QRO HF-2000 Linear Amplifier
The big warm glowing tubes still perform.  http://www.qrotec.com
by Bob Hutchinson, N5CNN

OK, let's get this straight right away - I love the big glass tubes. I like the warm glow. I grew up with the glowing tubes shinning their warm light into a subdued room.  As a kid I would take the cover off of anything that had tubes so I could feel the glow. My first real stereo equipment - - yep, tubes. Lots of tubes and covers always off. Dyna-Kit and McIntosh.

My amazement and adoration of the first 3-500 tube was a motivating factor in getting into Ham Radio. I have seven amplifiers with glass tubes and most of them have a pair of the 3-500s standing tall. The 3-500 tube style amplifiers are still offered by many amplifier manufacturers to satisfy the undying demand of those afflicted with OCTD, known in the medical community as Obsessive-Compulsive Tube Disorder.

I have used and modified enough Heathkit SB220 and SB221 amplifiers to correct design or production shortcomings or to improve performance to be qualified to review any amplifier of similar design. Our well-equipped test bench is not equipped with a spectrum analyzer so we don't get into that at all.

Ordering the HF-2000 and dealing with Ray Connin, QRO owner, was truly a pleasure. Ray,  KB8VU, is easy-going and easy to talk to. The boxes came when he said they would and I opened the larger box with the amp, sans the tubes and Peter Dahl combination plate, filament and control transformer. Transformer and tubes were shipped in their own boxes.

This HF-2000 was first partially disassembled and photographed in our light tent, which was designed just for this purpose. The photography results of the interior parts are presented here. I was impressed with the quality of the fasteners, all stainless steel. The top cover is 1/8" aluminum, which is about twice as thick as other amplifiers. (More about this at end of article.)
 

Big Switch Please note the very robust band switch. This switch is of a higher quality and rating than other amps having two times the power. This switch is rotated via a fiberglass rod connecting the front of the switch to the knob. This insulator provides an extra measure of safety. The operating knob is normal size for Ham size linear amplifiers but could have more leverage to operate this big band switch.

As you browse the pictures, I believe you will agree that components and circuit boards are neat, uncluttered and easy to get to and adhere to the first design considerations for an HF linear amplifier.

The product has user serviceable parts in a potentially lethal, high voltage environment.

And the power supplies?
The circuit board area includes the rectifier, filter capacitor, meter and control circuits and components. The plate power supply is the tried and proven full wave voltage doubler design made famous by the Heathkit SB220 and used today in over half of all new tube type amplifiers. Rectifier is populated with the industry standard N5408, 1000 V. PIV diodes providing 5000 V. peak inverse voltage protection capacity. Filter capacitors are 470 uh/450 V. computer grade electrolytics in two banks for proper operation of the doubler circuit. Eight of them provide about 55 uh filtering and a 3600 V. working capacity.

The no load voltage on the meter is 3200 V. Other secondary power from the transformer provide voltages for the requirements of metering, control, bias, soft start and QSK T/R circuits.

Vacuum relay QSK
QRO made the design change a while back to include their vacuum relay, full break-in QSK circuit, of a design similar to the Rich Measures circuit, at no additional cost. I have modified several grounded grid tube type amplifiers (SB220) with this QSK feature. Although I am not a CW full break-in type of guy, I can say that this feature provides wonderful VOX enhancements for telephone-like SSB conversations.

Component carefully inserted
The transformer was unpacked and I was surprised to see a rope for assistance in lifting it into the case. There is an electrolytic capacitor in the inrush protection (soft start) circuit board sticking out that could interfere with the insertion of the heavy transformer. Well, I believe it would be the other way, the transformer could just smash the little capacitor so I stood a sheet of .060 Plexiglas up to guide the transformer properly without damaging the cap. After the transformer is bolted in, it clears this capacitor by about 3/8 inch.

All fasteners are stainless. I inserted the 1/4-20  hex cap screws with washers from the bottom and installed washers and nuts on top. With the right tools and patience its easy. Next I mated the primary and secondary plugs and, following the manual, made sure nothing could possibly get into the box fan blades.

The tubes are RF Parts private labeled Graphite type and were installed with ease into the steel (appears to be stainless) and ceramic sockets with silver plated pin grippers. The anode straps with parasitic suppressors were attached to the anodes.

As you can see in the many pictures, everything is neat and the workmanship is superb. No shortcuts, anywhere. Whether you are a 25-yea-old Ham or Mr. Average Ham at age 59, this top quality amplifier will probably outlive you.

I installed a 15 amp 234V. type AC plug on the AC power cord. The three wires were nicely tinned for ease of proper plug installation. I hooked up the RF input from the Icom exciter and RF output to the Bird and Coaxial Dynamics watt meters, in series with the antenna switchbox.  (Article about my 4x vacuum relay, remote antenna switch design  soon). Then the T/R relay control and ALC feedback cable and ground.

But does it run?
Well, let's see. On start-up for the first time - no sounds. I found, later in life, that I was not as indestructible as I believed in my youth. Hearing not quite up to standards now but noisy amplifiers irritate me. QRO knows how to cool an amplifier without fan noise. What a pleasure. I have installed several two speed thermal switch fan controllers, with adjustable low speed, in amplifiers attempting to bring the noise down to this level. This amp is quiet. The tubes light up.

Bench
Our test bench is equipped with a 2000 watt fan cooled dummy load, low pass filter, Bird 43 and Coaxial Dynamics watt meters with 2500 W. slugs and antenna switch access to 75M & 40M dipoles and 20M, 17M and 10M vertical antennas. A watt meter was used to measure exciter drive power.  I first tuned and tested each band for function on the dummy load. All as expected. Then I went through my systematic load test, band by band, on the dummy load. Results below coincide with the tune up table furnished with the amplifier. The QRO was optimized for the phone section of 160 and 80 meters. Drive signal is CW.

 

Freq.	Drive	Plate Voltage	Plate Current	Power Out
3.900  Mhz	110W	2700V	700MA	1350W
7.200 Mhz	110W	2700V	700MA	1400W
14.200 Mhz	110W	2700V	900MA	1500W
18.120 Mhz	110W	2700V	800MA	1400W
21.350Mhz	110W	2700V	850MA	1350W
28.400 Mhz	110W	2600V	850MA	1275W

Grid current never exceeded 220MA in final tune during any test.

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