QRO HF-2500DX Mark III
3.5 KW Watt Strong Man Amplifier
Also Dummy Load Destroyer
by Bob Hutchinson, N5CNN

  Export Model

For a few Dollars more - an extra Rusky tube, a really powerful Peter Dahl plate transformer and a foreign address, you can have a BIG signal from this beast. Like horses? Horsepower? Mo Power? Follow me along as I see if this one can live up to its reputation as the Big Strong Man of Ham Radio amplifiers. We can compare it to it's bad boy little brother, the QRO HF-2500DX.

After testing the two smaller QROs, Ray Connin and I talked about testing the more powerful export Mark III. We see these big amplifiers on web sites, but seldom get to actually fondle them. With OSK this one goes for $4,195.00 US, plus shipping to the exporter of your choice. OK, OK. It's the day before the Fourth of July and I can't wait. I've got a big box, a little box and a medium sized box and a nice screw gun.

                                                     Click any picture larger:>>

Some assembly required
I opened the large box first to take care of "some assembly required" as the tubes were in their own box, packed in the plate transformer space. Many components are the same as those used in the HF-2500DX. The case is the same size. With some differences for the additional power, the RF components are similar. I see now why the smaller HF-2500DX two holer seems to be built for bigger things.

I removed the cover and removed the screws holding the left side panels so I can get in close with the camera. Yep, sure enough, it's got holes for three of the Russian military 4CX800A/GU74B tubes. Takes a plate full of tubes to fill this one.  After inspection to make sure all pins are straight, I got em all aligned, carefully applied force to seat all tubes and attached a high voltage harness to each tube. No surprises.

Physical size is 20" Wide X 19" Deep X 8" High. After installation of all component this QRO will weigh 90+ lbs., so I will install the plate transformer and ancillary transformer after the photographic session and after I move it to the test bench.

After the first picture session I moved the unit to the bench, reattached the left side panel and removed the right side panel so I could set the transformers in and plug em up. Everything plugs together as it should. I slide the unit toward the back of the bench, under the shelf with all the metering, walk abound back and cable everything up. Yes, it's that kind of setup. Can't everyone walk around to the back of their station? Well, this is a test bench.

As mentioned in the other QRO reviews, QRO uses the very best components, usually of much greater capacity or quality than necessary. Every component, assembly or harness installed or secured to military specs.

Mains, soft start, in-rush protection stuff
I mentioned in the little brother review that QRO used the real stuff for mains AC functions including 50 amp solid state relays for start-up softly. Well, those superior AC components are here too.

RF section
f you own an HF-2000 or HF-2500DX by QRO, it's nice to know that most of the components were designed, rated and selected for an amplifier much more powerful. Again I am impressed with the band switch. No miniaturization here, Ray uses the real thing, a big, rugged ceramic Radio Switch or equal band switch that will handle the job. Big coils, big capacitors, big toroidal 4:1 transformer output network, big choke, big everything. The air variables have fine 6:1 vernier drives that make tuning and repeatability to the numbers easy.

Centrifugal squirrel cage type blower. Noise?
Well, I have written lotsa words about centrifugal blower noise in amplifiers. This amp requires 50% more air capacity than little brother. However, Ray has included the two speed circuit arrangements here also.

Vacuum relay QSK
Vacuum RF relay circuits provide T/R switching with great hi-voltage potential reserve, fast make and break on the order of two or three milliseconds, and less noise. QRO utilizes the premium Jennings or Kilavac relays to efficiently handle the speed and power requirements of QSK in an amplifier of this power. We use the same relay x 4 here at our test bench in our antenna switch.

Non-glowing 4CX800A/GU74B Tubes X 3
Most amplifiers made today in this legal limit ++ class utilize either 3CX800 Eimac ceramic/metal triodes or Svetlana ceramic/metal 4CX800A/GU74B tetrodes. It appears that the future is the Rusky tubes. Even though the Russian tubes, which have flooded the world market, are Russian military ugly and without the Eimac brilliant bright finish, they have economically captured much of the amateur legal limit+ amplifier market. I recently noticed replacement Eimac 3CX800 tubes for $525.00 on a couple of web sites. The Rusky ugly tubes are about $125.00 at the high end and $50.00 at street prices.

Screen grid over-current protection
Like most amplifiers utilizing the 4CX800A tetrodes, this one has the necessary over-current trip circuit to protect the screen grids. See below for more info.

Plate current, plate voltage, screen grid current, screen voltage, everything but power out monitoring. Tuning with just the screen current meter is much easier than I thought it would be. Normal amateur wattmeters are useless for testing this amplifier. Not enough capacity. See below.

The manual
Superb, best of any amplifier maker.

And the power supplies?
Well, robust, of course. Plate and bias AC voltage is furnished by a lotsa watts (must be about 5.3 KW ICAS) black epoxy
Hypersil from the El Paso King of amateur radio transformers, Peter Dahl. Ancillary power for cathode heaters, control, relay, metering, etc. is provided by a separate robust transformer, also from the King in El Paso.

The power supply HV rectifier board contains twenty N5408 rectifier diodes in a bridge arrangement providing 5,000 PIV capacity for HV DC smoothed and filtered by eight 470uf, high grade electrolytic capacitors. Total smoothing and filtering for the no load 3,400 VDC B+ is about 58uf.

Circuit boards
The larger control board above the HV rectifier and filter capacitor boards provides LV bias, screen trip and control circuitry. The smaller circuit board mounted on the divider is the screen supply board. The RF I/O and ALC circuit board resides in the ventilated box attached to the rear wall behind the tubes. Metering board is mounted out of camera range just under the meters and in front of the rectifier board.

But, does it light up? Does it run?
Will it provide major presence on the bands? Will it transform timid stations into commercial power? Does it have neighbor frying, bad-boy, bad man, capabilities for - uhhhh - export - or to be modified as an industrial, scientific, or medical device as specified in CFR 47, Part 18, Subpart 18, 121? Is it really a QRO beast? We'll find out.

Our test bench is equipped with a 2000+ watt fan cooled dummy load, 3,500 watt low pass filter, Bird 43 and Coaxial Dynamics watt meters cabled in series, each with 5,000 watt slug for this test. The bench has a superb 7,500 w. antenna switch utilizing Jennings vacuum relays, designed and built by this writer for access to 75M & 40M dipoles, 20M, 17M and 15M verticals and the dummy load. Various putt-putt watt meters are used between exciter and amplifier to measure drive power and SWR.

Accuracy of measurement
We use the average of the two wattmeter mentioned above. Both manufacturer's spec. an available inaccuracy of + or - 5% of slug rating at a mid-scale reading. This works out to be an error factor of + or - 125 watts at mid-scale. The maker's don't even mention accuracy at close to full meter right deflection. Remember, these wattmeter are the bottom of their line, most inexpensive of the manufacturer's offerings. See recent wattmeter article here.

Data below is grouped:
Screen grid Current MA / Plate Current MA / CW Average Power Out Watts
50W. Drive
Loafing Column
70W. Drive
Overdrive Column
100W. Drive
1.950 Mhz 80/1300/2,600W 75/1500/3,000W 100/1600/3,300W
3.925  Mhz 75/1200/2,650W 80/1500/3,000W 100/1600/3,600W
7.245 Mhz 65/1300/2,950W 80/1600/3,400W 100/1600/3,600W
14.250 Mhz 80/1500/3,300W 85/1600/3,500W 100/1800/3,750W
18.140 Mhz 80/1500/3,200W 90/1600/3,500W 100/1700/3,600W

Notes: No testing was done on 15 meters or higher frequencies. See below for reason. This Strong Man exceeds USA legal limit with minimum exciter output on most bands.

The testing above was done with 15% duty cycle side band yammer, yammer, yammer in mind. The HF2500DX manual warns against continuous operation at screen grid currents in excess of 75ma.

It's not like tuning the glass tube amplifiers. Tuning for maximum output (max. smoke) is not applicable here. There is no forward or reverse power meter in this amplifier. Focus is on screen current, so I tuned it just like the manual prescribes. I set Tune to 50 and Load to zero, applied 40 watts exciter drive, got slight negative screen current reading, advanced Load clockwise until slight screen current indicated. Then Tune clockwise until 75 to 100ma indicated, Load counterclockwise until 25ma indicated, Tune clockwise until 75 to 100ma indicated, Load counterclockwise until 25ma indicated. After back and forth adjusting while screen current monitoring a few times the Tune control will no longer increase screen current. At this point, if your power requirements are for SSB, the Load control can be set for screen current between 25ma and 100ma. It's tuned to resonance for the drive level just like the manual describes. For conservative operation or for continuous high power modes, limit screen current to 75ma.

After performing the start from zero procedure several times it goes quite fast, nothing to it. I even quit watching the watt meters. I tuned by the book, watching screen current only, then referenced the watt meters to find that resonance and maximum power for the drive level had been achieved. For 70 watts and 100 watts drive I just continued the procedure. I recorded the numbers for future reference.

Excessive screen current instantly trips the over-current circuit and the fault light comes on. I tripped it several times while learning the technique for tuning this beast. A slight counterclockwise rotation of the Load control and a touch of the fault button/lamp will get things going again. Retuning by the numbers to overdrive settings and reducing the drive to suit power requirements is a snap.

By the way, this testing was done on the Fourth of July. The 18,000 BTU air conditioner for the shop became completely inadequate trying to deal with the Houston heat and the heat from these tubes and the overloaded 2,000 watt dummy load. I had to stop several times and let the place and the operator cool down.

Find the weak link(s)
Perhaps you have had conversation with Hams on the air about really big powerful amplifiers and how one of these might affect your operations. I have had those discussions about lotsa power finding weaknesses in systems, connectors, balans, connections, switches, neighborhood relations, etc. Well, testing this beast found some weaknesses here.

When the power out got serious the 20 amp domestic breaker on the single phase 240 VAC mains circuit popped. Hmmmm. I reset it. Ya know, once one of these cheap domestic breakers trips it will never hold at the same amps again. After a couple of more trips outside to reset it, I tripped down to Lowe's for a bigger breaker for mo power.

Lotsa cabling and connectors at the test bench. Three watt meters, low-pass filter, antenna switch, etc. and the 2,000 watt dummy load with an amplifier in this power class is dummy. Well, you guessed it, too much power for this test bench designed not for the QRO Mark III beast. I suspended the testing after 17 meters. I'll have to update some equipment upward in capacity before continuing testing any amplifier in this power class.

If one gets an opportunity to run this amplifier or one in this power class legally, everything from the antenna down will have to be beefier than what we have here. The exception is the Coaxial Dynamics wattmeter, the Bird wattmeter and the vacuum relay antenna switch.

Weak links? Porsche components
I once bolted a 140 horsepower Chevy Corvair engine up to a Volkswagen transaxle of the 36 horsepower variety for a mo-horsepower project. Oh yea, my mo-horsepower desires exceeded the 36 horsepower Volkswagen transmission capabilities, finding the weak links just as the Volkswagen dune buggy specialist guy predicted. Also as predicted, I bought higher capacity, wallet smashing Porsche drive train components to better serve my mo-power desires.

The first thing I tested after lighting up this strongman up was the no load B+. It was 3,400 VDC. The normal no load B+ voltage for the
4CX800A tubes is 2750 to 2950 VDC. I knew this voltage from the massive Peter Dahl transformer would provide big power. Additionally, the third tube atop the plenum chamber kinda changes the sound to a different, powerful sounding frequency.  On any band tested this is a 3,500+ watts Strong Man machine.

Click Here for QRO web site. Be sure and tell-em you saw their fine product at HRM. Oh, and I'm afraid I need to advise you per the below:

Notice Regarding HF-2500DX Mark III to U.S. Amateurs: This device has not been approved by the Federal Communications Commission. This device is not, and may not be, offered for sale or lease, or sold or leased until the approval of the FCC has been obtained.

Bob Hutchinson, N5CNN
President and Founder
Wireless Industry Association
713 467-0077

If you would like to publish an article here contact Bob Hutchinson, N5CNN.



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