mercury IIIS is present here (24-bit color), but with improved software that provides much more personalization. The LUX uses sub-folders in which every parameter of your amp’s operation can be adjusted, down to the appearance of the screen or the type of metering you’d prefer. I like the classical analog meters a great deal and sometimes use those (see photo) but my standard is the linear readout which offers Power, Reflected Power, SWR, Temperature, Voltage, Current and a warning panel for the protection algorithms.



Software allows you to program each of the antenna outputs (there are three of these) so that your antenna of choice is immediately selected for a given frequency. In a nutshell, the amp is designed to be operated with very little human involvement, which is something I appreciate a great deal. Once you step out of line, a buzzer and a graphic reminder of the issue will alert you. Upon hitting the reset button, you’re back on the air.


For this article, I have personally checked the protection circuits for each of the covered parameters. One of those is to protect your amp from high amounts of reflected power in an SWR mismatch situation, and it worked flawlessly anytime I had something over an approximately 2.0 to 1 on my antenna analyzer (both the LUX and the Mercury IIIS determine this fault in terms of >125w of reflected power, and not actual SWR). Of course, trying to pump out power into a non-existent antenna is always a good test, but more than a bit scary. It can be a deal killer with many solid-state amps, but here the protection circuits catch it. And, since I don’t operate with the ALC on, I am reminded now and again that I’m overdriving the amp. I’ll get the buzzer and a screen notice to back off a bit, and a moment later I’m back on the air. In a QSO, it comes across as no more than a brief hiccup.


The amp will operate on either standard household voltage or on 240V, depending upon your choice of cable, as it has a self-contained switching power supply that senses the power input. For this reason, there is no power cord provided with the Mercury LUX. I’m sure that many before me have had to make a trip to the hardware store upon taking it out of the box. It seems unnatural and a bit unfair that when you buy a $4K+ piece of gear you must go to the store and buy a power cable. On the other hand, I just bought a new Samsung washer/dryer and had to do the same thing.


Full performance across all bands is guaranteed on 240v. On household voltage, you’ll get somewhat less but it will otherwise be fully operational. I’ve chosen to run at lower voltage as a change to 240v at my shack could cost several thousand dollars.


On the Air with the Mercury LUX


Physically, this is a beautiful piece of gear. You can see the difference between the Mercury IIIS and the LUX in the fit and finish; the LUX is as nice as any piece of high-end audiophile equipment I own and ranks with the best of my amateur radio gear. The case itself appears to be powder coated and has a slight texture, as opposed to the stamped metal appearance that so many other radios have. It’s a stunning design highlighted by two brushed chrome handles built to sustain the 29-pound weight of the unit. Some users have apparently found a way to remove the handles, but for me they make it a thing of beauty. The riser feet in front allow you the option of a bit of an angle for better viewing of the touchscreen.


The specs appear to be very conservative, and even on household voltage it performs well. Here are some examples of what I am seeing with a mostly-resonant multiband OCF dipole antenna.


6M: 48w drive = 1KW

10M: 45w drive = 1480w

12M: 45w drive = 1420w

15M: 33w drive = 1500w

17M: 45w drive = 1420w

20M: 25w drive = 1500w

40M: 22w drive = 1500w

75/80M: 34w drive = 1440w

160M: 33w drive = 1500w


Based on others' comments, the required exciter output in 240v is dramatically lower. For example, Mario (KC8P) puts 10w in on most bands and gets a Kilowatt or more out of the LUX. In my situation, each of those 1500w frequencies in my chart would go into the 1800-1900w range if a full 50w input was applied.


Remote Operation


I’d like to thank both Ted (N4SQB) and Mario (KC8P) for assisting with this section of my review of the Mercury LUX, as I have no need for a remote amplifier at my shack. Both of these operators are early LUX owners and run their LUX remotely. Ted has his LUX in the barn, but when it’s winter, he runs the station from inside his house. He reports that the LUX is working great as a remote linear amp (he operates from within his home Internet router).


“The KM3KM software connects your amplifier through USB to a computer. There are several different software products you can use at the other end to control it, but I just use the remote desktop that is a part of the Windows 7 Professional Version. I’ve had lots of amplifiers. The LUX just sits on a shelf out there and does its job without drama. If I’m nearby, I have to strain to hear it while it pumps out the legal limit. The darn thing just works great.”


Mario (KC8P) takes this remote capability a few steps further, as he’s an IT professional and does considerable travel. On a recent trip to Poland, Mario ran FT8 with the LUX from 33,000 ft. despite some latency issues he experienced due to the aircraft net connection (See the photo from his aircraft seat). He uses the LUX on multiple modes – CW, FT8, and Sideband, with a Flex 6600 and a SteppIR antenna. Mario uses a wide variety of software with his remote setup, but includes the KM3KM software with every connection. (I’ve posted the roster of software tools that Mario uses with his remote setup . . . see the forum for this attachment).



“The software that came with the LUX is crucial. The metering is very helpful and gives you reflected power readings and so on. I can monitor how my station is operating due to the metering I get from the LUX remotely. This software is crucial to this method of operating. It might be possible to use the LUX without that, but I wouldn’t really know what is going on if an alarm occurs and I wouldn’t be able to reset.” Mario’s only negative is that the power switch is mechanical, and he still needs to find a solution to turning the amp on and off from a 240v outlet.



In Conclusion


You’ll find my interview with Kenny Martinez of KM3KM Electronics in issue three of Trials and Errors. I asked Kenny for that interview because he is clearly an innovator who is building a new ham radio company, but as he’s only been in the market 3-4 years, he’s still a bit of a mystery. If Martinez keeps turning out products like the Mercury amplifiers, he will be an important figure for the future of our hobby. What makes KM3KM Electronics a company to watch is that their designs are so different. When you combine that with what you hear via word-of-mouth through a fanatical user base, it is clearly a powerful combination for sales growth.


Is the LUX a perfect product? No, it’s not, but like Mario’s comment, these are small complaints. Every time I need to reach my hand to the back of the amp to turn it on, I scrape my skin on some other piece of gear and wonder why the master power switch is located on the back of their amp. (Note: This is not just KM3KM, as other manufacturers have done this for years.) And it’s certainly not inexpensive, either. The retail price of this linear amp is presently set at $4300, while the original Mercury IIIS is still priced at around $2800. Still, those are very competitive prices in their market segments (against companies like Ameritron, ACOM, RF-Kit, Flex and so on). The bigger question would be whether the LUX is worth the upgrade from the Mercury IIIS.


You’ll get 80-85% of the way by buying the Mercury IIIS. But after owning both, I’m glad I settled on the LUX. There is something very satisfying about owning a product that is built this well. When you turn it on, it just hums with power. I’ve beat the daylights out of this amp in the last few weeks and trust that my LUX will allow me to continue to do so no matter where my ham radio hobby takes me! I rate it an A+ in build quality and performance, with an A- for my user experience, due to a couple of knuckles scraped by reaching for that rear power switch, and a trip to Home Depot for a power cable.


Don't forget - there's a new "Short Takes" Ham Accessories review this week, at this link


73 for now,

Dave, W7DGJ


PS - Please join our Forum discussion on this column topic, by visiting this link. There are other posts detailing some of the features of the amplifier.


Dave Jensen, W7DGJ

Dave Jensen, W7DGJ, was first licensed in 1966. Originally WN7VDY (and later WA7VDY), Dave operated on 40 and 80 meter CW with a shack that consisted primarily of Heathkit equipment. Dave loved radio so much he went off to college to study broadcasting and came out with a BS in Communications from Ohio University (Athens, OH). He worked his way through a number of audio electronics companies after graduation, including the professional microphone business for Audio-Technica.  He was later licensed as W7DGJ out of Scottsdale, Arizona, where he ran an executive recruitment practice (CareerTrax Inc.) for several decades. Jensen has published articles in magazines dealing with science and engineering. His column “Tooling Up” ran for 20 years in the website of the leading science journal, SCIENCE, and his column called “Managing Your Career” continues to be a popular read each month for the Pharmaceutical and Household Products industries in two journals published by Rodman Publishing.

Articles Written by Dave Jensen, W7DGJ

This page was last updated February 1, 2023 16:56