AL-811H tube swap to 572Bs

Discussion in 'Amateur Radio Amplifiers' started by KW4GT, May 12, 2019.

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  1. N8FVJ

    N8FVJ Ham Member QRZ Page

    Well, after 1 year due to a move, I just got my HF antenna up in air today. Winters in MI are a little tough or I would have had the antenna installed earlier. No time last fall due to work. I am retired now. I been talking on 75 meters tonight. So much fun. The AL-811 makes about 800 watts PEP out with 80 watts drive. Grid current is about 100ma on peaks. I assume ok.
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    It is only rated for 600 Watts PEP.

    The tubes would last a lot longer running about 500 Watts.

    No one would even notice your drop in power. (Unless you tell them.)

    You will still be 5-9 +. :rolleyes:
    Last edited: May 15, 2019
    WA7PRC likes this.
  3. N8FVJ

    N8FVJ Ham Member QRZ Page

    The 572B are loafing at 800 watts PEP out with three tubes. You are thinking a pair. I use three 572B. 600 watts CW is tougher on tubes than 800 watts SSB PEP out. I may reduce to 600 watts PEP out as you state no one would notice the reduction. All I know is it beats the heck out of 100 watts.
    KK4YPQ and KA9JLM like this.
  4. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    You should know, by now, that antennas erected during the worst weather work the best!

    In high school, one December, there was a contest on 160-meters and I wanted to get in on the action. I didn't have enough wire for a 1/2-wave antenna so I unwound the filament windings from several TV power transformers to add to what I already had. Then, I got 270-feet of wire in the air and fed it with transmitting 300-ohm twin lead as a true Zepp. The antenna worked very well on 160, 80, 40, 20, and 10-meters. Not very well on 15-meters. However, I had a 2-element Yagi for 15-meters so the fact that the Zepp didn't work on that band was not a problem.

    At the time, I lived in the "snow capital" of Indiana, LaPorte. The antenna was erected at about 3:00 AM, with over 3-feet of snow on the ground, and snow falling at a rate of between 4-inches and 5-inches per hour. That was the winter of 1960. The antenna was still there when my mother sold the property in the early 1990s.

    Glen, K9STH
    N8VIL likes this.
  5. N8FVJ

    N8FVJ Ham Member QRZ Page

    If I was 16 years old.
  6. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    You need to find a couple of 16-year old neighborhood kids and talk them into working on your antennas!

    There is a 16-year old, who lives 5-houses up the hill, who always offers to help with any of my projects be they antenna projects or doing things around the yard.

    Glen, K9STH
  7. N8VIL

    N8VIL Ham Member QRZ Page

    Sounds like how I do my projects. I am 48. My wife thinks I am nuts.
  8. KW4GT

    KW4GT Ham Member QRZ Page

    Another question.... has anybody had to re-neutralize their amp when swapping in 572Bs?
  9. N8FVJ

    N8FVJ Ham Member QRZ Page

    The three tube AL-811 amps do not need or have neutralization. Four tube AL-811A are neutralized with a non adjustable metal plate across from the tube plates. I have not read about any neutralization issues switching to 572B tubes.
  10. KW4GT

    KW4GT Ham Member QRZ Page

    I've read the procedure for neutralizing the AL-811H, it's been awhile but as I recall it involves energizing the T/R relay and injecting a small signal into the output while moving the plates either towards or away form the tubes. I'd rather not mess with that, so it's good to hear that it's not needed.

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