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.
ACOM 1000 The ACOM 1000 is a self-contained amplifier with athe RF deck and PSU in one box measuring 42.2W x 18.8 H x 35.5 cm and weighing 18 kg. It uses a single GU47b / 4CX800A to deliver 1000W output on all modes from 50-60W of drive with no time limit on SSB or CW, although for continuous-carrier modes like RTTY, an auxiliary externally-mounted fan is necessary for extended transmissions longer than 15 minutes. The amplifier uses conventional tune, load and bandswitch controls, and these are fitted with large knobs for ease of use. The primary mains power switch is located on the rear panel. which enables the microprocessor-based control system. The amplifier power switch is located on the front panel, together with an LCD display and three other buttons via which a whole host of set-up and monitoring functions are accessible. The amplifier is switched from the transceiver via a "ground-to-transmit" line and the linear also provides a "ground when ready" feedback line into the transceiver to prevent hot switching for radios that support that facility. A vacuum relay is used and fast T/R switching to enable effective QSK operation.
    The valve operates in grounded-cathode with a  cathode resistor to proviede some RF negative feedback. The drive power is applied to a 50 ohm resistor across the control grid with some additional matching to tune out the valve capacitance. This gives a broadband match without any input bandswitching yielding a low input VSWR across the whole frequency range. The amplifier uses a Pi-L output tuning network for good harmonic rejection and this will match into antenna VSWR's up to 3:1, increasing the useable bandwidth of many antennas, and removing the need for an antenna tuner in certain situations. The tuning network is followed by a low-pass filter with a cut-off frequency of 55 MHz to reduce harmonics in the VHF range further. This is particularly important with the second harmonic of 6m falling inside the FM broadcast band.
    The mains power supply unit is inrush current protected and provides the 2.8 kV anode supply, the screen, control grid and heater supplies, and the supplies for the relays and fans. A separate PSU powers the microcontroller-based control system so this is available when the amplifier is turned off.

    Extensive circuitry is incorporated to protect the amplifier from a host of potentially damaging conditions. With this auto-protection system, sensors are incorporated to allow the control circuitry to monitor forward and reflected power, drive power, anode DC voltage and current, peak RF anode voltage, screen current, control grid current, control voltages and exhaust air temperature. From these measurements other parameters are computed, for example antenna VSWR, output power, relay closure, and a novel arrangement to detect the presence of arcs. The results of all these measurements are available for display, one line at a time, on the LCD.
    Associated with these measurements are limit levels, which if exceeded are flagged immediately to the LCD and may operate trips. The first level of protection reports when limits are getting close, eg "reduce drive". The second level trips the amplifier back into standby condition in the event of a soft fault, eg high VSWR or excess drive. The third level shuts off the AC power in the event of a hard fault, eg a major problem, or supply failure. the status of these faults is stored in non-volatile memory and the last seven conditions may be recalled to the LCD for analysis.




    The ACOM 1000 incorporates a handy tuning aid to allow simple and rapid tuning of the amplifier. This is called TRI (True Resonance Indicator). Tuning Pi networks is usually a two-handed process as the tune and load controls interact. By suitable processing of the sensor signals the correct settings of the Tune and Load controls are separately identified and displayed on the LCD. A bargraph indicator is used to peak the tune control and on a separate scale a marker shows where the loading control should be correctly set and whether to rotate the knob to the left or to the right. Initial tuning is done at reduced power, and a 6 dB attenuator is automatically inserted into the drive path when the settings are away from optimum. This is disabled when the tuning is near optimum.

    The amplifier is superbly constructed on an anodised aluminium frame with a wrap-around case. Interlocks disconnect the AC power and short the HT to ground when the case is removed. A dividing screen separates the RF deck from the power supply / control circuitry and the RF deck is also separately shielded. The output network components are very substantially rated with much silver plating in evidence on the inductors and connecting straps, and chunky "door knob" ceramic capacitors.     Full cabinet forced-air cooling is provided with the centrifugal blower mounted internally on the side of the grid box. Air is drawn in through the rear panel, past the transformer and PSU circuitry, through the output tuning components and into the grid box. The air is then forced through the valve anode via a rubber chimney, past the temperature sensor, and vented out through the top of the case. For high duty cycles or high ambient temperatures an additional fan can be fitted on the rear panel.

    Measurements were made on the amplifier under CW and two-tone SSB conditions. As the linearity of the amplifier is potentially better than most transceivers, care must be taken to use a low-distortion two-tone drive source. This was fabricated using two transceivers operating on CW with a 5 KHz frequency spacing coupled together with a high-power hybrid coupler. This arrangement yielded around 80W PEP drive power with residual intermodulation products at -50dB.

    The ACOM 1000 delivered 800 to 900 watts output with 60 watts drive power, and 1 kW output with 70-90W drive dependent on frequency; the gain was actually highest at 50 MHz. The bargraph meter was somewhat optimistic reading about 10% high on low frequencies, risng to 20% on the upper bands. The harmonic rejection was well within specification, better than 55dB at low frequencies, rising to 75dB on 24 MHz and above. The input VSWR was 1.3:1 or better over the frequency range. Two-tone distortion levels measured at -34dB worst case for 3rd order products at 1kW PEP output and -50dB for 5th order.

    When the mains power is switched on "ACOM 1000" is reported initially omn the LCD. After switching on the amplifier, it takes 2.5 minutes for the valve to heat up before the amplifier is ready to use, this countdown being reported on the LCD. This can seem interminable if you are in a hurry. The amplifier performed very well. It was docile and easy to tune, if a little critical on 50 MHz, and the TRI tuning aid enabled the correct tuning to be rapidly and simply set. The blower was very quiet in operation. When the exhaust temperature reaches 90deg C, which it can to under hectic contest conditions or RTTY, the blower engages a higher speed,  but, even in this case, the increase in noise level is not particularly great. The amplifier follows well in QSK operation and the relays are virtually silent in operation due to their special mounting. The amplifier will not allow hot-switching, it will not switch to transmit if RF drive is already present but will report this on the LCD.

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. We have some corrections to the review, which, although they were accepted by the RSGB, did not meet the publication deadline

1. In the introduction to the review Peter Hart states "Both amplifiers are manufactured in Bulgaria from parts sourced in Eastern Europe and this results in a top-quality amplifier at a very attractive price." ACOM have asked us to make clear that the vast majority of components are sourced from Western components producers. The vacuum tubes used - 4CX800A's - are made at the Svetlana factory, in St Petersburg, Russia (the oldest vacuum tube factory in the world). ACOM produce the bandswitches and variable capacitors for the tank circuits at their own factory.

2. In the description of the ACOM 2000A, the QSK is discussed "The amplifier follows well in QSK operation but the relays are quite noisy. They would benefit from the same mounting method as adopted in the 1000." In fact the ACOM 2000A reviewed was an early model and the current units use the same rubber-mounting method as used in the ACOM 1000.