Passively and Safely Neutralize the Ameritron AL-572 and Other Amplifiers
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- Опубликовано: 22 дек 2023
- Please forgive the few audio warts.
In this video of an AL572 Ameritron amplifier, I show an amplifier that was grossly out of neutralization.
This cell phone video shows how to tell if an amplifier is out of neutralization. If adjusting the plate control (resonance) causes a large change in exciter SWR and a large change in exciter power level, the amplifier is out of neutralization. Improper neutralization will significantly impact fifteen-meter through ten-meter performance.
I designed the AL572 many years ago. The AL572 uses a manufactured coaxial capacitor to balance out the 572B anode to filament capacitance. The construction of this capacitor is critical. It must have a smooth rounded end #8AWG wire that extends to the stop depth of the blind hole in the Teflon insert. The Teflon insert must be correct. This capacitor can have over 3000 volts peak across it. The capacitor cannot have insulation flaws and the tubing and wire must be free of sharp edges.
The #8 AWG wire should be just over 3 inches long.
In this amplifier, MFJ-Ameritron cut the wire too short. It never should have left the factory this way because it is impossible to neutralize. The wire they used was only long enough to null two tubes.
I replaced the #8 with a smooth rounded end wire of the proper length. The amplifier performance radically improved on 15 through 10 meters.
This method works perfectly in the AL811H only if the two tubes nearest the butterfly capacitor flapper plates remain connected, or if only the flappers are bent in and out. The test setup also works on the AL1200 and other amplifiers. 
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For lack of an inch of wire neutralization is lost. You just have to shake your head. At what point will MFJ be sufficiently embarrassed to correct such simple QC practices. I can only hope I live long enough to see that day. Thank you Tom for your amazing insight and sharing with us mere mortals. Merry Christmas to everyone.
Tom, Thanks again for sharing your considerable expertise with a clear explanation and possible fix what may be a reoccurring problem. You are a definite asset to the ham radio community! Bob W7KD
Super, Tom! Your explanations are clear and concise. Thanks!
Wow. That certainly is not a "fault" one would expect from the factory. Thanks for sharing this method with the ham community Tom. 73
I'm just a tad confused here. I understand that the amplifier was disconnected from AC power when you were doing this passive neutralization and that you were sending a voltage in across the relay which I presume is the relay that's affected when you go from standby to on is that what causes the light to light up on that switch because you were handling the caps on the tubes which I presume is the connection to the plate. Normally I would not be wanting to touch that to be honest with you I don't care about that piece of plastic. So when you went from standby to on can I assume that the power of the relay is what caused that lamp to light up I just want to make sure that's what's going on because clearly there's no way you had that amplifier on
The red light is in the 12V system. It is an LED. It illuminates when the relay is "keyed".
I am sure a lot of them are like that. This is not the first one, we have found that in every amplifier except the real old ones.
Thanks, Tom. Very instructive!
Awesome! Thanks!
As always I really appreciate you taking the time to share your knowledge. You have been so very helpful in other places! :) I had chased a high input SWR on a SB-200 for quite a while. Turned out to be a multi-point failure, mostly my only SWR meter was to blame and bad tubes. I didn't want to install new tubes until I had the amp sorted out as someone had their heathen fingers in it before I got it. I'm still new to RF, but had wondered why I needed to neutralize the HW-101 finals but not the SB-200 finals. A google search didn't find me a real answer either. If you have time please share any knowledge you can on neutralizing a SB-200.
Very interesting 🤔 thank you very much
Thanks Tom, very informative video! Keep them coming, please! Merry Xmas & 73 DE SV8YM
A little confused by the graphic of the setup. The Radio receiver "in" is connecting to the the input of the Amplifier and the output of the Signal generator is going to the presumably RF OUT of the AMP. Could you explain this?
Neutralization cancels the unwanted anode-to-grid feedback. The best way to adjust it is to have a cold amplifier, feed a stable signal backward into the output circuit, and look at the feedthrough at the amplifier's normal input with a detector of some type. You are looking through the cold amplifier backward like the unwanted signal would be flowing.
You could also look at it forward but because the system has unevenly distributed loss it is not exactly bilateral, and the generator or source sees a much better match looking back into a tuned tank than it does into an open input.
Just do it the way I show. It is the best way.
I can't do a reply to what you wrote me because I keep getting an error from RUclips so I will just have to continue on with your statement regarding the 811 tubes that I wish to replace with the 572 B's so here is what I originally tried to write as a reply but was not able to do so because of RUclips issues. It isn't just replying to you but actually any reply that I try to do.
"l thank you for the info. I am using Penta 572 B's that are matched set of four from DX engineering. I already have them and intend on putting them in there. I will check the cooling fan situation and I will also check to see if there is a neutralization problem. The serial number is 20871 for my al811h but I don't know what that corresponds to for the year of manufacture. I will do all the checks you ask before I even thinking about swapping out those 811s but I have never been able to get more than 500 watts out of this unit even past the 70 w Dr power paired at some point it just seems to top off at about 500 W maybe a little less"
As soon as I have a little time I will take a look at the issue. I don't get to spend anywhere near the time in fact I very rarely get into my radio room but I'm trying to make that better and I finally retire in June so I'll have a lot more time I hope. I think I've turned my radio on four times in the last nine months
Could you do me a favor also and post a link when you reply to me on your web page that has the modified tuning procedure for the Al 811h amplifier. Also could you answer me a question because I'm going to be replacing the 4811 tubes that I have in there currently with 572 B's and I was wondering if any change in neutralization must take place assuming the neutralization was any good on the 811 tubes?
Unless you have tube issues melting the tube anodes, I would stay with 811 tubes. All new 572 tubes come from one of two places in China. I was buying tubes from the best Chinese source. Despite carefully weeding out bad tubes, probably far better than anyone else does, I still saw a 20-30% premature emission failure rate.
572 tubes require more flapper plate spacing than 811 tubes. You really should check your particular amplifier for abnormal behavior. If you see the input SWR moving around significantly it needs neutralized. MFJ has also moved an output coaxial cable ground point from the chassis to the input PC board. This shield grounding causes a ground loop that deteriorates ten meter stability.
Also, check the fan. If it has a Cooltron fan, the fan is too small.
The damage worry in 811 tubes is not from drive power or output power, it is from tube anode heat. If you watch anode heat the 811 tubes will last a lot longer than 572s.
This is the summary of a lot of investigative work...
Thanks for sharing this vital information with us.
De K4WRF