[TenTec] Centaur 411 report

[Rick Denney]

My new (to me) Centaur arrive on Friday, and this is a quick report
from putting it in operation and comparing it to my AL-811.

Executive summary: The Centaur is MUCH nicer, and even compared to a
used AL-811 is only a little more expensive.

The seller warned me that the QSK/PTT switch had a bent handle, and I
foolishly tried to straighten it. Of course, it broke, and I replaced
that and the bypass switch with mini-toggle DPDT switches from Radio
Shack. They look the same as the (admittedly low cost) originals and
they work. I may order something a bit nicer from Mouser in the
fullness of time.

Working on the Centaur is a pleasure. It comes apart easily, unlike
the AL-811, and the panels can be removed without unsoldering
anything. Just like the Omni V. A lot of disassembly is required to
remove the front panel, but it's all obvious and easy to reassmble,
especially compared to the AL-811.

Yes, the fan is noisy. But I never felt much warmth coming off those
Chinese 811's, even after heating up my dummy load quite a bit. And I
barely heard it at all when wearing headphones, as I routinely do.

The Centaur has a forward power meter, which the manual claims is only
calibrated in the 400-600 watt range. I doubt that it is calibrated at
all. I was reading 600+ watts consistently on 80-15 meters and 500 on
10 meters into a dummy load, with grid current well below 125 ma and
plate currents well below 500 ma even key down. I measure output using
an Ameritron AWM-30 true peak-reading power meter, which also allows
me to tune up using CW dits instead of key down.

The archives reported many who were unable to get the full rated power
out of the Centaur. I think I know why. The power meter just plain
reads low. When putting out 600 watts key down (as measured by the
AWM-30), I was also seeing over 600 watts on my 229 tuner's power
meter, but only a little over 400 watts on the Centaur's power meter.
I wonder if those who didn't think they were getting full power really
were.

The Centaur tunes quickly and easily compared to the AL-811, and runs
at rated power with lower grid and plate currents.

Something I can't explain (based on the usual descriptions of 811's in
grounded-grid AB configuration), but that agrees with the ARRL review,
is that it reaches rated power at an exciter power of 80-90 watts,
instead of 55-60 watts as with the AL-811. This is a good thing, in
that it largely prevents an overdrive-based Big Mistake. Tuning is
easy-schmeazy: Set RF Power on the transceiver to the middle of the
dial, send a string of dits, tune the amp for maximum power, turn the
RF power up to about 9 on a scale of 10, and tune for maximum power on
another string of dits. Keep an eye on grid current, keeping it below
125 ma. Plate current should be below 450 or 500 ma, but may be a bit
higher during tuning. The Centaur tunes in half the time of an AL-811,
at least with my hands on the controls.

The circuit design seems safer. For example, the bleeder resistors are
in pairs and paralleled. Ditto for the parasitic suppression
resistors. The design resists a serious fault from a failed resistor.
The execution is considerably more robust. The voltage rating of the
power-supply caps is 2200 volts against a high-voltage supply of 1800
volts. On the AL-811, the caps are rated to 1800 volts, which depends
on everything being perfectly matched to keep one of the caps from
experiencing a bit higher voltage than the rating. The design includes
fault protections such as prevention of hot-switching, which the
AL-811 does not have.

The transformer in the Centaur is substantially beefier (and heavier)
than the transformer in the AL-811.

I could not find a way to overdrive the tubes, except I suppose by
running it at full exciter power in an untuned state. The notion of
putting 572b tubes in this amp just for improved forgiveness seems
really unjustifiable. I'm leaving the Chinese 811's in it until I have
a reason not to.

And then there's the QSK. I am not a CW operator, but I could easily
hear the band between the dits at 30 or 40 WPM. It works so well it
makes me want to work on those CW skills. A pair of audio cables to
connect the Key In and Key Out with the TX Out and TX EN plugs on the
Omni was all that was required.

Ten Tec's manual is typical of Ten Tec: Informative and complete.

The cabinet is lower and wider than an AL-811, which raised an issue.
Unlike the AL-811, it would not sit atop my 229 tuner, so I had to
build a shelf for it.

I now have three different Ten Tec style periods in the shack: The 229
tuner is a style match, I think, for the old Omni A-D series. The
Centaur is all gray with white lettering and the front panel sits
under the lip of the upper and lower cases with no trim, and seems a
newer style than the Omni V. This seems a bit strange, since the
Centaur dates from when the Omni VI was current and the VI has the
same style as the V. But they all say Ten Tec on them, and I'm glad.

Rick, KR9D