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HV Bridge Rectifier

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Blower attached to sound and vibration absorbing pad.

Superb workmanship.

Proper switch work.


Alpha 99 Linear Amplifier
Alpha's latest made-in-USA HF amplifier utilizes the workhorse Russian 4CX800A ceramic-metal tubes. 
by Bob Hutchinson, N5CNN

Is Alpha King of the hill in legal limit + Ham radio linear amplifiers? Well, there's a lot of information in this article - so you be the judge.

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Crosslink, Inc. of Boulder CO is the new manufacturer of Alpha linear amplifiers. In keeping our promise to review amplifiers we ordered this model 99 from Crosslink, Inc. on September 7, 2001, planning for a November article. At the time of our order it appears that none of the new model 99s had yet been shipped. This one was received just before Thanksgiving.

Ordering the new Alpha 99 and dealing with Scott Ehrhorn and John Tuttle was a good experience. Everything was well documented and email acknowledgements were as expected - good business practices throughout the process. Delivery was a few days over two months. I understand that delivery is improved since production of this unit.

To my knowledge, this is the first Alpha made in the USA to be equipped with the Russian Svetlana 4CX800A/GU74b ceramic-metal tetrode tubes.

Warm glow
OK, lets get this straight again. As you may have read in the review of the QRO HF2000 amplifier, I love the big warm, glowing glass tubes. After publishing the QRO review I came into possession of a nice Ten Tec Titan legal power + amplifier that utilizes the 3CX800 type Eimac tubes. Well, there is some to be said about indirectly heated cathode tubes - dramatically reduced heat and lotsa power in a small package.

After removing the model 99 from the big box my memory worked as it should, it reminded me that Alpha has been doing this for a while. First impression is the small size of the amplifier. Upon removing the cover and briefly examining the innards, my memory again reminded me that Alpha has been doing this for a while. The lay out is standard for most single box amplifiers made today. It's a U shaped pan with front to back divider/stiffener to separate left RF section from right side rectifier, power supply and circuit board section. The cover becomes an integral structural member when completely attached.

In it's class, legal limit +, it is a small, compact amplifier. It stands eight inches above the table with the front support bail folded. Other: 17 inches wide and 16 1/2 inches deep plus hardware attached to the rear panel, such as the extra fan. I have not weighed it but it is the lightest in it's class, shipping weight is 80 lbs. for both cartons. Even though the amplifier is small, the components inside don't seem to be crammed in. Everything is orderly and there is plenty of evidence that service will be easy, if ever needed. Circuit boards and components utilize quick disconnects and plugs so removal or replacement will require minimum solder melting, if any.

Examine the pictures to see the quality components, design and construction. I have been into quite a few amplifiers in my days and know the good stuff when I see it. During the photography session in our light tent I get to see everything and photograph it. The first, and most apparent, item is the front to rear divider wall. It's an extensively ventilated quality fabricated aluminum divider. Concave clearance for the tune and load capacitors are stamped into it.  The fast vacuum T/R relay is mounted in a large rubber grommet and has shrink tubing on the soldered leads to effectively dampen noise that normally would be transmitted to the case. The squirrel cage type blower motor and cage is mounted on a dense, sound insulating foam rubber pad to reduce noise while the black housing is firmly mounted to the box

The AC primary, nine conductor connector for the transformer is firmly mounted to the back panel as a receptacle. Great attention has been paid to wire and cable containment, routing and security. All is first rate throughout the amplifier. Although I have not been inside the patriarch of the Alpha family, the 87A, I suspect the 99 and 87A share some components.
The Rusky tubes
The tubes appear to be larger in diameter than the 3CX800 type tubes. The 4CX800A/GU74b tubes are operated in a grounded-cathode, grid driven tetrode configuration. Because of the low drive voltages required, input from the exciter can be loaded with a non-inductive 50 ohm resistor while a fairly simple circuit compensates for tube and circuit reactances in the higher frequency bands. This provides a very low input SWR situation across the Amateur HF bands. No tuned input circuits are required. This is a cost saving plus for manufacturer and purchaser.

Tune & Load different
The tune and load capacitor do not have the normal mechanical vernier drives. Instead, the two-section capacitors are switched for the bands to provide an "electrical vernier" function.

Auxiliary muffin fan added
Alpha claims that this amplifier provides 1,500 watts peak power output on SSB, keyed CW, SSTV, RTTY, digital modes or FM, with no time limit. They also mention that "continuous key-down" , as in brick-on-the-key operation, for more than five minutes requires additional cooling. Well, I knew that. You knew that. We all knew that, didn't we? This unit was shipped with the optional auxiliary muffin fan attached over the rear air inlet. The fan is also required when the model 99 is powered by 50 Hz AC mains.

I suspect the centrifugal blower is completely adequate for the tubes, but, in high power output situations better circulation is required to the front of the transformer where the rectifier/filter and control circuit boards are located. Especially, if transformer efficiency is reduced when using 50 Hz AC power. For me and most of those trying to wear out microphones, the normal model 99 cooling system is more than adequate.

Vacuum relay QSK
Full break-in QSK circuit provides wonderful enhancements for telephone-like SSB conversations or efficient high speed control for all other modes.

The manual
As good as the best I've seen. Nicely bound, complete with schematics.

And the power supplies?
The circuit board area includes the HV bridge rectifier, filter capacitors, meter and control circuits and components. Alpha has stuck with the full wave bridge rectifier. The filter section is populated with nine 220mf electrolytic capacitors, each rated at 400 v., providing about 25 mf filtering and a working PIV protection of 3,600 v.

A special Peter Dahl Hypersil transformer of the 3.5 kw + capacity provides plate and other AC voltages. The design no load B+ voltage should be 2,900 VDC. Our instruments measured 2,969 VDC with 234 VAC from the mains. Other secondary power from the transformer provide voltages for the requirements of metering, grid, control, bias, soft start and QSK T/R circuits.

Some assembly required
Not much though - the Peter Dahl transformer has a nice strap for lifting. It also has the 1/4"-20 nuts attached to the top side of the transformer mounting plate so the furnished stainless cap screws with washers can be quickly run in from the bottom of the case, thank you. The pan has several guides to locate the transformer and all three electrical plugs are pre-fitted, pre-shaped and in place for quick installation. It's a snap. The cover fits great. I put it back together sans one screw and no extras came with the unit.

Our test bench is equipped with a 2000 watt fan cooled dummy load, 3,500 watt low pass filter and Bird 43 and Coaxial Dynamics watt meters cabled in series, each with 2,500 watt slug. 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 wattmeters are used between exciter and amplifier to measure drive power and SWR.

But does it run?
Well, let's see. Yes, it lights up, lotsa lights, red, green, green LED bars, flashy stuff. A red LED stays on until the internal gyros wind up to speed. Kidding, no gyros, just indirectly heated cathodes being heated up. Tuning is easy and straightforward. If you've developed good tuning habits or can read the manual, it's a snap. The green LED instrument bars do a fine job of feedback to the operator. The continuous green LED bar displays of forward and reflected power are especially impressive. Reflected power green bar display runs to red when it exceeds 10% of forward power. The forward power green bar display runs to red when power out exceeds 1500 watts. *(The illuminated red "Bad Boy" sign pops up and the audio alarm message, "Bad Boy", "Bad Boy", "Bad Boy" sounds until power is reduced to legal. Of course if you're on a phone band with VOX, other Hams will hear this and know you for what you really are.)

I went through my systematic load test for the following bands on the dummy load. Results below coincide with the tune up table furnished with the amplifier. Drive signal is CW.
Freq. Drive Plate Current Power Out
3.90  Mhz 55W 900MA 1500W
7.200 Mhz 55W 900MA 1500W
14.225 Mhz 55W 900MA 1500W
18.140 Mhz 55W 1000MA 1500W
 21.350Mhz 55W 1000MA 1500W
28.400 Mhz 55W 1100MA 1500W

Over drive it
The above test table reflects the loafing facts. For you bad-boy wantabees, this amplifier is just loafing at 1,500 watts CW output. On each band during the tests, except for 10 meters, I continued tuning on the dummy load in short segments and increasing drive until full output of the 110 watt exciter was reached or power out exceeded the full scale capacity of both wattmeters (2,500 watts). In all cases we were beyond the accurate measuring capacity of both meters. This means we could not accurately determine the maximum power output but it's 1,000 plus watts beyond legal limit in CW mode.

Superb legal limit + HF linear amplifier. For a competitively priced, single box legal + amplifier, the model 99 is smaller, lighter, more compact and more powerful than any Amateur Radio amplifier this writer has handled, used or examined. Wonderful design, excellent components and quality workmanship throughout. But - - - is Alpha still King-of-the-hill? Well, hell yes. I knew that. You knew that. We all knew that. See all the Alpha amplifiers at their new  web site.

Other Conclusion
At the beginning of this article I mentioned that I love the warm, glowing tubes. I still do, but may use one of these powerhouse ceramic-metal jobs in warmer weather - less heat, ya-know?

* Satire

Bob Hutchinson, N5CNN
President and Founder
Wireless Industry Association
713 467-0077

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