Wednesday, June 15, 2016

Ameritron ALS-600 SPS with switching power supply- In Depth Review


This is the long version of the ALS-600S review. An abbreviated version of this review first appeared in the March 2005 issue of QST. See QST for lab test results.

Ameritron ALS-600S with switching power supply - Walt Stinson, W0CP

Is measuring "Watts Per Pound" any way to judge amplifiers? If so, the ALS-600S just moved way up in the ranking. The overall weight of the amp and power supply has dropped almost in half, to 22.5 lbs from 44.5 lbs. Wary of back strain from moving amps around the shack, my first reaction upon picking up the amp from UPS was "where’s the other box"? The amp and power supply were shipped in one outer box, with a total shipping weight of just 36 lbs.

The ALS-600S is an optional upgrade of the popular ALS-600 (previously reviewed in QST, August, 2001). The only difference between the two models is the power supply. In fact, the new power supply is perfectly compatible with all ALS-600 RF decks and can be purchased separately. If that were all there is to it, this review could end here. However, the ALS-600S is so much lighter than its predecessor that it deserves a fresh look.

The ALS-600S utilizes four MRF-150 MOS FET RF output transistors. This device is used by many manufacturers in both transceivers and solid state amplifiers (including the Icom IC-7800 & IC-PW1, Yaesu Quadra, and TenTec Hercules). It is a "tried and true" transistor, originally developed by Motorola and now produced by MA/COMM. It's rated at about 300 watts of power dissipation, so there’s some welcome headroom in this design. The ALS-600 RF deck was designed for Ameritron by Tom Rauch, W8JI, who also had a hand in engineering the noise filtering for the new SPS power supply.

ALS-600 vs ALS-500. Though not the subject of this review, Ameritron makes another solid state amplifier, the ALS-500M. This amp utilizes 2SC2879 bipolar output transistors, another popular output transistor that runs with 12 volts. While this amp is undoubtedly used by many hams in the shack, its intended application is mobile. The ALS-600S, with its 50v supply, has about 10dB lower IMD than 12 volt models like the ALS-500M, which means a significantly cleaner signal. Moreover, the ALS-500M requires a hefty 80 amps for full output. Such engineering trade-offs are necessary and appropriate for an amplifier running 12 volts in mobile operations. However, for use in the shack, output transistors that run at a higher voltage are desirable.

Solid state FET amplifiers have several obvious advantages. Because the output network is broadband, they do not require tuning. Operation is as easy as setting the correct frequency range on the band switch. Moreover, unlike tubes, the output devices do not deteriorate with use. High voltages are not present, so arcing is not a problem, and the MRF-150 is said to have about the same high-order inter-modulation distortion and momentary overload tolerance as vacuum tube finals. No warm-up operation provides an instant boost in power at the press of a switch.

With such advantages, one wonders why solid state amps aren’t more popular. One problem has been the power supply. Apparently, it is easier and less expensive for manufacturers to build a high voltage, low current supply for tubes than to build a high current low voltage switching supply (with low RF noise) for transistors. Ironically, until now, the weight advantage of solid state designs was lost in the conventional power supplies they employ. In fact, solid state amps often have a lower "watts per pound" ratio than their tube counterparts when the weight of the power supply is factored into the equation. Now that Ameritron has addressed that problem, others will certainly follow and the popularity of solid state amps might improve.

Tunable tube amplifiers have some clear advantages of their own. The adjustable output networks they employ can be adjusted for SWR changes and buffer the tubes from the "outside world", which in amateur service can be pretty harsh at times. In solid state amps high SWR can cause current and power dissipation to exceed the rating of the output devices. So, RF power FET’s, like those in the ALS-600, require protection from excessive SWR to prevent heat damage. Of course, a manually tuned antenna coupler can always be used with antennas that do not present a nominal load, just as with solid state transceivers.

The ALS-600S employs several important protection schemes to improve reliability. At 70 watts of reflected power, the amplifier will fault and automatically switch to standby. At 600 watts output, this will occur when the SWR exceeds 2:1. Reducing the power output to 300 watts, the amplifier will fault at about 3:1 SWR. Toggling the standby switch puts the amplifier back into operation. One way to avoid this faulting is to utilize the ALS-600’s ALC (automatic level control) system to automatically reduce exciter power when the SWR rises. This requires the connection of a shielded audio type cable between the exciter and the amp. The manual does a good job of explaining how to set up the system. The amplifier will also fault if the band selector is set below the operating frequency. A thermal protection circuit will force the amplifier into standby mode when it senses excessive heat. The amp will remain in this mode until the operating temperature drops to a nominal range. Overheating can occur with extended duty cycles that tax the cooling system, or by exceeding the power dissipation rating of the final transistors – a factor of output and SWR.

Ten and twelve meter operation requires the addition of a filter board (MOD-10MB), which was obtained from Ameritron for $29.95 and a copy of my license. After removing the top cover, the plug-in board was quickly and easily mounted to the main filter board with four screws. When reinstalling the cover, I was careful to put the vent holes on the same side as the heat sink, as the proper alignment is not otherwise indicated. QSK operation is provided by an external accessory pin-diode switch - model QSK-5 ($349.95). We didn’t test the QSK option.

I like to compete from outside the US in DX contests. DXpeditioners have long put a high value on reducing weight, and amplifiers present a problem. My travel amps have included the Yaesu FL-2100, MLA-2500, Alpha 76A, ETO 91B, and ACOM 1000 – heavy beasts all. Years ago, Finnish hams introduced the concept of lightweight DXpedition amps with the FinnFet solid state amps and matching switching supplies they used on several major treks. Available in various power levels, these amps are in the 20 lbs weight class. In late 2001, I decided to find out if the ALS-600 could fill the same niche by mating it with a switching supply and testing it out on a trip to Belize.

I acquired a commercial 50 volt switching supply and mated it to the ALS-600 RF deck, just for fun. W1LLU and I used this setup in the 2002 ARRL DX Phone contest, racking up 4,500 QSO’s as V31DJ. Switching power supplies are now standard fare for powering 12v gear, but they haven’t been used much in amplifiers. Aside from the 50 volt requirement of the MRF-150, a big stumbling block is radiated noise. At V31DJ, interference from the commercial switching supply was a problem on some frequencies, especially with the Yagi directly over the shack, and it cost us a few Q’s. This experiment made it quite clear that switching power supplies intended for amateur-use require significant filtering that is absent in general purpose commercial units. It also convinced us of the practicality of mating this amplifier to a switching supply for DX trips - the entire system fit into a case that was previously used to transport the transformer of my previous amplifier, and weighed less. When we returned, I let Ameritron know all about our experiences and encouraged them to look at developing the ALS-600 into a more DXpedition friendly product. They listened!

Operating 160-10 meters, multi-single, we unintentionally abused the ALS-600 in just about every possible manner. We ran it into the wrong antenna, put the band switch in the wrong position, and experienced an RF feedback problem that caused excessive drive. We ran it for 48 hours straight with no auxiliary cooling. Nonetheless, I am happy to report that the amp faulted only when it was supposed to. It protected itself from our boneheaded, sleep-deprived band changing maneuvers, and otherwise kept on trucking. One quick toggle of the standby switch was all it ever took to reactivate the amp after a fault. Outside the contest, this amp demonstrated its attributes on the WARC bands, as well. We also gave it a good workout on CW and PSK-31, throttling the power back to 500 watts, as recommended in the manual. Unlike past trips, after a while I found myself not worrying about damaging this amp. It seems quite capable of looking out for itself.

The ALS-600SPS power supply provides 50 volts in switch mode at 25 amps maximum (1250 watts) for the MRF-150’s and 14 volts plus and minus at 2 amps for other circuits. There is a separate 12 volt supply for the current surge relay, which also supplies +12 VDC at 200ma maximum to an auxiliary jack on the rear panel of the RF deck. Jumpers allow input voltages between 90-135 VAC and 185-260 VAC. In the USA, the correct setting will almost always be 120 or 240 volts and all my operation was at 120 volts on an ordinary 15 amp household circuit. The SPS interconnects with the RF deck using a heavy duty 6’ cable with Jones connectors. This power supply uses active regulation, as compared to choke regulation in the conventional unit. As a result, the supply voltage is more stable, which keeps the output transistors happy. The supply has a meter on the front panel and, in operation, no sag was apparent in the voltage at full output. I found that the fan noise from the SPS was higher than from the conventional supply or the RF deck. Fortunately, the interconnecting cable is long enough to locate the SPS on the floor. This arrangement frees up some deck space while reducing any annoying fan noise.

I am happy to report that the ALS-600SPS power supply is a big improvement over the off the shelf switching power supply I used 3 years ago. The benefits of engaging a talented RF engineer/ham to refine the power supply design are readily apparent when comparing the radiated noise level of the SPS to the commercial unit. I couldn’t hear any noise at all from the SPS on the vertical or Quad located in the yard next to my shack. I then connected a piece of wire to my transceiver’s antenna jack and laid the wire on top of the SPS. I could hear every wall wart in my shack loud and clear, but tuning through the bands and switching the 600S on/off it took me a few minutes to find one barely audible signal from the SPS. Kudos to Ameritron.

On the air contacts reported the expected S unit increase in signal strength when the ALS-600 was switched on line. One S unit is nominally 6dB, although the actual calibration varies quite a bit among various receiver models. At 600w output, the calculated increase in strength over a 100 watt transceiver will be about 8 dB. Notably, a 1500w legal limit amplifier will only provide another 4 dB of signal strength – less than an additional S unit. It is important to keep this in mind when considering which amplifier to purchase. I suspect that for many hams, an amplifier in the power class of the ALS-600S provides all the power needed for most situations.

Using the 600S at home with my Lightning Bolt quad, which exhibits a very low SWR across five bands (20-17-15-12-10), it was easy to drive to its rated output of 500 watts output on CW with a 100w transceiver. On SSB, I obtained 600 watts output on SSB voice peaks. The theoretical output limit of the amp is about 700 watts, but it’s a good idea to stay 100 watts below that in order to keep the distortion products at a low level. Adjacent operators will thank you. Operation on RTTY is possible, but it’s advisable to keep the power below 300 watts unless an auxiliary fan is utilized (Papst 4530Z, part # 410-3140, $43). With the fan, the rating goes up to 500 watts (with a duty cycle of 2 min on, 1 off). The standard power supply has a front panel RTTY switch to reduce the high voltage to 30 volts and the maximum output to about 275 watts. The SPS lacks this feature, so it’s necessary to reduce the drive power when using digital modes.

It is bad practice to overdrive any amplifier, particularly solid state units. They tend to distort more severely than tubes when overdriven. Ameritron uses a 35 ohm, 50 watt, snap-in attenuating resistor to match the input sensitivity of the 600S to the output of a typical transceiver (older versions used a 45 ohm resistor). This also serves to bring the amp into compliance with FCC regulations that limit the drive sensitivity of amplifiers capable of transmitting on 10/12 meters. The value of this resistor assumes a 100 watt output exciter and yours might be higher or lower. If higher, you’ll have ensure that you throttle back the drive.
Over time, I came to greatly appreciate the size, weight, reliability and simplicity of this amplifier. My XYL, K0ZV, also enjoyed using the amplifier, especially when chasing DX. Still, there are a few improvements I would like to see. Automatic band switching capability would leverage the convenience of "no-tuneup" and add greatly to the appeal of this amplifier, as would a built-in QSK option. I’d definitely like the SPS to be smaller. Both cabinets have side vent holes, but the SPS is slightly longer and narrower than the RF deck and stacking is awkward. The circuit board on the SPS has unusually wide component spacing when compared to its commercial counterpart and occupies about the same desk space as the old style power supply.

There appears to be enough room in both the RF deck and SPS to shave another 10-20% off the dimensions, plus the corresponding weight. The SPS meter is a nice touch but it seems unnecessary and could be sacrificed in the interest of reducing size and weight. While some weight improvement could also be obtained by combining the two units into one chassis, getting the power supply off the desk saves space and reduces fan noise. I’d welcome an integrated unit, if the fan noise was reduced along with the combined size.
The lamps in the RF deck and SPS meters are overly bright and mine burned out after a couple of years. I replaced them with super reliable LED’s. Now that white ones are available, LED’s should be standard for meter illumination. The owner’s manual, while including comprehensive schematics and operating instructions, lacks a concise specifications page. It includes a brief addendum for the SPS (no schematics), but otherwise has not been updated for the 600S.

In designing any amplifier there are engineering trade-offs. In the real world there is no "perfect" amplifier. In the past, the desire to achieve high performance and low price has necessitated trade-offs of space and weight. As a teenage ham, I owned a Heath Apache that weighed 100 pounds. I doubt that a 120 watt, 100 pound transmitter could sell today. A steady improvement in technology through the ensuing years has permitted size to shrink and weight to drop. For the most part, these advances have not been as evident in amplifiers as they have been in transceivers. For the sake of our backs, it’s time for that to change. Even tube amps can now be designed with a switching supply. Hopefully, we’ll see more examples in the future. There’s nothing fun about lugging around a 60 lbs amplifier – or paying excess baggage charges. But now, fellow Contesters and DXpeditioners, the ALS-600S makes it possible to pack a transceiver and 600 watt amp that together weigh less than 30 pounds.

(c) Copyright 2004 by Walton Stinson. All rights reserved.

No comments:

Post a Comment