Rusky tubed bad boy transformer???
Bob Hutchinson, N5CNN
to examine QRO Technology's legal limit ++ transformer and see if it is
capable of transforming Hams wallowing in low power mediocrity
on the HF bands into Hams of presence. (Presence meaning, or associated
with: attendance, occurrence, charisma, attainment, persona, appeal,
allure, magnetism, achievement, realization, - - or, in the Ham Radio
world, bad-boy RF
Technologies has been building fine HF linear amplifiers for eight
years. Ray Connin, QRO owner, has been building them a lot longer. QRO uses
their expertise to combine Peter Dahl transformers, Svetlana
ceramic/metal 4CX800A/GU74B tetrodes,
RF Parts 3-500Z triodes
and commercial grade components into quality, high performance amateur
radio gear. The HF-2500DX price with OSK is $3,145.00 plus shipping from Bryan, OH,
which was about $65.00.
Click any picture larger:>>
This HF-2500DX arrived in three boxes, small, medium (heavy) and large. 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. I removed the cover and removed the screws holding the
left side panels so I can get in close with the
camera. After inspection of the Russian military 4CX800A/GU74B tubes to
make sure all pins are straight, I got em all aligned, carefully applied
force to seat both tubes and attached a high voltage harness to each tube. No
Both sides of the amplifier are doubled walled. After removing the cover these two inner walls are
removable to allow great access to components. This feature sure makes it
handy for pictures. I only remove one side at a time because for structural
reasons. When assembled with the 1/8" thick cover, the whole
box is very sturdy.
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.
OK, it's tent duty
cover and left side inter panel removed there is good access to all of the
amplifier. Large windows in the right inner side panel allow good camera
access. I like the picture taking part. Instead of trying to compose a
picture on the tiny screen on the tiny camera, I cable to a 13 inch color
TV monitor just to the left of the light tent. This is the one pair
method, requiring only one pair of glasses. You younger Hams might be
After the main
picture session I reattached the left side panel and removed the right
side panel so I could photograph the Peter Dahl transformers in place, but
not bolted in.
I have examined and
photographed the innards of many amplifiers. I get to see the
construction, design and components up close, and I share all of it with
HRM browser and readers. All can see from the pictures that 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. I believe you will agree that, if you or I were to design
and choose components to build our own amplifier, we would build using the
same good stuff that Ray Connin uses in these amplifiers. QRO's slogan is
"Built by Hams, for Hams."
Mains, soft start, in-rush
No sissy, putt-putt 117 VAC household circuit can drive this one.
The mains power required is 200 or 240 VAC at 20 TO 25 amps. If you have
used a 240 VAC circuit for your smaller amplifier in the past on a shared
basis with some other appliance such as an air conditioner, you may need
to make dedicated arrangements to run this amplifier in bad boy mode. Soft start
current inrush protection is provided by two big, healthy, 50 amp, not 25
state relays that eliminate damaging inrush currents for all components.
Like the QRO HF-2000 I reviewed last year, upon removing the cover
I was duly impressed with an item than denotes the care, attention and
quality built into a linear amplifier, the band switch. No miniaturization
Ray uses the real thing, a big, rugged
ceramic Radio Switch Model 86 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.
Well, any time you have to force air under pressure around corners,
through a plenum chambers and through two axial/radial heat exchangers,
you can't get around the centrifugal blower. The blower must rotate at
high speed to develop the characteristic of their name. Ray has design a
two speed circuit utilizing a big resistor to slow the slightly
overcapacity blower down to a slower and less noisy speed for side band
yammer, yammer, yammer and 50CFM for the high speed switch position. For
the demanding, demanding mo-power and duty cycle, the high speed will keep
plate choke is big and out of heat's way. I have seen a few plate chokes
burned up. They were too small and not out of heat's way. In glowing tube
amplifiers a small diameter choke adjacent to the tubes must be in the
direct air low. In metal ceramic tube amplifiers, the plate choke is not
usually in a direct air stream, therefore it needs to be bigger in
diameter so it wont overheat from operation that is not as it should be
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 a circuit similar to the one
made famous by amplifier expert Rich Measures, AG6K, whose circuit this
writer has installed and used on three Heathkit SB220 amplifiers. If you
are not intimidated or bored by really technical amplifier writings and
projects click here to the
Measures web page.
I love the warm
glowing glass tubes. They radiate and conduct heat from the anode, which
gets cherry red or white hot, and the cathode, in all directions to every adjacent
object or component. These surrounding objects and components - case, plate choke, wires,
tube sockets, pins and coils accept and buffer or store the heat as parasitic heat exchangers.
I guess the professor would want me to use terms like exothermic (gives
off heat) and endothermic (absorbs heat), huh? These components then give up their heat to the cooling air that slowly and quietly moves past
and makes it way through the exhaust outlet.
serene, doesn't it? Metal ceramic, indirectly heated cathode tubes have an
anode that gets really hot also. But the heat is efficiently conducted
direct to the metal parts of the tube and the radial/axial heat exchanger fins. Little heat is
transferred elsewhere, it's all going to the aluminum fins. Serene can't
describe the aggressive process of high velocity pressurized air rushing though the plenum and tubes, taking the heat right out through the exhaust. Works well.
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.
What's the difference?
Well, I believe the inter-modulation distortion and some other specs are
better for the Eimac tube. But, the Rusky tube satisfies the FCC specs
required for amateur amplifiers of this type. They also supply the owner
with peace-of-mind satisfaction of low replacement cost, if ever needed.
Screen grid over-current
most amplifiers utilizing the 4CX800 tetrodes, this one has a trip circuit
to protect the
screen grid from excessive current. 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. Use your outboard watt meter for
status, power and self esteem feedback, and to keep big brother from
beating down your door and confiscating your bad-boy QRO.
manual & extras
Superb, best of any amplifier maker. Extra case screws came with this
amplifier. Since this is the first time I have checked out this amplifier
I don't know if it has always shipped with extra screws for the case.
Except for Ameritron, I have never received extra screws with any
amplifier. Thanks QRO.
QRO guarantees it
Warranty: Two years 100% percent parts &
labor. Tubes are covered by the manufacturer for one year.
the power supplies?
robust, of course. Plate and bias AC voltage is furnished by a lotsa watts
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
Peter Dahl transformer.
power supply HV rectifier board contains twenty N5408
rectifier diodes in a bridge arrangement providing 5,000 PIV capacity for
DC smoothed and filtered by eight 470uf, high grade electrolytic
capacitors. Total smoothing and filtering for the 2,750 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
Stainless steel and Teflon dominate here. Pictures tell the quality
But, does it light up? Does it run?
Will it provide presence on the bands? Will it transform
timid stations? Does it have big brother attracting, bad-boy capabilities?
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 2,500 watt slug.
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
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.
Data below is grouped:
Screen grid Current MA
/ Plate Current MA / CW Power Out Watts
QRP Settings Column
Bad Boy Column
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 50ma.
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 both Tune and Load to zero, applied 50
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 50ma max.
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
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.
Radio status decisions
In the first paragraph presence was mentioned as a possible benefit of
ownership. Well, this one's got it. Just take your pick:
occurrence, charisma, attainment, persona, appeal, allure, magnetism,
achievement, realization, - - or, just bad-boy RF power. With little drive
this box can truly QRO. It is a "transformer" too. Including the two Peter Dahls the total is three.
Ray Connin, QRO
owner and designer, suffers from the overkill syndrome even though he has
never been a hunter. It appears he opts for excessive capacity in
components as a standard design practice. Example of this standard can be
found in items such as the 50 amp solid state relays for soft staring
everything. Most makers would use 20 amp or 25 amp component for "just
enough" capacity. The big band switch is three times the size used in
For a 10 watt
dissipation requirement Ray specifies premium 50 watt gold resistors with
finned heat sink, bolted to the case, where most makers would use less
expensive 15 to 25 watt types air mounted. This "overkill" can be seen
everywhere in the amplifier. If you and I were making an amplifier we
the best components such as those found in QRO amplifiers.
Click Here to buy QRO amplifier at ebay. Be sure and tell-em
you saw their fine product at HRM.
Bob Hutchinson, N5CNN
President and Founder
Wireless Industry Association
If you would like to
publish an article here contact Bob