A Few ALS-600 Projects - W1AEX

I picked up one of these great little amplifiers in the summer of 2013 and have found it to be my amplifier of choice during the warm weather months here in Connecticut. I drive it with my ANAN-200D and also my Kenwood TS-590S and have found the ALS-600 to be a very reliable amplifier that delivers everything Ameritron promised to my hex beam and 190 foot open-wire center-fed antenna. I use it for AM linear, sideband, CW, and a little bit of RTTY and it has proven to be very rugged and reliable. With that being said, there were some behaviors that were a little bit off in my opinion and so after a little investigating I decided to address the annoyances that were compromising the amplifier's performance. None of the annoyances was particularly difficult to resolve and once I had completed the changes the amplifier has been as close to perfect as anything I might put on the bench. The links below will take you to an explanation of what I did to get my amplifier squared away.

Loud buzzing sound coming from the linear power supply

Poor IMD performance on all bands even at modest output levels

Much higher drive required to reach full output on 75/80 meters

Random failure of the TX/RX open frame relay to transition

Loud buzzing sound coming from the linear power supply

When I first installed my amplifier the linear power supply was perfectly quiet, however, within a few hours it started to make a slight buzzing sound. By the end of the first day, the noise got louder and louder until it sounded like a grade school science project. Removing the top cover quickly revealed that a spacer in the PS filter choke had broken free of the lamination core and was vibrating without restraint. The white arrow in the picture below shows the location of the problem.

Having worked on a lot of vintage equipment I have run into this kind of thing before and knew there were about a dozen ways to deal with it. I chose one of the quick and easy fixes and simply grabbed some black electrical tape and folded it onto itself a few times to create a thin and flexible wedge which I jammed between the spacer and the lamination core. That worked like a charm and the noise has never returned. If you encounter this annoyance I suppose you could contact Ameritron and demand a new PS choke but I'm sure they would require that you box the whole amplifier up and ship it back to them first. Seems like a waste of time and money so I'd suggest using a simple fix since the problem is annoying but not likely to cause any future problems. At any rate, the spacer wedge solution has worked perfectly since the summer of 2013.

Much higher drive required to reach full output on 75/80 meters

The problem with the need for far greater drive to get the amplifier to perform on the 75/80 meter band is quite well documented in many forums where the ALS-600 is discussed. I found that 40 to 50 watts would easily drive my ALS-600 to either 500 watts of CW or 600 watts of sideband on all but two bands. The bands where more drive was needed were 75/80 meters and 10 meters. My issue on 10 meters was easily resolved by adjusting the spacing between turns in the 10 meter filter. One of the inductors looked like it had been squashed and several turns at one end might even have been shorted. At any rate, I removed the filter and after spending a few minutes "re-shaping" the malformed inductor to make it look nice I re-installed it and found that instead of needing around 70 watts to deliver full output it only required a bit more than 50 watts to get there. That was definitely worth the 10 minutes of adjustment.

The issue on the 75/80 meter band was a little more interesting. Depending on where I was in the band it could take as much as 85 watts to reach full output and from what I could see of the current being drawn at that power level the efficiency was terrible. In addition, it was immediately apparent when using my ANAN-100 with the adaptive pre-distortion linearization protocol enabled that something was seriously wrong on that band. Normally, when the RF sample from the output of an amplifier is displayed in the OpenHPSDR AmpView window, non-linearity will be displayed as a gentle curved line (distortion) with the linearity correction (pre-distortion) shown as an exactly opposite image of the distortion, as seen in the third image below. When the algorithm applies the correction, the actual transmitted signal from the amplifier is a nearly perfect (linear) straight line that is seen as a diagonal between the distortion and the pre-distortion representations. The first two images below show the anomalies observed on 80 meters, first at 300 watts output where adaptive pre-distortion looks a little confused and then at 600 watts output where the adaptive pre-distortion algorithm is unable to predict what the amplifier is doing because the amplifier is not behaving normally. The third image shows the same amplifier
behaving normally at 600 watts output (SSB) after applying the 2.3 - 4.4 MHz output filter modification discussed below.


After a lot of discussion by several owners in the Ameritron forum at Yahoo W8JI (Tom) said he would take a look at a late model amplifier to see what was going on. He noted the same higher drive issue and after some time on his bench he found that adding some padding across the 680 pF input (C305) and output (C311) capacitors
of the 2.3 MHz to 4.4 MHz filter helped to alleviate the problem. He suggested soldering a 160 pF transmitting mica across each capacitor in his summary post in the Ameritron Amplifier group at Yahoo:


 Note that these capacitors need to be transmitting mica caps capable of handling high RF current. He noted that you must use snubber caps in a DM19 or larger case value. You can buy suitable caps directly from Ameritron since these values are used in the 14.5 MHz - 21.6 MHz filter of the ALS-600.

Ameritron part # 208-6161  (160 pF 1000V CDV-30)

I found that these capacitors are too large to mount side-by-side with the 680 pF caps in the output filter assembly. The only solution was to solder them to the underside of the output filter board and this is very easily done by removing the four screws holding down the board and carefully sliding the board back slightly to clear the remote connector that extends through the front panel. Once you have cleared the front panel tilt it up and insert a small screwdriver to hold the board up while you do surgery. The picture below shows what this looked like after I installed the padding caps. Due to the proximity of the metal shield under the filter board I wrapped some black electrical tape around the capacitors for a little added insulation. Note that this probably is not necessary as one side of the cap is connected to ground anyway, but there are other traces that the caps will be pressed against that gave me a little pause for caution. When I lowered the output board back down and positioned it so the four holes for the fasteners were aligned I noticed that the caps were fat enough to cause the board to flex very slightly. It didn't look like it would be an issue so I decided to let it ride that way.

After bolting the output filter board back down and powering up the amplifier I found that the amplifier reached full output with just under 55 watts, which is very close to the drive requirement for all the other bands. I ran the amplifier this way for about 6 months without any issues at all, but it bothered me that those capacitors were squeezed under there. Eventually, I ended up browsing Digi-Key and Mouser to see if they carried mica snubber caps in the 800 pF to 900 pF range. As it turned out, they did not list any caps using the CDV-30 case as used by Ameritron but they did sell DM19 (CDV19) 1000V caps with values of 820 pF and 910 pF. You can also find them at Digi-Key by clicking the Cornell Dubilier part number in parentheses below:

Mouser part number:     598-CDV19FF821JO3    820 pF 1000V DM19   (Cornell Dubilier Electronics (CDE) CDV19FF821JO3)
Mouser part number:    
598-CDV19FF911JO3    910 pF 1000V DM19    (Cornell Dubilier Electronics (CDE) CDV19FF911JO3)

I grabbed a pair of each of those values and opened the amplifier back up when they arrived. After removing the padder caps on the underside of the board and the original 680 pF caps, I installed the 910 pF DM19 snubbers from Mouser. You can see in the picture below that they are physically smaller than the original CDV-30 case capacitors.

I found that the 910 pF value was slightly more efficient than the 840 pF total of the original 680 pF caps and the 160 pF padding caps so that 50 watts of drive will produce 500 watts of CW output or 600 watts SSB across the 75/80 meter band. I was happy enough with the result that I put the amplifier back together and called it a day. I completed the modification at the end of June 2015 and the amplifier has performed flawlessly since then on the 75/80 meter band.

Poor IMD performance on all bands even at modest output levels

One of the rigs that I use to drive the ALS-600 is my ANAN-100 software defined radio. A very slick feature of this rig is that it is the only 100 watt transceiver that offers pre-distortion linearization. This feature, known as Pure Signal, allows you to place an RF sampler at the output of any station amplifier for the purpose of feeding a low-level sample of the signal into one of the ANAN's receivers. An algorithm in the OpenHPSDR software analyzes any distortion (IMD) that is present in the output signal and then pre-distorts the input signal to cancel out the distortion at the output. This is a simplification of what is going on but it works brilliantly and results in a "correction" of your signal to allow your entire transmit chain to reach 3rd order IMD levels better than -50 dB down. I found that this feature worked beautifully with my AL-82 amplifier on all bands and the pre-distortion protocol didn't have to work very hard to produce results exceeding -50 dB down. The picture below shows my ANAN's SSB signal as received on a separate SDR while running adaptive pre-distortion linearization (OpenHPSDR Pure Signal) as it's transmitting 1500 watts PEP output with my AL-82. The straight up and down filter skirts and lack of artifacts above and below the signal are evidence of excellent linearity as the Pure Signal algorithm does its work.

When I switched over to the ALS-600 I was surprised to see that the Amp View display in OpenHPSDR indicated that the correction factor was enormous compared to my AL-82. Additionally, the algorithm would randomly stop correcting whatever the ALS-600 was doing at power levels above 400 watts PEP. A quick two-tone test showed that the 3rd order IMD of my ALS-600 was in the range of -25 dB to -28 dB with the worst results at full output. This was a bit surprising as the MRF-150 output devices are easily capable of producing signals that exceed -30 dB without any problem at all, assuming they are biased for optimal idling current and well balanced. As it turns out, my ALS-600 was not biased for optimal idling current and the device pairs were not idling close enough to each other for satisfactory operation. I found some chatter about optimal bias settings for the MRF-150 devices and suggestions ranged from 100 ma to 300 ma for each device. When in doubt, in my opinion the best place to look is the specification datasheet of the manufacturer of the device. I found that Motorola used an IDQ setting of 250 ma per device in the test setups found in their datasheet. That was good enough for me so I decided my target idling current per device would be 250 ma. It has been reported by some users that Ameritron is setting each device for 300 ma when the amplifiers are sent in for service. I don't know if that's true but the devices can easily handle settings in the 250 to 300 ma range. Whatever target you aim for make sure you set all the devices to the same idling current.

The only equipment needed to set the idling current for each MRF-150 is a current meter that can measure at least 2 amps of current, a few clip leads, and an insulated adjustment tool. With the linear power supply, the problem of finding a convenient place to insert your ammeter is easily resolved by pulling the fuse (F3) and attaching clip leads to each solder tab of the rear panel fuse holder. Both of my digital ammeters are fused (with 2A fast blow fuses) so if something fails during the adjustment and sends an MRF-150 into full conduction the meter fuse will blow at a point that is quite safe. I'm unsure whether or not a B+ fuse is present at the back panel of the optional ALS-600 switching supply so you may need to improvise if you don't have the linear power supply. The narrative and pictures below should make it clear how to proceed.

One other thing that you must contend with during this adjustment process is the slow rise in idling current due to thermal effects. I found that if I keyed the ALS-600 while observing the idling current that the devices settled down after the amp was keyed for several minutes. After a few runs at this procedure I found that I got the most consistent results by keying the amplifier and waiting for the idling current to stabilize, and then turning the bias po
t for MOSFET #1 clockwise to the cut-off point. After noting the idle current I adjusted the bias pot counter-clockwise to set the chosen idling current for MOSFET #1. Proceed to MOSFET #2 without unkeying the amp and then repeat the process for MOSFET devices #3 and #4. This seemed to give far more consistent results than zeroing all the devices to cut-off and then setting all the devices in sequence.
The step-by-step adjustment directions are listed below:

NOTE:  No RF drive is applied during these adjustments so disconnect the coax cables from the RF input and RF output connectors of the ALS-600 before beginning.
1. Remove the cover from the amplifier and the power supply.
2. Remove the 10 meter filter from the amplifier deck to allow access to bias adjustment ports 2 and 4.
3. Remove the fuse from the power supply.
4. Use clip leads connected to the two solder connections on the fuse clip as shown below to insert your (fused) ammeter at this point.

5. Power up the amplifier
6. Key the amplifier and let it sit until the idling current stabilizes (usually several minutes) and then record the total current draw at idle on a notepad.

7. While observing the current meter, slowly turn bias pot #1 clockwise to assure that the current is dropping. Continue turning the pot clockwise until you reach the cut-off point where the total current does not drop further.
8. Record the total idling current so you can establish where that device was idling from the factory and then slowly turn bias pot #1 counter-clockwise until the idling current is increased by 250 ma.
9. Repeat the adjustment procedure in numerical sequence for bias pots 2, 3, and 4. As noted in step 8 above, record the idling current when you reach cut-off for each device to establish how each MRF-150 was set from the factory.

When you have completed the adjustments, power down the amplifier, remove the clip leads, install the fuse, and put the covers back on. When the amplifier is re-assembled slide it back into its normal operating position and you are good to go. My adjustment notes below show how my amp was set up from the factory. The imbalance between devices 1 and 2 which form one push-pull pair probably explains why it was performing less than optimally. After running the amplifier for several months I repeated the adjustment procedure and found that the idling current for each device was still within a few milliamps of my first setting. After correcting the idling current issue I found that my ANAN with OpenHPSDR pre-distortion linearization engaged had no issues correcting the ALS-600 on any band. Making sure that your ALS-600 amplifier is idling correctly will help keep you from ending up on this page!

------------------------------------------------W1AEX notes during bias adjustment--------------------------------------------

Total idling current before adjustment:  610 ma

With device #1 at cut-off:    535 ma (610 ma - 535 ma = 075 ma idling current for device 1)
With device #2 at cut-off:    415 ma (610 ma - 415 ma = 195 ma idling current for device 2)
With device #3 at cut-off:    465 ma (610 ma - 465 ma = 145 ma idling current for device 3)
With device #4 at cut-off:    435 ma (610 ma - 435 ma = 175 ma idling current for device 4)

Total idling current after adjustment:  1020 ma  (Each device @250 ma x 4 devices = 1000 ma + 20 ma for residual current draw by devices other than the MRF-150 MOSFETs = 1020 ma)

If you total up the mathematically derived factory idling current for all 4 devices you will see that it comes up to 590 ma. So... what about the other 20 ma? I found that if I set all the devices to cut-off there was a residual current draw of 20 ma. At any rate, you can certainly do the bias procedure by zeroing all the devices and then going back and increasing the idling current by 250 ma for each device (be sure to observe the numerical sequence in the photograph above) but the upward idling current drift problem is more of an issue that way. I found it easier to get more precise results by keying the amp, letting it sit at idle for several minutes, and then zeroing one device at a time and then setting the idling current for that device and repeating this for each of the remaining devices. Either way will work fine as long as you are careful.

A two-tone test showed the amplifier delivering better than -34 dB third order IMD which is quite respectable. With the Pure Signal pre-distortion linearization protocol engaged the amp reached -45 dB or better on all bands without any difficulty at full output.

NOTE: During the summer of 2017, as part of my annual amplifier clean-up, I put the ALS-600 back on the bench to check the bias settings. The previous settings were holding very well but I decided to set each device to idle at 200 ma for better cooling. There has been a lot of discussion about what the optimal resting current is for this amplifier and the truth is that the range for acceptable IMD performance is pretty generous. If you are going to use the amplifier for RTTY or AM linear the 200 ma setting is a good compromise as it will still produce a very clean signal on SSB and thermal stability will be greater when running continuous duty modes. On hot summer days, with the amplifier set to idle at 300 ma or even 250 ma, the thermal protection switch would sometimes flip the amplifier into "standby" if I transmitted longer than a couple of minutes. With an idling current of 200 ma per device the thermal switch never actuated during the continuous duty modes so it looks like it's right in the sweet spot. Following the adjustment, I checked the amp's linearity by using the AmpView utility in the OpenHPSDR mRX PS software and found that the amplifier was still correcting perfectly on all bands.

Total idling current after the June 2017 adjustment:  820 ma  (Each device @200 ma x 4 devices = 800 ma + 20 ma for residual current draw by devices other than the MRF-150 MOSFETs = 820 ma)

Random failure of the TX/RX open frame relay to transition correctly

In the summer of 2014, after a year of operation, my ALS-600 began to randomly fail to properly transition from RX to TX as the open frame T/R relay would not reliably close. As you would expect, this resulted in a very high SWR for the exciter as it looked into the open connection it was presented with. Mashing the PTT function of the exciter would clear the problem but it would randomly show up again after a day or two and as time went by it began to happen more frequently. Browsing the forums where Ameritron amplifiers are discussed revealed quite a few similar complaints other owners have had with the open frame relay. This kind of surprised me because the AL-80B that I owned for years used the same open frame relay and it never gave me any problems at all.

At any rate, I grabbed the ALS-600 and put it on my work bench to see what was going on. After pulling the cover off the RF deck and powering it up on the bench I cycled it between RX and TX while watching the relay. Sure enough, after about 10 successful transitions the contact pivot arm got hung up and did not fully close. Cycling it a few times would clear the problem each time it happened. After watching this behavior through a few cycles I determined that it was a simple mechanical problem with the contact pivot arm. As you can see in the photo below, there is a spring at the top of the relay that keeps the RX contacts tightly engaged until the coil is energized and the magnetic pull overcomes the spring tension to pull the pivot arm inward so that the TX contacts are firmly engaged.

Closer inspection of the relay pivot point the next time it failed to transition revealed that one side of the pivot arm was not pushed completely onto its pivot tab, causing the pivot arm to be slightly misaligned. The second picture below shows where the pivot tabs are located on the frame of the relay. I grabbed the pivot arm and carefully slid it off the pivot tabs and then slid it back on. I noticed that one of the pivot tabs seemed to fit much more tightly than the other and so I slid the pivot arm on and off the tabs several times, which seemed to free up the movement nicely. I suspect there might have been a little burr or other irregularity on one of the tabs that prevented the pivot arm from successfully moving through its full range. The failure probably started to appear when the pivot arm eventually walked a few millimeters away from its fully seated position and got hung up on that rough spot. At any rate, the relay has worked flawlessly since the summer of 2014 so I consider the problem to be fully resolved. If you run into TX/RX transition problems with your open-frame relay and want to try this "fix" just be careful to avoid stretching the return spring excessively as you slide the pivot arm on and off the tabs.