By Dave Jensen, W7DGJ

A few issues ago QRZ published my column about great ham radio gear that originates in Eastern Europe. That article focused on companies like RigExpert and others from Ukraine or surrounding countries. Because I reviewed a neat antenna switch from Bulgaria, a number of readers wrote and asked, "Hey, did you forget about ACOM?"

Well, that's exactly what happened. The brand is so well established in the linear amplifier market (with a generally fine reputation) that I assumed it was from Japan, Germany or America. Nope, these products are produced in Bulgaria by a company that global users have admired for years. I remember a friend of mine and his comments about his ACOM amplifier. He loved it, even though it was extraordinarily expensive and it required two hams to get it up on his shack desk. No kidding . . . that amp, and in fact most of the amps that ACOM has had over the years, is a bruiser. You wouldn't want to try lifting it yourself, or even taking out the transformer and doing some modifications without a partner around to help out.

So, when the CEO of ACOM offered to send me a loaner for review of a new "light weight" Solid State Amp, I jumped on the opportunity. The unit I discuss below is one of the first 500S produced and it was shipped to me directly from the testing laboratories for FCC certification (which it has now achieved). The ACOM 500S amplifier will be sold in the USA by DX Engineering, and has a retail price through DXE of $3395.00. A search of the world market for this amplifier shows that other countries have announced it as well and are taking deposits -- but at a substantially lower retail price. While it might save considerable money to order from a British ham shop, ACOM has their dealers set up as providers of warranty service and it may not be worth the savings to take the risk of possible international shipping for later repair. Why is it that the amp is so much more expensive in the USA? 

Of course, the size and heft of earlier ACOM amps has also been an indication of quality, and with hundreds of reviews on eHam.net and other locations around the Internet, I'd say that ACOM has earned that reputation. They've had a few negative and some balanced reviews as well, where the poster gripes about tubes gone South, but even in most of those comments the poster states that the matter was handled well by ACOM's customer service. They are relatively new to the Solid State amplifier business, but this 500S is a totally redesigned (and I would say very positive) addition to the product lineup. The 700S remains in the line, but not the 600S.

A Mid-Range Amp with Some High-End Features

I love new gear, and unboxing the 500S was a treat. It arrived in its original ACOM shipping container, which is customized to fit the amplifier perfectly. Holding the two pop-up handles on the interior layer of protection brings up the 17 lb. amplifier, with edge protection tape and the usual "peel it off" protective coverings. The amplifier is supplied with a wired-in cable, unterminated, which can be easily modified with a plug for either household voltage or 240v. I would have preferred a cable that unplugs, so you can have a quick exchange of cables to make a swap between 240v and 120v, but that's a small matter and easily resolved with a screwdriver or a soldering iron. There's a fuse matter as well, depending upon which power type you are using.

There is no manual provided, but a newly published ACOM 500S manual is provided at this link. I found the instruction manual to be complete and fully intelligible, and as I'm a writer and copy editor by background, I appreciate the lack of English language gaffes. The whole document comes off as very professional and is a must-read before operation. (China, take note! If Bulgaria can pull it off, why not your country?). I'd recommend to ACOM that they provide a thumb drive with this excellent PDF, as well as perhaps some photos of the inside of the amp as hams love to pour over that info.  I didn't open the unit up as it was headed to Hamvention after my testing.

As with any piece of Ham gear, there are certain elements of the radio which are labeled differently, or which need to be considered before hitting push-to-talk. One of those is the fact that of the two Antenna outputs on the back of the rig, only the Antenna 2 connection is suitable for a tuner by another manufacturer. ACOM makes a beautiful matching tuner -- which this reviewer has not been able to get his hands on -- that mates with the Antenna 1 output. It's made clear in the manual that a resonant antenna can be attached to either Antenna 1 or Antenna 2 output but that if you're going to hook up a tuner such as an MFJ-998, you'll need to use only Antenna 2. My testing on this linear amplifier was conducted using two tuners (MFJ as well as the Mercury AT) and two resonant dipoles.

The installation at my shack was uneventful, which is what you're looking for when interconnecting fine pieces of amateur radio gear.  The PTT function out of the ICOM IC-7300 went directly into the "Key In" RCA input on the back of the amp, the RF output on the ICOM into the RF In on the amp, and the Antenna 2 served as my exclusive location for antenna switching. I used my "matching" Bulgarian antenna switch, produced by my friend Miro (LZ2RR) from that output to change antennas. 

The amplifier has a master power switch at the back of the unit, which can be left on at all times if desired, allowing you to switch the amp on or off (standby mode) from the front power control button. The screen at the front of the amp is attractive and useful, at 4.25" x 2.75." It is not a touch-sensitive screen as are some competitive products, so you'll be using the front panel switches to sort through the menu and select your parameters or settings of interest. The menu is not excessively deep and is intuitive. I had no problem getting on the air in a few minutes. The fan that cools the single LD-MOS, in combination with the fan in the internal switching power supply, could be heard but the sound was not obnoxious; my ears rarely even acknowledged fans running as I worked the bands.

The amp will provide a full 500W output on its preferred 240v. On household voltage here Stateside, I was able to get easily into the high 400's and I could have pushed it to 500w easily with a bit more drive, but common sense says that I'd prefer to have a clean signal, operating at a bit less than "full" output.

6 M - 25W in = 465W output on SSB

10 M - 18W in = 475 W output on SSB

12 M - 22W in = 450 W output on SSB

15 M  - 18W in = 425 W output on SSB

17 M - 20W in = 400 W output on SSB

20 M - 25W in = 440 W output on SSB

40 M - 25W in - 440W output on SSB

80 M - 25W in = 445W output on SSB

160M and 4M not Tested

Weight Advantage, Protection Circuits, and Remote Capability

The ACOM 500S may be a mid-range amp in terms of power output, but that's forgiveable if you've got a specific need for it's size and shape. Keep in mind that this is a very compact piece of gear. It will fit into even the tiniest shack's footprint, and is not going to require significant manpower to move it around. At just over 17 pounds (and 11.5" x 10.7" x 6.2") it represents a desireable addition to a DXPedition gear roster, giving your team's signal a significant boost without breaking anyone's back. 

Other amp features include three levels of protection circuits, some of which I appreciate but which I'm still trying to figure out . . . The first warning level is when an error has been detected but the ACOM knows it's not necessary to shut down, so it gives you a few seconds to read the warning and take action. My warnings came because I had "guessed" a bit high on the maximum exciter drive I should use when dialing into a new frequency, and all I needed to do was to dip the power by a couple of watts on the ICOM. The amp followed me nicely around the bands, generally by the sound of the first "dit" of my key or my first breath of air on the mic. One odd first warning about "High power coming too early" popped up a couple of times and I'm still trying to figure that one out. If you don't take action, you'll get a second warning that puts the amp into standby. Finally, a third level of protection drops the power supply, so the amp shuts off, storing the reason for the outage in its memory.

The various parameters that are monitored include the output relay contact state and switching times, the RF drive frequency and drive power (RF power in), the final transistor's DC current and DC voltage on the drains, as well as the gate bias voltage and the heat sink temperature, the power supply component temperatures, the RF output both forward and reflected, and some other derivative parameters. All of those elements are stored in an internal non-volatile memory which you can access via the front panel screen.

While I wasn't able to keep the ACOM 500S long enough to experiment with remote operations, the capability is definitely there. A hardware item from ACOM, the ACOM eBox (available soon at DXE), can be connected to the 500S RS232 port and the Internet. Then the amplifier can be controlled from any location by PC, Mac, phone or iPad by the standard Browser. Another way to operate this amp is by a remote control software package running on a remote PC connected to the amplifier's RS232 port. ACOM users have developed software called TRX Manager and ACOM Director, both of which can be used for this purpose -- I had no experience with those, but ACOM support tells me that user groups provide these working solutions for remote capability.

In summary, this is a fine addition to the ACOM lineup, with an A+ build quality and an A user experience rating. I didn't give the amp an A+ in user experience because there's a 10ms switchover time when operating CW semi-break in and that is too long for full QSK.

While I don't understand the need to have both a 500W and a 700W amp in the company's lineup, the newly engineered solid state design in the 500S will surely work its way upwards in the future. I expect we'll see more from ACOM in solid state if this "little guy" strikes a positive reception with hams.

73 for now,

Dave, W7DGJ

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