AL-811H tube swap to 572Bs

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

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  1. KW4GT

    KW4GT Ham Member QRZ Page

    I'm contemplating at some point putting 572Bs in my amp for increased 'ruggedness'. (I've seen a few claims of somewhat increased output, but I'm skeptical of that).

    I've seen some people who have replaced all 4 tubes, and I've read other places that say to just use 3 572Bs..... which is it?

    Also, if you just use 3 tubes won't it mess with the amp's input impedance?
  2. KC9PFH

    KC9PFH Ham Member QRZ Page

    A couple of years ago I replaced all 4 811A tubes in my 811H with Taylor 572Bs. I was replacing the 811 tubes every year as they went bad. The 572Bs DO NOT produce increased output, their output is limited by the relatively low plate voltage in this amplifier. But they hold up much better. I recommend the conversion, it has worked very well for me. de KC9PFH
    KW6LA, KU3X and K7TRF like this.
  3. DL3LK

    DL3LK Ham Member QRZ Page

    The input resistance of one 572 is 215 ohms. So if your amp has 4 tubes your input resistance is 215/4 = 54 ohms. If you put only 3 of these tubes in a 4 tubes amp, then your input resistance is 215/3 = 70 Ohm. You have to tune the input filters to match the 70 ohm. If you have a amp without a input filter you will have a missmatch in input R and not a good input swr. You see the best is replace all 4 tubes. My 4 572Bs are now 30 years old and fresh like the first day i used it.

    K9AXN, KU3X, WE6C and 1 other person like this.
  4. N8FVJ

    N8FVJ Ham Member QRZ Page

    Changing from fragile 811A to 572B are much more durable. Use four 572B in your amp to keep the tuned input correct. I have an AL-811 with three 572B. It produces 800 watts PEP out with 80 watts input. Three 811A would likely melt the plates at 800 watt PEP out. 3 or 4 572B makes same power output due to lower than best HV on the tube plates. Four Shuguang 572B from Hong Kong is $215 plus shipping on shown on ebay. US based quad of 572B tubes are $288.
  5. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    As several have pointed out, the real limiting factor is the power supply when replacing 811A tubes with 572B tubes. It is the same when replacing the 811A tubes in amplifiers like the Collins 30L-1 or Heath HA-10 Warrior. The amplifier is not going to put out more power.

    However, there is one slight exception and that is with the Ameritron AL-811. Although the factory ratings are for 600-watts output, a more realistic output is 450-watts and with the AL-811H the factory ratings are for 800-watts output but a more realistic output is 600-watts. At least for any decent tube life.

    Both the AL-811 and the AL-811H use the same power transformer. Therefore, when the 811A tubes are replaced, in the AL-811, with 572B tubes, the output can safely be increased from 450-watts to 600-watts without any significant reduction in tube life. Some operators do try to get more output from both the AL-811 and AL-811H beyond around 600-watts when 572B tubes are installed. Unfortunately, that does put more stress on the power supply. Basically, the station, on the other end of the QSO, is not going to be able to tell any difference in the signal strength. But, you amplifier is going to know about the higher output and what you really are doing is to increase the profit of your electric company while realistically not doing anything that improves your effective signal strength.

    Glen, K9STH
    WD4IGX likes this.
  6. N8FVJ

    N8FVJ Ham Member QRZ Page

    Correct Glen and especially so about true output with 811A tubes. But at 800 watts PEP out or 600 watts CW out that is more stressful than SSB 800 watts PEP the 19lb power transformer is really not stressed. Fortunately Ameritron used a large enough transformer. It is about the same weight as a Heathkit SB-200 transformer. Neither are known for failures.
  7. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    And the Heath SB-200 needs to be kept down to around 600-watts output for best tube life on 80, 40, and 20-meters, less on 15-meters and 10-meters. Having 3, or 4, 572B tubes does remove the tube life from the equation and then the power supply becomes the limiting factor with the Ameritron amplifiers

    Glen, K9STH.
  8. N8VIL

    N8VIL Ham Member QRZ Page

    Since I see many posts on the durability of 572B tubes in AL-811 series amplifiers, I decided to take the plunge. I threw a low ball best offer for 3 Shuguang 572B tubes at $40.00 each on a whim ($120.00 total with free shipping) on Ebay with a seller with "make offer" and offer was accepted. From China of course. Same price as 3 Taylor 811's with shipping from RF Parts. My 811's are good but I figured why not. I shall see what happens with the quality of the tubes. Might be nice not having to worry about keeping an eye out for glowing plates on the 811's. I was not about to spend regular 572B pricing on an AL-811 amplifier. I have the 3 tube AL-811 so impedance should not be an issue or maybee a slight touch up.
  9. N8FVJ

    N8FVJ Ham Member QRZ Page

    I use three NOS Cetron 572B in my AL-811 and pleased with the outcome. Amp tunes close to same with 811A and has full range tuning on all the ham bands. I also changed the load capacitor to a 1200 volt rated capacitor vs the original 500 volt rated capacitor (receiving type) as I heard the original capacitor arc over a few times. It is same physical size as the tune capacitor. It never arcs over now. Found the replacement capacitor on ebay for $30. I get 800 watts PEP out with 80 watts drive on 75 meters. Slightly less on 10 meters.
    Last edited: May 14, 2019
  10. N8VIL

    N8VIL Ham Member QRZ Page

    A few years back I had 4 NOS Centron 572B tubes gifted to me. I had them in the AL-811 for a bit until I acquired 2 Yaesu FL-2100B amps and needed the tubes for them. Remembering back, they worked quite well in the AL-811. Good idea on the load capacitor. Those plates are quite close together. I think I will loaf it along at 600-650 watts.
    Last edited: May 14, 2019

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