HF-2500DX Mark III
3.5 KW Watt Strong Man Amplifier
Also Dummy Load Destroyer
Bob Hutchinson, N5CNN
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
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.
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
sized box and a nice screw gun.
Click any picture larger:>>
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
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
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
Mains, soft start, in-rush
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
If 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
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
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 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.
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
most amplifiers utilizing the 4CX800A tetrodes, this one has the necessary
over-current trip circuit
to protect the screen grids. See below for more
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.
Superb, best of any amplifier maker.
the power supplies?
robust, of course. Plate and bias AC voltage is furnished by a lotsa watts
(must be about 5.3 KW ICAS)
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.
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.
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
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.
For HF RF
power out our test bench is equipped with:
Coaxial Dynamics 83000A peak reading directional RF wattmeter.
Bird 43 directional RF wattmeter.
Coaxial Dynamics 81000A Directional RF Wattmeter.
These with same slug values, usually 2500 watt, connected in series with a 3500
watt low pass filter, a 7,500 watt. antenna switch utilizing Jennings vacuum
relays for access to 3000 watt fan cooled dummy load located outside of
building, or 75M & 40M dipoles, and verticals for higher HF frequencies.
Testing of wattmeters is performed by placing in series with the wattmeters
mentioned above for comparison.
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
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.
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
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
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
links? Porsche components
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
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
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:
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
Bob Hutchinson, N5CNN
President and Founder
Wireless Industry Association
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Bob Hutchinson, N5CNN.