66
June 1997
Product Review
Edited by Rick Lindquist, N1RL• Senior Assistant Technical Editor
By Rick Lindquist, N1RL
Senior Assistant Technical Editor
The Centaur represents Ten-Tec’s at-
tempt to go the extra mile in offering a low-
end HF linear amplifier. The Centaur cer-
tainly is not the first “three-hole” 811A
amplifier on the market, but the folks in
Tennessee reckoned they could go a few
steps beyond what was already out there, at
the same time keeping the price in the same
general ballpark. The result is an economi-
cal 600 W class amplifier that offers QSK
as a standard feature—the only one in this
price category to do so—plus hot-switch-
ing protection, pi-section input filters and
other niceties.
Let’s face it. Not everyone can handle
the $2500 cost of a big-power amp—and
not everyone needs that kind of power any-
way. At this point in the sunspot cycle,
though, some ops would just like a little
leverage on the bands. Sometimes—just
sometimes—the few hundred additional
watts an amp like this can offer will make
the difference between snagging that new
one, making a sked or continuing an enjoy-
able ragchew under changing or less-than-
optimal conditions. A little desktop ampli-
fier like this can certainly fill a lot of gaps.
In typical Ten-Tec fashion, the Centaur
is a fairly compact, straightforward box—
nothing fancy. On the outside, it has ver-
nier-driven
LOAD
and
TUNE
controls (cali-
brated with 0-10 markings on each knob’s
apron); a bandswitch (marked for 160, 80,
40, 20, 15 and 10 meters, but the amp shares
positions for 30, 17 and 12 meters); two
lighted multimeters (one reads either grid
Ten-Tec Centaur Model 411 HF Linear Amplifier
current or power output in watts; the other
reads either plate voltage or plate current);
a row of mini-toggle function switches; and
a big rocker-style
POWER
switch to turn
the unit on.
Inside the box (and visible through the
ventilation holes in the top of the gray steel
cabinet) are three Svetlana 811A tubes, a
fairly husky transformer (1.5 kVA CCS and
accounting for about half the weight of the
amplifier), the tuning capacitor, and other
components (see photo). The parallel
811As are operated in class AB
2
grounded-
grid configuration with a nominal 550 mA
of plate current and approximately 1700 V
of plate voltage (at full load). The Centaur
can operate satisfactorily from either 120 or
240 V ac input. A four-inch muffin-type
fan moves 100 CFM of air across the three
output tubes, so the amplifier runs quite
cool (much cooler than my Collins 30L-1,
which has four 811As in the output but a
rather insubstantial-looking cooling fan).
The excellent cooling should keep the
811As running for a long time, assuming
you don’t otherwise abuse them. The major
tradeoff here is in fan noise (more on that
subject later).
The Centaur is rated at 600 W output on
SSB and CW on 80 through 15 meters,
500 W output on 160, 12 and 10 meters, and
400 W output on FM, RTTY or SSTV.
(Ten-Tec says that by tuning up for full SSB
or CW power then setting the carrier output
at 150 W, the amp can be used for AM as
well.) It takes the better part of the output
of a typical transceiver to drive the Centaur
to full output—90 W or so would do it in
most cases, we found. We got the rated
power on all bands except 17 meters, where
it was much closer to—but not quite—500
W. While the Centaur manual advises
keeping the plate current at or below 550
mA during tuneup, we found that the tubes
drew up to 750 mA on some bands when
the linear was tuned for rated output. Ten-
Tec said this is normal. (My personal phi-
losophy with an amplifier like this would
be to run it at 450 to 500 W. There’s pre-
cious little to be gained by pushing it to its
absolute limits.)
Setting up the Centaur is pretty easy. It
comes from the factory wired for 120 V ac
use, but if you’ve got 240 V available in
the shack, it’s quite simple (and probably
advisable) to change two jumpers—acces-
sible via a rear-panel “inspection plate”—
plus a couple of fuses and the wall plug for
240-V operation.
As it comes from the factory, the Cen-
taur does not operate on 12 or 10 meters
(per FCC requirements). To add these
bands, users first must send a photocopy of
their Amateur Radio license to Ten-Tec
requesting the free modification package.
In return, they’ll receive a small circuit
board that installs via the “inspection plate”
access panel. It takes about five minutes
and involves no soldering.
The rear panel has SO-239 connectors
THE BOTTOM LINE
An economy-class “half-gallon”
811A linear amplifier with QSK stan-
dard. Works great, runs cool, but some
ops might find the blower a little loud.