ACOM 1000 Amplifier
Features, functions, a howto and opinions by Jason Buchanan - N1SU
My quest for a big amplifier started before I put up my first antenna at my new house. Of the players out there, the QROTEC amps looked to be the best product available and whatever question you might have about their construction was washed away once you saw the great color photos on their site. On the other hand, a company in Bulgaria named ACOM had their own big guns, but not so well known. However it only takes a few minutes to figure out why ACOM is the leader in HF and 6 meter amplifier technolgy.
The True Resistance Indicator (TRI) tuning aid display will visually show you how to tune for optimum linearity and least stress to the tube. 88 watts of drive will produce approximately 1250 watts output, even on 6 meters!
If you switch from one band to another without retuning the unit will automatically insert a 6dB attenuator to reduce drive. The attenuator allows you to tune up without touching your rig's output levels - if you are already close to a proper tuning match the attenuator does not engage. Depending on how fast you are, you will be ready to go in under 5 seconds.
Once tuned you have about 100KC of room to move around without needing to re-tune. My experience so far has shown that you can move quite a bit more than that with little expense to linearity or power output. The TRI display will tell you if you need to touch up the tuning - operation is effortless and worry-free.
The list goes on but its performance is the icing on the cake. The True Resistance Indicator (TRI) tuning aid shows a graphical pointer on the LCD display indicating which direction to rotate the LOAD capacitor knob. The center of the scale corresponds to the proper LOAD capacitor tuning which presents an optimum load resistance to the tube and optimum linearity. 60 watts input power yields 1000 watts output continuous load.
While you're operating, a bunch of other nifty stuff is going on in the background. The tube chimney has a small thermistor that constantly monitors the exhaust air temperature - when it reaches 130F the fan speed increases slightly, but you'll never know it because you can't hear it!
There's no need for a giant antenna tuner - 3:1 SWR is easily matched by the very big load and plate capacitors. Arcing would be difficult to achieve, and because the 4CX800A tube operates at a lower plate voltage under load (2400V) you don't have the same worries as a 3-500Z amp has.
The more complex digital modes like MT63 are tough for some radios to handle. Surprisingly, the amplifier was able to perfectly handle MT63 1000Hz Long at full power out - over 1200 watts PEP! I made 11 MT63 contacts at full power and no one knew that I was actually running a linear with occasional 1400 watt peaks - truly unbelievable. I had equal success with BPSK31 and BPSK63 modes at 1200 watts output - with no awful sidebands splattering the band.
Running digital modes heats things up but the amplifier takes it in stride. The exhaust air temperature rose to 180F during a very long BPSK31 QSO but even though the fan speed sped up it was impossible to hear the difference.
Some photos of the back of the unit - notice the Key In and Key Out connectors - for QSK operation this is a great feature to have.
These things are built tough. The transformer is bolted in with four very strong Phillips screws. No disassembly or re-assembly required. The unit I am using was on a DX expedition with a few scrapes and scratches typical of an expedition but the case is not warped or bent. There was a fair amount of salt water splash on the top of the cabinet but a little elbow grease took it off.
The top of the unit has a single exhaust port - all of the air brought into the unit circulates through the entire chassis and is exhausted through the top of the cabinet. Cool air flows across the transformer and HV compartments, through the tank compartment and then into the tube compartment. Air is blown through the bottom of the tube and out of the top of the cabinet. A thermistor monitors the exhaust temperature and adjusts the fan speed to keep the tube cool. A single squirrel cage fan pulls air into the rear of the unit and is forced out through the cooling fins of the 4CX800A tube. The placement of the fan is excellent because it is next to the tube compartment and a considerable distance from the air intake at the back of the unit. The noise is almost completely eliminated due to the double-walled chassis that surrounds it.
The cabinet has 2 interlocks including a HV grounding if the top cover is removed. The RF compartment has a second aluminum enclosure that surrounds the entire RF deck. VHF parasitics and radiation are eliminated with this configuration. Sound dampening is enhanced - the vacuum QSK relay is very quiet. The anode HV handoff is filtered to prevent RF feedback and VHF parasitics finding their way into the HV supply.
VHF parasitic suppression for 160 through 6 meters operation is maintained in a variety of places throughout the unit. The tank components are silver plated.
The base of the tube compartment houses the SWR bridge and various monitors with additional temperature monitoring components.
More to come - I hope to finish this page in a few days!
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