I recently decided to sell some of the older equipment at my station and try something new. Previous amplifiers were an Alpha 9500 and Alpha 8410. Initially, I considered changing from an automatic, to manual tune amplifier. I acquired a used Alpha 8410. However, I quickly discovered I missed the automatic capabilities of an auto-tune amplifier.
I seriously considered the Acom 2000A, but when a conversation with a known source for replacement match pairs of GU-74 tubes indicated "out of stock", "not sure when we will receive more" and "I am sure there may be some out there somewhere???" left me a bit leery. I decided to avoid purchasing an amplifier that uses surplus out of production devices. I know there probably are plenty of GU-74's around, but with my luck, that will come to an end the day I would need a new tube! So I decided to look elsewhere. Based on my research, the OM Power amplifiers are comparable to other top of the line amplifiers offered from Acom, Alpha, and Emtron. I note, the quality and construction is just as good as the Alpha's I've owned.
The OM 4000A utilizes a pair of still in production FU-728F tubes. That tube is compared to the Eimac 4CX1500. However, claims are the FU728F can handle considerably higher dissipation. I don't doubt that claim. OM Power rates the 4000A amplifier for 4000 watts PEP and 3000 watts continues duty cycle. Testing into a 5 KW dummy load, indicates the amplifier can easily reach advertised output. Legal limit for the U.S.,(1500 watts PEP) is achieved with very little drive power. It has more than enough headroom for any mode/any duty cycle at 1500 watts. I have no doubt, that someone could literally transmit nonstop for days at legal limit without the OM 4000A breaking a sweat.
The OM 4000A weighs over 100 pounds, and it takes a bit of muscle to place it into position. I have my station operation integrated with HRD 6.3, for rig control and logging, using common RS232 for CAT. The 4000A was easily added to my station using standard straight DB9 cables. The amplifier includes a built in RS232 device that passes data from the transceiver to the computer, that accommodated my switching arrangement. The amplifier has a single input and output RF connection, but also has additional data ports to control other devices for band/frequency segments, such as remote antenna switches, or band pass filter networks. My station consists of Yaesu and Kenwood transceivers, which are supported using RS232, along with many other transceivers. Users of Icom equipment are also accommodated. The 4000A includes a CI-V port for direct connection to them as well.
It takes a bit of initial effort to set up the 4000A amplifier for automatic tuning. Setting communications between the transceiver and the amplifier is very easy. Once they are communicating, the amplifier will keep apprised of the frequency data from the radio. I want to note, the OM Power 4000A is different from a Alpha 9500 I previously owned. The Alpha 9500 is a true FULLY automatic amplifier, much like a fully automatic antenna tuner. On the Alpha 9500, one can simply set the amplifier into automatic mode, send RF, and it will tune itself for whatever frequency/band you are on to achieve the best combination for output.
However, the 4000A uses presets, and automatically adjusts the Tune and Load to predetermined settings, ONLY after the operator has performed an initial manual tuning sequence for each band/segment, and loaded those settings into the memory registers.
Operationally, the amplifier works well. It's very well protected, and will fault out for the following conditions
- VSWR too high
- Input power too high
- Output power too high
- Plate voltage too low
- Anode current too high
- Screen current too high
- Grid current too high
- Mistuning of PA
- Hot switching protection
- Soft start for protecting your fuses
- “switch-on blocking “ at opened amplifier.
Manually, it's very easy to tune if needed. In auto mode, tuning is repeatable and consistent. I like the fact, I can step through various bands tuning around for stations to contact, and the amplifier does not retune until I apply RF. That feature saves on wear and tear of the stepper tuning motors. Nevertheless, just momentarily depressing a foot switch I use for PTT, initiates the tuning sequence that lasts for a mere second or two. Noise from the switching relay is noticeable, but it's not objectionable.
My only complaint concerning the amplifier is the blower for the tubes. It's quite a bit more noticeable than I am used to hearing. Although I have not received any reports that blower noise can be heard when I am using Phone. I contacted OM Power to see if something could be done to adjust the blower to reduce the noise. OM Power is very responsive, but ultimately it was explained to me the blower speed is factory set to dissipate 3000 watts of heat from the tubes at maximum output. Thus, there is no way to slow down the fan without unauthorized circuit changes.
Overall, the OM Power 4000A is a well built amplifier with exceptional reserve power. It does everything that OM Power advertises without a glitch. IMO, it's the best amplifier in this power class, and it's the only one that is auto-tune. I can easily see the 4000A serving in the role as a perfect amplifier for a serious contest or club station. It's ability to remotely assign antennas switches or other devices based on frequency is a useful attribute. Another feature is the ability to remotely use the amplifier. There is an optional remote unit that allows the amplifier to be remotely controlled over a network.
The OM Power 4000A is an exceptional amplifier. I occasionally notice other operators still purchasing 30+ year old two tube Alpha 77's for nearly the same price as this one. IMO, this is the modern replacement that takes it to the next level, adding remote control capability, auto tuning, CAT interface, and the ability to implement external control of many outboard antenna system devices based on frequency.
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