Ameritron 811h fan mod

Discussion in 'Amateur Radio Amplifiers' started by W5DWR, Oct 10, 2010.

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  1. W5DWR

    W5DWR Ham Member QRZ Page

    I read on a review about the 811h that there is a drop-in replacement fan sold by Radio Shack. Does anyone happen to know what the part number of the fan is?

    Thanks and 73,

    Don - w5dwr
  2. W8JI

    W8JI Ham Member QRZ Page

    The much better question is, "Why would someone want to replace the fan with one from Radio Shack and what is that fan's ratings?" :)

    I found the review but I would not change the fan uless you change to 572B tubes, and unless you are sure the fan really does have more airflow. More air flow will NOT improve ratings or life of the 811A tubes.
    Last edited: Oct 11, 2010
  3. W5DWR

    W5DWR Ham Member QRZ Page

    The amp does have 572b tubes, and if I can find the part number I can check the specs for it and make sure it moves more air before I do any changing. It sounded like a good idea at the time :rolleyes:

    Don - w5dwr
  4. W8JI

    W8JI Ham Member QRZ Page

    It could use more air if you are going to hammer the 572's.

    I would make sure, against about .025 inches of backpressure, it has more airflow.
  5. W5DWR

    W5DWR Ham Member QRZ Page

    Thanks, and I don't hammer the 572b's. Like I said, it sounded like a good idea at the time. Maybe not :cool:
  6. W4AFB

    W4AFB Guest

    The amp has more then adequate ventilation as long as you dont block the intake on the side of the amp. And unless you arent tuning the amp correctly your only going to run the 572b's so hard with 1800 plate volts. No need to re-invent the wheel.
  7. W8JI

    W8JI Ham Member QRZ Page

    More cooling is always better for components, although the difference is probably not ever noticable in normal use.

    When I design things like that (I designed that amp and many others) for amateur, noise is always a major concern. Noise is always a major complaint no matter how quiet an amp is.

    Because of that, I always set airflow the minimum required at 5000 ft altitude. Unless you run the 811H hard with bigger 572B tubes, the extra cooling won't do much. If you change the tubes and hammer it the weakest link is tank coil cooling and transformer temperature, so more airflow helps that.

    73 Tom
  8. W5DWR

    W5DWR Ham Member QRZ Page

    When I tune the amp, I tune to 600 mA of plate current and 150 mA of grid current, so pushing too hard is not a problem. I think I'll leave everything as is. Thanks to everyone for your input.

    Don - w5dwr
    Last edited: Oct 11, 2010
  9. W0UZR

    W0UZR Ham Member QRZ Page

    As long as we're talking about the 811H cooling system, I have a question. My fan in the back is not blowing the air out. It's blowing the air in. And the hot air comes out the vent holes on the side. Is my fan in backwards?
  10. W0UZR

    W0UZR Ham Member QRZ Page

    When I tune my amp, I turn up the drive to where both of the meters are almost pegged, then when I adjust the plate and load then they come down, then turn drive up again to where they are almost pegged again and adjust plate and load again. But DO NOT spend anymore than 2 or 3 seconds on each adjustment and unkey right away! Then when it's all tuned up, I turn my drive down to where the meters say where the book says to have it during operation, about 475ma current and 100 grid, then I turn the load up a little more (Not Much) to get the grid to drop, then adjust the plate again for peak and also look at the grid when retuning the plate to adjust for lowest on the grid while keeping the peak power output up. The lower the grid, the better your tubes will last.
    Last edited: Oct 12, 2010
  11. AD6KA

    AD6KA Ham Member QRZ Page

    No, I believe that standard practice with amps
    is to "pressurize" the cabinet with fresh air going in,
    so the hot air goes out the vents. More efficient.
    Caveat: I don't own an 811.

    Thanks for that interesting tidbit about the altitude.
    Why that number, just curious......

    73, Ken AD6KA
  12. W4AFB

    W4AFB Guest

    Do not operate the amplifier in excessively warm locations or
    near heating vents or radiators. Be sure air can circulate freely
    around and through the amplifier cabinet. Provide an
    unobstructed air inlet for the blower. Do NOT place any books,
    magazines or equipment that will impede the free flow of air
    near the sides of the cabinet.
    The AL-811H ventilation system has been designed and tested
    to maintain the 811A tube temperature safely below the tube
    manufacturer's rating at 500 watts output with a 100% duty
    cycle. To insure proper ventilation in your installation, observe
    the following:
    1. Do not block or restrict the ventilation holes in the cover.
    2. The exhaust air flow is over 20 CFM. Do not "assist" the
    air flow unless the fan exceeds the AL-811H fan CFM by a
    factor of 2:1.
    3. Do not mount additional fans on the AL-811H cabinet.
    4. The exhaust air will become warm at high power levels. Do
    not place any heat sensitive objects in the exhaust air


    No need to redesign the case or the fans and in my opinon 572B's are a waste of money for this amp.

  13. KR2C

    KR2C Premium Subscriber QRZ Page

    I have an older al-811h. I'm also using the 572b tubes. The fan in my amplifier is starting to make noise like the bearing is going bad. What would be a good replacement for the fan?
  14. W8JI

    W8JI Ham Member QRZ Page

    Fans always work better pressuring rather that pulling, plus the fan is not subjected to heat when pushing cool air in.

    5000 feet is about the highest altitude expected.

    The fan is a standard physical size and so there are dozens of replacements. I wouldn't know a current part number.
  15. W0UZR

    W0UZR Ham Member QRZ Page

    Whatever you get, I would make sure it moves the same amount of air as the original and it's the same RPMs There are plenty of replacements and I assume as many variations of the speed of the fan, the volume of air it moves.
  16. LA4AMA

    LA4AMA Ham Member QRZ Page

    AL-811 Fan mod


    I just opened my AL-811H amplifier and the fan is a 115V AC - 12w PAPST 8800N. Data here:

    I am looking into finding a fan with same capacity (or more) and maybe less fan noise. I am wondering if I can use the 12V DC supply that goes to the metering lights. I see there is a 1.5A 12V rectifier circuit in the AL-811H so I believe there is enough current available for a 1.3w 12V DC fan (Papst 8412NGM):

    Best regards
  17. W0UZR

    W0UZR Ham Member QRZ Page

    I have that fan. And if it will fit in the amp, then is't smaller than mine. I ran it without being in anything, and it's the noisiest fan I came across except for the fan over my stove.

    That isn't going to be the fan for being quiet to say the least.
  18. LA4AMA

    LA4AMA Ham Member QRZ Page

    The Papst 8800N in my AL-811H is making some sort of vibration noise, but then the amp is from 1997. Probably just worn out this fan.....

    The Papst 8412 NGM is rated at 26dB A at 2600 rpm, actually more quiet than the 8800N rated at 28dB A at 2500rpm.

    Best regards

  19. LA4AMA

    LA4AMA Ham Member QRZ Page

    I just tried a 12V DC fan on the front meter 12V DC circuit. The fan is only 1.3w and 110mA, but the voltage dropped to 9.3V on this circuit when I inserted the fan.

    So this is a no go, not possible with a 12V DC fan unless one builds a separate DC supply, but there is little room for extra circuits in the AL-811H.

    Installed the original fan again, I'll leave this as it is.

    Best regards
  20. W0UZR

    W0UZR Ham Member QRZ Page

    How about calling Ameritron and order a fan from them? The fan that came with mine is so quiet, I hardly know it's on. If it needs more air flow, then ask them if the fan in the 572 amp is the same or different. If it's different, them maybe it's the same size physically and it will fit in there.
    Last edited: Nov 5, 2010
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