AL-811 Unknown mod help

Discussion in 'Amateur Radio Amplifiers' started by KE8CPD, Jun 26, 2019.

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  1. KE8CPD

    KE8CPD XML Subscriber QRZ Page

    Good Evening All,
    So i really new the whole amp thing, and dont know much about the design of them. But here goes.
    I picked up what i though to be a clean AL-811 amp. Got it home and started to learn the ins and out of tuning it. I got as far a pushing 600 watts with about 60w in. But it resulted in a high grid current. So i all Ameritron and try to get some help. They said it sounds like it is not tuned right, but the also said the check D16 which controls the grid current. I did some research and checked it with a meter and it doesnt seem to be bad. But looking at the coil (dont know the correct name, or what it does) and it looks like the taps have been changed around a bit.

    Attached is the photo, the pen shows the two taps that have been moved. So what does this do to the amp, I was gonna move them back but figured i would try to get some help before i did.

    Thanks for any help. IMG_20190625_192909.jpg
     
  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Was the amplifier previously owned by a "CBer" or "freebander"?

    Sounds like some modification done to "improve" the operation outside of the amateur radio bands.

    Glen, K9STH
     
    K9ASE likes this.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    D16 doesn't control grid current, but the amplifier LOAD control does.

    Make sure to keep key-down (tuning) times very short with that amp: 10 seconds maximum is recommended.

    But assuming you're doing that, when everything is peaked if grid current is high, increase (clockwise) the LOADing and watch the grid current go back down.

    Still, what usually destroys the tubes is excessive PLATE current, over time. The 10-second MAX tuning time recommended is a very good guideline, and wait 30-60 seconds before keying the amplifier again after that.
     
  4. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    ^^^ Good points above ^^^

    Here's a picture of an unmolested coil.
    It sure looks like someone moved those two wires over.
    I'd move them back. Use a bit of heat and flux to make good joints and clean up with alcohol. (Get competent help if you don't solder).
    600 watts for 60 in is all you can reliably expect out of this amp (or even a little less if you want the tubes to last a while... like 500W PEP max is more like it).

    [​IMG]

    This picture is borrowed from W8JI's website. He designed the amp. The one in this pic is the 811's big brother, the 811H.
     
  5. KE8CPD

    KE8CPD XML Subscriber QRZ Page

    As far as i knew it was not owned by a CBer or freebander

    I will make sure i only key down for 10 sec or less, and leave lots of time for it to cool down. I miss spoke on the D16 comment, thanks for the clarification on that.

    Thanks for the photo and advice, i will resolder them tonight. I dont plan on running more than 500 amps through it. I have a second set of tubes but hate to have to replacement. During my research i have found that if you over drive these amps the tubes will fry.
     
  6. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Sounds like a great plan.
    Get a bright light and take a good look around while you're in there for other soldering (monkeying around) that doesn't look "factory." :confused:

    Don't be afraid to post more pictures if you think we should peek at something. :)
     
  7. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    With the AL-811, with only 3-each 811A tubes, do NOT try to get 600-watts out of the amplifier! You can, but your tube life is going to be drastically reduced. Every other amplifier that has 4-each 811A tubes is rated at no more than 600-watts out (and that is on 80-meters through 20-meters) and Collins rated the 30L-1 at only 500-watts output. The AL-811 has only 3-tubes and needs to be de-rated by 25% from the amplifiers that have 4-each 811A tubes.

    For maximum tube life, no matter what Ameritron and W8JI say, 450-watts, maybe 475-watts output on 80-meters through 20-meters, 400-watts maximum on 17-meters and 15-meters, and 350-watts maximum on 12-meters and 10-meters.

    If you replace the 811A tubes with 572B tubes, then you can get 600-watts out of the AL-811 because the power supply is the same as in the AL-811H that has 4-each 811A tubes in it and can do 600-watts output on 80-meters through 20-meters, 500-watts output on 17-meters and 15-meters, and 450-watts output on 12-meters and 10-meters.

    Higher power output is really limited by the power supply when there are 572B tubes in the amplifier. The reason for lower output on 17-meters through 10-meters is because the tube efficiency drops off at the higher frequencies. The input power is going to be pretty much the same but the plate dissipation is going to increase.

    The station, on the other end of the QSO, is not really going to be able to tell the difference but your tubes are definitely going to know it and your electric company will thank you for purchasing more electricity!

    Glen, K9STH
     
    VK2WP likes this.
  8. KE8CPD

    KE8CPD XML Subscriber QRZ Page

    So i got it resolder but havent power it up yet. Ran out of time for the night. The solder joints were dirty and i cleaned them up as best as i could. I uploaded a couple more photos. they look factory but not totally sure. I know the green wire was cut by the previous owner, but figured i would upload it. But the other photo shows what i think is a ferrite, but its lose. I assume that is normal, there is a few more that i saw. But as far as the rest i didnt see much of monkeying around.
    IMG_20190625_220803.jpg IMG_20190625_220830.jpg

    Good to know. I will make sure i will not push it.I may try to find some 572b tubes but its not high on the priority list.
     
  9. W1QJ

    W1QJ Ham Member QRZ Page

    When Ameritron told you to check D16 and said "it controls grid current" they mean that if D16 is shorted it will seem that your grid current is unusually high. This is because when D16 shorts out (to protect the grid meter) they meter is now reading the same as the plate meter. The easy way to determine if D16 is shorted is to key the amp in the SSB mode with no drive. If the grid meter rises off zero and is the same distance away from zero as the plate meter (in other words, grid meter tracks the plate meter) then D16 is shorted and needs to be changed. It is no longer reading grid current. It is now responding to plate current.
     
  10. KE8CPD

    KE8CPD XML Subscriber QRZ Page

    Does this look like D16 is shorted out? IMG_20190626_211039.jpg
    Photo was it keyed up on SSB with no drive.

    So i tuned the amp according to the W8JI.com tuning supplements, everything seem to tune as described. Expect for the high grid current, and it only takes 60 or so watts to drive it to 500 watts. I tried turning the drive input down to bring the grid current down. The gird current does fall under the specs but only at 20 watts in and about 200 out.
     

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