Time Honored Powerhouse
Bob Hutchinson, N5CNN
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Durable "Commander" legend
I believe the HF-2500 Commander may be the longest
in-production Ham Radio HF amplifier model utilizing indirectly heated cathode, metal-ceramic
tubes. It has been on the market since 1990 and has developed a grand
reputation over time as a reliable, compact, lightweight desktop legal
limit++ powerhouse that doesn't break. Pat Stein of
Command Technologies says
the Commander is in-use in
hundreds of countries, Worldwide.
Over the years I have heard many Hams on the
air make casual comment that they were using a "Commander", when
asked. In the 90s I learned that "Commander" was some sort of an
established Ham Radio linear amplifier legend for power, reliability and
quality. Command Technologies also produces amplifiers for 6 and 2 meters.
I believe the Commander
HF-2500 was first to
use the proven voltage doubler power supply design, made famous by the
legendary Heathkit SB220, in a modern legal limit++ amplifier utilizing
the metal-ceramic external anode triodes. After talking several times to
Command Technologies owner Patrick Stein, we arranged for
an HF-2500 to be Browned to HRM for review. The current going price for
this amplifier is $3,295.00.
follow along as I examine this mother of light weight legal limit++
amplifiers, up real close, to see if it's legendary reputation is
Seven boxes arrived from Brown Truck Company today but no amplifier. Wait
a minute - - here's a box from Command Technologies, Inc., but it's too
small to be a legal limit ++ amplifier. Well, two boxes, one small and
heavy and one not large enough for an amplifier, but it is an amplifier.
Out of the box and
on the bench. With plastic off I noticed it has a two piece cover similar
to the Titan II, nice. Since the first thing is photography in the light
tent it's off with the top cover, but this is different. The cover is
secured with stainless steel socket head machine screws with clear plastic
washers. This gives a professional appearance to the product.
The proper size
allen driver for my screw gun seams to have been "loaned" out of my tool
collection so I clamped the short end of the allen wrench that came with
the amplifier in the vice and scored it with the edge of a file. I then
snapped it off and chucked it in my nice screw gun. Off with the top and
bottom covers. Everything inside is orderly, neat and well laid out. No
acres of wasted space in this amplifier.
OK, it's light tent duty
for the Olympus camera
This is the fun part. Neat and orderly makes good product pictures.
Open frame T/R relay behind the tubes, under the tuned input housing,
control circuit board on the right side panel and another board under the
wait a minute - - - there are some parts missing, the rectifier/filter
section. Where's the rectifier/filter section?
The big robust band switch and coil are mounted on top of a backward
and upside down L shaped plenum
chamber. The load
capacitor is mounted under the tune capacitor and both
are beside the plenum. The rectifier/filter circuit board is inside the
plenum. Huh? Yes, inside the plenum, in the breeze. Components will stay
cool there for sure. This design certainly makes efficient use of space.
The only other amplifier I have reviewed with major components inside the plenum
is the Ten Tec Titan II. It has all circuit boards except
the rectifier/filter components inside
the air chamber.
From an old
article I found: The tank circuit here is a pi network on all bands that
transforms plate impedance from 1400 ohms to 200 ohms. A 4:1 ferrite core
transformer provides the final 50 ohms impedance. In the event the anode
dc blocking capacitor should short, this arrangement provides a dc shunt
path to ground instead of to the RF output connector.
squirrel cage type blower?
Well yes, one can only move the required air into the plenum, swirl some
around the rectifier diodes and filter capacitors and force it up through
the radial/axial heat exchangers surrounding the external anodes with
a centrifugal blower. A fan just won't cut it here. The blower AC
circuit has a resistor to slow the blower some as it has more than enough
capacity for the job.
no noise at all, quiet as a mouse - - kidding, of course it makes
noise. However, Command has recently altered the design to reduce the
blower noise to the outside of the case. Normally, air flow is through the
large air intake area on the right side of the amplifier, into the right
side chamber, around the transformer and circuit board and into the
There is a
bottom pan, front to back central panel, front and rear panels and side
panels with large windows as found in many amplifiers. A bottom/sides
cover with lotsa holes for air circulation on right and left side has been
standard with the HF-2500. This arrangement provides two bottoms, pan and
outer, bottom cover with about 3/4" space between.
design has air circulation inlets holes in the bottom pan under the blower
and similar air inlet holes in the bottom cover. No air inlet holes on the
right side at all. Air is sucked from under the amplifier, into the space
between the pan and cover, into the right side chamber and into the
blower. With no holes on the right and double bottom with 3/4" air space,
blower sound is better trapped inside and less noise makes its way outside
to the user's ear bulbs.
Yes, beautiful, high
performance, shiny Eimac 3CX800A tubes with the external, in-the-breeze
- - - not really beautiful, well, maybe beautiful in design, but not
beautiful like the big, warm, glowing glass tubes.
other makers of this legal limit++ class of amplifier have switched to the
4CX800A/GU74b tetrode tubes, mainly for economic reasons, Command Technologies has stuck to the
proven and more expensive Eimac 3CX800A7 premium metal-ceramic triodes.
The Eimac tube specifications, consistency and quality control are
attributes the Rusky makers are hard-pressed to match.
With triode vacuum tubes a
tuned input is require for each band to match the impedance of the exciter
output. The tuned input circuits are mounted in the box on the rear panel,
behind the tubes. Adjustment holes in the rear are available for each
band, if ever needed.
Well done as most are today. Good diagrams. Final test sheet furnished
with starting tune and load settings for all bands.
is the full time grid current meter. Right is the switchable plate
current/plate voltage meter. Forward and reflected power is measured with
external equipment, not supplied.
the power supplies?
The rectifier circuit is the
proven full wave doubler rectifier/filter arrangement utilizing 10 N5408 3
amp, 1000 PIV diodes and 8 270 uf, 450 v. premium computer grade
capacitors. This text book circuit provides about 33 uf of smoothing for
the measured 2,990 VDC B+. Much less total filter capacitance is required
here because of the smoother DC form the full wave doubler rectifier
Upon removing the
transformer from the box I noticed it is the premium core type Hypersil.
Transformers in all other amplifiers I have examined, except for the Alpha
87A, are shell types having one winding segment and usually two steel cores banded together with
steel strapping. This one has two winding segments and one steel core.
Of course, this
premium core type transformer has a premium cost.
is good, reduces the weight
By using more than one winding with secondaries in series, the total
length of wire in the secondary is less, resulting in less weight and less
resistance. Transformers used in full wave voltage doubler rectifier
circuits have half the voltage of transformers used in full wave bridge
rectifier circuits and a lot less copper wire in the secondary. When the
voltage doubler rectifier circuit is mated with the light weight core type
plate transformer and it's reduced amount of copper wire, you have the
most efficient arrangement available today. This is why the Commander
HF-2500 weights only 65 lbs. but still has a 4KW+ power supply. An amplifier of this capacity 25 years ago weighed 2 to 3 times as much
as the Commander.
transformers, a filament transformer right next to the blower and the
control transformer mounted on the circuit board, right side, rear. The
control power supply provides power for all other functions such as
relays, lamps, trip circuits, etc. The QSK version of the HF-2500 has more
circuits mounted here. By the way, the vacuum relay QSK option adds couple of hundred
dollars to the price.
Mains power control
is handled by a single 20 amp fuse. A pair of robust solid state relays
handle the switching and soft start or surge control during start up.
Some assembly required
box with the tubes comes in the plate transformer space. Each Eimac tube,
not like a real tube, no glass, just shinny metal, has a small notch on
the outer ring that correlates with the
gap in the pin index of the tube socket. This gives somewhat of a head start
in feeling when the pins are aligned and ready for seating pressure.
I don't install the
transformer, I just set it in, after the picture session, and mate the one plug. Assembly
Will the "Commander"
confirm it's grand reputation?
see if it will light up. Let's run it. Well, the tubes don't really
light up, but the meters do.
The Commander manual
says never operate this amplifier without ALC connected, but I will ignore
the warning. In all my years in ham radio I have never met or had
knowledge of a Ham that actually used or connected ALC. If our
exciter was capable of somewhat more than 90 - 100 watts I would consider
using ALC. As with most of these high gain amplifiers, 90 to 100 watts is
overdriving and tuning without exceeding grid current maximums is
Our test bench is
equipped with a 2000+ watt fan cooled dummy load,
wrecked by a QRO Mark III
and repaired, 3,500
watt low pass filter, Bird 43 and Coaxial Dynamics watt meters cabled in
series. Bird with 5,000 watt slug and Coaxial Dynamics with 2,500 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
watt meters are used between exciter and amplifier to measure drive power
and SWR. Mains voltage during this test was averaging 245 VAC.
Accuracy of measurement
We use the average of
the two watt meters 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 for the 2,500
watt slugs and + or - 250 watts for the 5,000 watt slugs.. The maker's
don't even mention accuracy at close to full meter right deflection.
Remember, these watt meters are the bottom of their line, most inexpensive
of the manufacturer's offerings.
Exciter signal is CW.
Bad Boy Column
90 - 100W. Drive
The grid current protection circuit appears to be set to trip at about
125ma. Using the starting numbers from the test sheet that came with the
amplifier, tuning was a snap. I tuned just like the manual says. A few
times, until I got the feel, the grid over-current tripped, but the trip
is reset by releasing the PTT.
Tuning with 50 watts loafing drive is a snap. Tuning with
90 to 100 watts requires more care while watching the grid current. There
is no way to exceed max. safe grid current in this amplifier, over current
trip works well.
if you've got this far you've been following along to see if the
"Commander" reputation is warranted - right? To see if it truly is
the mother of light weight legal limit++ Ham Radio linear amplifiers.
reputation is real and warranted and, based on the age and
longevity of the legend and
reputation, the "Commander" is the mother of light weight legal
limit++ Ham Radio amplifiers.
If you're looking for a
compact amplifier that is a tower of reliability and stamina and, perhaps
seeking to quickly power your way up and out of the noise mire to a real
presence on the bands, the
"Commander HF-2500" legend will do the grand job for you and your
Ham Radio heirs or future owners.
See the article about quieting the
centrifugal blowers in amplifiers. Hopefully, we will see some
automatic blower speed control circuits in amplifiers one day. Until then,
I'm afraid you will have to do-it-yourself. The Commander will be an easy
one to convert because everything is accessible.
Bob Hutchinson, N5CNN
President and Founder
Wireless Industry Association
If you would like to
publish an article here contact Bob