Download Alpha 8100 HF Linear Amplifier

PRODUCT REVIEW
Alpha 8100 HF Linear Amplifier
Reviewed by Norm Fusaro, W3IZ
Assistant Manager, Membership and
Volunteer Programs Department
The Alpha 8100 is the latest manualtune legal-limit RF power amplifier from a
company that has a long history of producing
brawny, high quality products. Using the
latest microprocessor technology to monitor
critical functions and other high quality components to provide smooth and easy operation, the Alpha 8100 is capable of producing
a clean, robust signal with minimal drive on
all amateur bands from 1.8 to 29.7 MHz.
First Impressions
The first thing that I did after bringing the
Alpha 8100 down the basement stairs and
into the shack was to sit down and rest. This
desktop amplifier is no lightweight, tipping
the scales at 69 pounds. While catching my
breath I admired the sharp looking front
panel and large LED bargraph displays for
power and current/voltage/gain. The front
panel is very similar to the Alpha 99 that this
amplifier replaced in the product line. For
such a heavy box, the unit has a relatively
small footprint compared to other desktop
legal limit amps currently on the market.
ARRL Lab Test Engineer Michael Tracy,
KC1SX, had already unpacked the ’8100 and
installed the heavy transformer so he could
put the amplifier through its paces. The transformer is shipped separately to avoid damage
during shipment, and it’s a simple process
to install it. Installation involves removing
the cover, securing the transformer with four
screws and plugging in a couple of multipin
connectors. The well illustrated manual is
very clear on how to do this, and Alpha even
provides a piece of wood cut to the right size
to position the transformer and slide it into
place. The manual also recommends checking
the tubes, chimneys and hardware to make
sure nothing came loose in shipping.
Before plugging in the unit I removed the
cover to check for the proper voltage taps on
the transformer. The Alpha 8100 has a builtin power supply that is capable of functioning with a variety of ac input voltages. Line
voltage selections are easily accomplished
by moving the taps on the terminals located
in the power supply section of the amplifier.
As is typical of legal-limit power amps, the
user must provide a power cord connector
compatible with the station’s wiring.
While it is possible to run the amplifier
at reduced power from a 90 to 130 V line,
the manufacturer strongly recommends that
the amplifier be fed by a dedicated 200 to
240 V ac circuit with 10 gauge wire capable
of a minimum of 20 A. It’s well worth the
time and effort to install a proper 240 V line
for your amplifier.
Station Considerations
The manual offers a number of safety
notes. In addition to the many warnings about
electrical safety, there’s a section reminding
users about RF safety. If you are upgrading
your station from barefoot power levels, it’s
time to review the RF safety information and
do another station evaluation. See www.arrl.
org/tis/info/rfexpose.html for details.
Legal limit amplifiers require that you
use heavy duty station components, and
the manual devotes several pages to station
considerations. In addition to the previously
mentioned power requirements, consideration needs to be given to antennas, feed
lines and connectors, switches, antenna
tuners, relays and grounding. Every component must be rated for legal-limit operation. You can read more on this subject
Bottom Line
The Alpha 8100 effortlessly delivers a clean 1500 W of RF from 160
through 10 meters. Consider it a long
term investment in your operating
pleasure.
Mark J. Wilson, K1RO
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in the July 2006 issue of QST.1
The Alpha 8100 is rated to work into
antenna systems with an SWR up to 2.5:1.
The manual cautions you to tune the amplifier carefully when SWR exceeds 1.5:1. As
noted in the Station Engineering Considerations section, high SWR can cause excessive voltages and currents at various points
in your feed line system. You need to watch
for signs of arcing or other damage when
running high power into a high SWR.

Product Review Editor
A Look Under the Hood
While under the cover of the ’8100 I took
notice of the clean layout of components and
the quality construction that Alpha has long
been known for (Figure 1). Interior partitions
ensure good RF shielding and add to the
structural rigidity. The Alpha 8100 is built
around a matched pair of rugged 4CX800
(GU74b) tetrodes from Svetlana. These
ceramic tubes are operated in Class AB1
and reliably provide 1500 W output with
approximately 50 to 60 W of drive power.
Although there are several “workhorse”
amplifiers in the amateur market that use
4CX800 ceramic tetrodes, the Alpha 8100
distinguishes itself in several ways. The
sturdy cabinet construction, vacuum relays
for fast full break-in (QSK), heavy duty
switches and components and an extremely
quiet blower used to cool the unit are just a
few worth mentioning.
The ’8100 makes use of microprocessors
to monitor all critical currents and voltages in
the amplifier, as well as to control relays and
1D. Hedin,
“Learning to Live with a Linear,” QST,
Jul 2006, pp 37-40.

[email protected]
April 2007
2/22/2007 1:25:38 PM
operating parameters. If the microprocessor
detects any significant changes in operating
parameters it will quickly enter into a protective “fault” mode. Depending on the nature
of the problem, the amplifier either switches
to standby (“soft faults”) or shuts down completely (“hard faults”). Front panel LEDs
indicate the type of fault so the operator can
find and correct the problem. For soft faults,
within a few seconds the amplifier will reset
itself and return to normal operation.
A serial port on the rear panel provides
some information about the amplifier’s
setup and operating parameters that may
be useful during troubleshooting or remote
monitoring. Using HyperTerm or another
terminal program, you can read two “sentences” produced by the ’8100’s control
microprocessor. The first sentence shows
identification and calibration data, and the
second shows operating parameters such as
input and output current, plate and grid current, drive power, band switch setting, and
so on. You’ll need the manual to decipher
the various codes and a USB or DB-9 RS232 cable to connect the amplifier to your
computer.
Protection Circuitry
Alpha’s advertising refers to their amplifiers as having a “bombproof” RF deck. Of
course I didn’t pack the thing with TNT, but
I was not able to harm the unit even when
I intentionally made a few of the common
mistakes that happen to sleep-deprived operators in the heat of a contest. These included
transmitting on the wrong antenna or no
antenna at all and overdriving the amp. The
’8100’s protection circuits reacted quickly
by shutting down the amplifier and flashing
the appropriate combination of front panel
LEDs to indicate the cause of failure. In a
few seconds the amp automatically reset
itself and was ready for operation again.
I also tried continuous key down into a
dummy load. The ’8100 can handle long key
down intervals at full power and not even
break into a sweat. This is great for running
high duty cycle modes such as RTTY, SSTV,
digital modes or AM, with no time limit.
To reduce heat, extend tube life and
improve overall efficiency of the amplifier,
the ’8100 employs electronic bias switching
circuitry that increases the grid bias between
dits and dahs or in pauses in speech. Under
normal operating conditions this process
has no noticeable effect on the transmitted
signal.
Table 1
Alpha 8100, serial number 8106410008
Manufacturer’s Specifications
Measured in the ARRL Lab
Frequency range (US units): All amateur
frequencies, 1.8 to 29.7 MHz.
As specified.
(not tested on 60 meters)
Power output: 1500 W PEP or carrier.
As specified for SSB and CW.
Output matching: Up to 2.5:1 SWR.
Not tested.
Driving power required: 50-55 W nominal.
As specified.
Spurious signal and harmonic suppression:
50 dB below rated output or greater.
55 dB, worst case.
Meets FCC requirements.
Intermodulation distortion (IMD):
Greater than –30 dB.
3rd/5th/7th/9th order (worst case):
–52/–48/–53/–61 dB PEP.
Primary power requirements: 90-130 or 190-250 V ac, 50/60 Hz (5 user settable taps).
Size (height, width, depth): 7.5 × 17 ×18.25 inches; weight, 69 pounds.
Typical retail price: $4850.
tor. The exciter should be capable of sinking
the transmit relay current to ground. The
’8100 does not use an automatic level control (ALC) connection to the transmitter.
Once the amp has had time to warm up,
typically 150 seconds, the WAIT LED goes
out. Switch the OPR/STBY switch to OPR,
and the amp is now ready to operate.
The manual includes detailed instructions for two different ways of adjusting the
amplifier. Regardless of the method used for
tune-up, the procedure is quick and easy.
After doing it a few times an operator should
be able to make rapid band changes.
The classic “dip and load” tuning method
is recommended. After the selecting the
band of operation, simply move the TUNE
and LOAD controls to the preliminary settings outlined in the manual and set the
multimeter bargraph to display IP A (plate
current, 0 to 1.5 A). Each Alpha 8100 is
shipped with a table of actual settings that
were used to achieve full power output into
a dummy load at the factory. These setting
may vary slightly from those in the manual
but in either case they will get you in the
neighborhood for final tuning.
Apply about 20 W drive power and adjust
the TUNE control for a peak in output, which
should correspond to a dip in plate current.
Increase drive power for 1000 W output
tweak the TUNE and LOAD controls for maxi-
Tune Up
The Alpha 8100 easily connects to any
HF exciter using coaxial cable with PL-259
connectors and a transmit/receive (TR) relay
control cable with a standard phono connec-
Figure 1 — The Alpha 8100 has a clean layout and beefy components.
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mum power output. Then increase the drive
level until the amplifier puts out 1500 W.
To use the alternative “nominal gain”
tuning method, set the multimeter bargraph
to GAIN rather than plate current. With about
15 W of drive, adjust the TUNE control to
maximize the GAIN indication and then adjust
LOAD until the LED near the center (between
the white lines) is illuminated. Repeat these
steps, then increase drive for 1500 W output
and tweak TUNE and LOAD again.
The objective of the tune-up procedure
is to reach optimum efficiency and linearity at the desired output power, so the amp
will have to be retuned should the operator
decide to run the amp at less than full power.
The manual gives instructions for proper
tuning at lower power levels.
On the Air Performance
The Alpha 8100 performed great on the
air. It produced clean signals on all modes
with no distortion or any other negative re-
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prod.indd 72
ports. I ran the amp with two popular amateur
transceivers and into a variety of antenna
systems. I found the ’8100 quick and easy to
tune up, and it was a real performer even at
less than legal limit when conditions called
for a little less power. Tuning was smooth
and not particularly critical across all bands, a
welcome change from my normal station amplifier. That amp also uses a pair of 4CX800s
and is a little ticklish on 10 meters.
The blower was extremely quiet and
did not cause any distraction or unwanted
background noise in the transmitted audio.
Even during full duty cycle AM operation
the amp ran cool and quiet. The ’8100 keyed
quickly and cleanly when operated using full
QSK in CW mode.
The manual runs 56 pages plus 11 pages
of fold-out schematic diagrams. In addition
to detailed setup and operating information,
it includes sections on theory of operation
and troubleshooting. In a number of places
the manual encourages you to call Alpha’s
technical support line if you have any questions about setting up, using or shipping the
amplifier — a real plus.
Parting Shot
In today’s rapidly changing technological
world one would hardly consider any piece
of electronic equipment as an investment. A
quick look at the used market reveals that
Alpha amplifiers fetch a pretty high price,
if you even find someone who is willing to
part with one.
If you are considering the purchase of a
legal limit amplifier and you intend on keeping it for a while, then you should seriously
look at the Alpha 8100. Based on past reliability and performance, as well as the ease
of use, the Alpha 8100 should provide a nice
return for the serious DXer or contester.
Manufacturer: Alpha Radio Products,
6185 Arapahoe Ave, Boulder, CO 80303;
tel 303-473-9232, fax 303-473-9660;
www.alpharadioproducts.com.
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