Ameritron AL-811 amp problem

[tinkerer973WI]

I'm pretty new to the forum, as well as tube amps. I have an Ameritron AL-811, it still has the original 811A's, and has worked well till 3 days ago. I feed it with a Yaesu FT-847 and I do use the ALC line to limit the drive to 70W. Heres the problem, I tuned it up on 40m monthes ago and don't really use it on other bands, I turned it on and let it warm up for about 5 min, but when I keyed the mic to sign in to the net, my radio shut the power supply down. I had to turn the supply off and back on to get the radio back on. The radio seems fine, and amp will tune up and run normally on all bands except 40m, all bands will take full drive and generate about 550W output, but on 40m I can't push the plate current beyond 300ma or it shuts the radios power supply off... I'm figuring something is causing a higher than normal draw from the supply and overloading it because the supplies light stays on, just no output to the radio. only 40m is acting up, all other bands work fine. Any help is much aprieciated...... Kevin KC9OGB

 

[Shockwave]

Your power supply is suffering from RFI. When you're on 40 meters at full power, RF is getting into the power supply regulator circuits and tripping the overvoltage crowbar that shuts the supply down. The cure will be using RFI ferrite cores or chokes from Radio Shack to isolate the wires to the power supply. They simply snap around the wire as close to the power supply as possible to stop the wires from acting like antennas. You will have to experiment to determine if it's the AC or DC wires. You may need a ferrite core on both. One of these wires is probably close to resonant on 40 meters.

 

[tinkerer973WI]

Is this something that could happen do to rearange the bench, I had to temporarily set up in a differant place than my reg shack, and have the excess coax coiled and hanging on the side of the table, poss. to close to the supply? I will try moving the supply to the other end of the table tonight, and see if i get any differant results, I really hope its this simple.

 

[Shockwave]

Bingo! Changing the layout of the shack can certainly cause new RFI problems to crop up. You may not need the ferrite choke if moving the coiled up coax away from the power supply fixes the problem. It's quite likely it will but if it doesn't, don't hesitate to install the chokes on the wires. If it's possible to lay the extra coax outside on the ground, this will help remove any RF that may be on the braid of the coax before it enters the house by capacitively coupling it to ground. It is this simple.

 

[tinkerer973WI]

Well, results are looking very promising so far, I slid the power supply to the opposite end of the table I'm using, and thus far it loads up full power, all bands again. Why I never think about things like RFI effecting my other equipment, I will never know, and only 40m should have given that away , it's the only band I still need to retune the antenna on....I'm so glad it was something stupid like that....every once in a while I forget to adjust the loading after switching bands and I'm so afraid I'm gonna burn or damage something beyond use and some of those parts aren't cheap.

Thanks Shockwave, for your timely help and spot on diagnosis, I wasn't sure how long I'd be waiting for help....


till next time, 73 and thanks again,

Kevin Tinkerer973wi KC9OGB<More audio>

 

[Shockwave]

Glad to be able to help out and that you're back up on 40 meters with full output!