RSGB Review of ACOM 1000 and ACOM 2000A amplifiers (RadCom March 2001)

By Peter Hart, G3SJX, and reproduced by kind permission of the RSGB.

"ACOM is a name which is probably new to most readers. Established in 1988, and based in Bulgaria the company specialises in developing HF power products for commercial, government and amateur markets. In 1999 Acom launched its 2000A high-power automatically-tuned HF linear. First unveiled at the Dayton Hamvention in May of that year, this 1.5kW amplifier has been acclaimed by many top DXers and contesters as the best on the market. Similar in many respects to the Alpha 87A, it has more features, is significantly cheaper, and yet an equal to Alpha in terms of quality of design and construction. More recently, ACOM launched its 1000 amplifier, a manually-tuned amplifier for 1 kW output and covering all HF bands plus 6m. I was particularly pleased to obtain the first production sample for review. Although Acom amplifiers have been available for only a couple of years, the fruits of its handiwork have been visible for much longer in the Alpha 91b, which was entirely developed and manufactured by Acom under contract from Alpha. This amplifier was one of Alpha's mors popular and successful. [a review of Alpha amplifiers by Peter Hart appears in the February 1997 RadCom - Ed]

Both Acom amplifiers are based around the Gu74b / 4CX800A ceramic-metal tetrode which is available at very low cost compared with the 3CX series of  ceramic triodes or the glass 3-500. These tetrodes need very low drive and have excellent distortion characteristics. Both amplifiers are manufactured in Bulgaria from parts sourced in Eastern Europe (please see note below) and this results in a top quality amplifier at a very attractive price.

For ACOM 1000 review, click here.

ACOM 2000A

The ACOM 2000A is an automatically-tuned high-power amplifier for the HF bands, delivering 1.5kW output on all modes with 50-60W of drive. The amplifier is fully controlled via a small remote control unit which is connected by a cable to the amplifier main unit connecting the RF deck and power supply. The main unit measures 44W X 18H X 50cm D, weighs 36 kg and contains no controls or displays other than the primary power switch. It may be mounted remotely, saving space on the operating table. The power transformer is packed separately from the rest of the amplifier for shipment as this makes transportation much easier with each package weighing around 19kg. The transformer is easily bolted in place, with plug-in leads and has a carrying handle to manoeuvre it into position.

The amplifier uses a pair of GU47b / 4CX800A tetrodes in a configuration similar to the 1000 amplifier (grounded cathode, grid driven via swamping resistor) with broadband resistive grid drive, cathode negative feedback and a Pi-L output tuning network which will match into antenna VSWR's up to 3:1 (2:1 on 160m). The output network tune and load capacitors and bandswitch are stepper-motor driven from the auto-tuning control circuitry. As with the 1000, the amplifier is switched from the transceiver via a "ground to transmit" line, and a "ground when ready" feedback line to the transceiver is also provided. A vacuum antenna relay is used with fast T/R switching and sequencing to enable effective QSK operation. The power supply arrangement is similar to the 1000 but up-rated for the higher power involved.

A key feature of the 2000A is auto-tuning. The whole frequency range of the amplifier is split into 250 segments ranging in width from 25KHz on 160m and increasing with frequency up to 300 KHz on 10m. Associated with each frequency segment are 10 non-volatile memories storing the tuning settings for up to 10 antennas. In operation, the frequency of the incoming drive is sensed and the appropriate tuning settings recalled from the relevant segment memory. It only requires a single morse digit or speech syllable to achieve this. The amplifier is provided with a default set of factory-programmed tuning settings which are applicable if the VSWR is less than 1.5:1. User settings may be optionally stored, particularly where the antenna VSWR is higher than 1.5:1. Unlike the Alpha 87A, the ACOM 2000A determines the correct tuning condition automatically at the push of a button. The amplifier responds very quickly to frequency changes. Segment changes within a band take just 0.2s, band changes 1s, and a full retune, 3s maximum.

To make full use of the multiple antenna capability of the 2000A, Acom also supplies a 10-way remote antenna switch (2000SW) and automatic antenna selector (2000S). These interface to the 2000A to provide fully-protected automatic antenna switching from the amplifier, the selector or from a PC. Control software is also available which supports the FT1000MP as a driver. .

The remote control unit contains two peak-hold bargraph LED displays permanently monitoring the forward and reflected RF power levels. The control system is very extensive indeed, and structured around a number of menus and sub-menus in conjunction with an LCD panel. Some 20 of the amplifier's operating parameters may be monitored including all the valve electrode voltages and currents. There are menus to assist in the tuning, servicing, trouble-shooting and help routines with many commented displays. The power-on message can be tailored eg to your callsign, run time in hours can be monitored, the use can be locked by a PIN code, and much more. The amplifier is fitted with a RS-232 interface for PC control and monitoring of data and fault conditions. Extensive protection is built-in: the amplifier is virtually indestructible. All protection as listed for the 1000 is included, and more. Some 55 seperate fault messages may be reported on the control unit and these can even be sent as RTTY code over the phone line for diagnosis at a service centre.

Construction of the amplifier is very similar to the 1000; it is beautifully constructed and engineered. The blower mounting and cooling airflow is similar, and as with the 1000, an auxiliary fan should be used with continuous-carrier modes for transmissions longer than 15 minutes.

Measurements on the amplifier showed that, depending on frequency, 1.5kW output could be achieved with 51-60W drive and 1kW with about 40W drive. The bargraph meter was very accurate. The harmonic rejection was better than 52dB at low frequencies rising to 75 dB on 24 MHz and 28 MHz. The input VSWR was 1.2:1 over the frequency range. Two-tone distortion levels measured -34dB worst-case for 3rd order products at 1.5kW PEP output, improving by about 4dB at 1kW output. An excellent overall performance.

Following switch-on, the initial self-test is carried out and an audio confirmation sends "TEST" in CW. Then commences the 2.5 minute countdown for the valves to reach operating temperature, following which an audio confirmation sends "R" when ready to use. Then generally there is nothing else to do; the amplifier follows the exciter and delivers the power with just a brief whirring from the tuning motors when the frequency is changed. The fan is fairly quiet but noisier than that of the 1000 amplifier (Note : the fan has since been redesigned and is signfiicantly quieter on current models). The amplifier follows well in QSK but the relays are really quite noisy. They would benefit from the same mounting method as adopted in the 1000 (Note : that has been implemented with current models). A 34-page operating manual accompanies the amplifier, and is well-written and informative. Circuit diagrams and full technical data are contained in a separate technical supplement.

The ACOM 2000A is really in a class of it's own and at £3695 (Note - now £4295), this is an excellent price for what must be the ultimate linear.

Conclusions (common to both 1000 and 2000A) 

The two ACOM linears reviewed can both be highly recommended as offering excellent performance and quality at a very attractive price. The UK importer, Vine Antenna Products, offers two-year warranty support for both amplifiers and the power valves.