QRO HF-2500DX
Rusky tubed bad boy transformer???
by Bob Hutchinson, N5CNN

I'm going 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 power.)

QRO 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.

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Some assembly required
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 surprises.

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
With 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 confused here.

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 protection stuff
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 amp, solid state relays that eliminate damaging inrush currents for all components.

RF section
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 here, 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 easy.

Centrifugal squirrel cage type blower. Noise?
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 things cool.

Good choking
The 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 (abusive).

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 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.

Non-glowing Svetlana 4CX800A/GU74B tubes
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.

Sounds kinda 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.

Rusky invasion?
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 protection
Like most amplifiers utilizing the 4CX800 tetrodes, this one has a trip circuit to protect the
screen grid from excessive current. 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. 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.

The 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.

And the power supplies?
Well, robust, of course. Plate and bias AC voltage is furnished by a lotsa watts 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 Peter Dahl transformer.

The 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.

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.

Hardware, wire
Stainless steel and Teflon dominate here. Pictures tell the quality story.

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? Let's see.

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 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 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.

Data below is grouped:
Screen grid Current MA / Plate Current MA / CW Power Out Watts

Band	QRP Settings Column 50W. Drive Grid/Plate/Power	Loafing Column 70W. Drive Grid/Plate/Power	Bad Boy Column 100W. Drive Grid/Plate/Power
1.90 Mhz	35/850/1,400W	60/950/1,850W	80/1200/2,250W
3.90  Mhz	35/700/1,400W	60/900/1,800W	75/1200/2,300W
7.200 Mhz	40/800/1,400W	70/900/1,800W	75/1100/2,300W
14.225 Mhz	50/900/1,800W	70/1000/2,200W	80/1100/2,300W
18.140 Mhz	60/1100/1,900W	65/1100/2,000W	70/1150/2,100W
21.350Mhz	40/1100/1,900W	60/1100/2,100W	70/1200/2,200W
28.400Mhz	50/900/1,500W	60/1100/1,800W	70/1200/1,900W

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 50ma.

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 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 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.

Ham 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:  attendance, 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.

Conclusions
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 other amplifiers.

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 would want 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.

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