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 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 addition
power, the RF components are similar. I see now why the smaller
HF-2500DX two holer seems to be built for bigger things, it
is.
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 an c able
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
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 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
military tube, which has flooded the world market, are Russian
military ugly and without the Eimac brilliant bright finish, they
have economically captured most 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
curent trip circuit to protect the screen grids. See below for more
info.
Meters
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.
Bench
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 wattmeters are used
between exciter and amplifier to measure drive power and SWR.
Accuracy of
measurement
We use
the average of the two wattmeters 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
wattmeters 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
| Band |
50W.
Drive
Grid/Plate/Power |
Loafing Column
70W.
Drive
Grid/Plate/Power |
Overdrive Column
100W.
Drive
Grid/Plate/Power |
| 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.
Tuning
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 you're 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 wattmeters. I tuned by
the book, watching screen current only, then referenced the
wattmeters 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 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 breaker 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 wattmeters, 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.
 Conclusions
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
mailone@wirelessindustry.com
713
467-0077
If you would like to
publish an article here contact Bob Hutchinson, N5CNN: mailto:mailone@wirelessindustry.com.
Last
Modified: October 04, 2011 05:17:28 PM
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