Al-1500 blower question

Discussion in 'Amateur Radio Amplifiers' started by KN4CTD, Jan 20, 2019.

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  1. KN4CTD

    KN4CTD Ham Member QRZ Page

    I have an al-1500 that I converted run a gs-35b. The tube needs more cooling than the factory blower can supply. The conversion works well and I’m happy with it because the tube can be replaced for $100 if ever needed and it puts out at least legal limit easily with full exciter drive. It seems that I ran across someone on one of the amateur gear classifieds some time back who had a blower for sale that had a much higher cfm rating and was made to be a “drop in” replacement for the al-1500. That being said, does anyone happen to know what this blower is, who makes it, or one that will do the job? Does anyone know who actually manufactures the factory blower for these amps?
  2. KN4CTD

    KN4CTD Ham Member QRZ Page

    I did some digging online and did find that Pabst makes these blowers and there is an upgraded 55cfm blower available that is supposedly a drop in replacement. I think the specs on this tube call for around 80cfm. It looks like I may have to go another route.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Unless the GS-35b is a pretty lousy design I'm not sure why it would take more air than an 8877 does. The 8877 creates substantial back pressure. I've never used a GS-35b but would think it does, also.

    Blower ratings in open air like "55 cfm" or "80 cfm" don't mean much. There should always be a graph showing actual cfm vs. pressure expressed in mm, cm or inches (for small blowers, fractions of an inch) using a liquid standard.

    For short duty cycle modes like SSB or even CW, an 8877 doesn't require a lot of cooling and maybe the GS-35b doesn't either. If you run legal limit RTTY or AM -- that requires a lot more cooling.
  4. KN4CTD

    KN4CTD Ham Member QRZ Page

    I’ve barely used this amp much and when I do, it’s only on ssb voice. I try not to get long winded when I do as I don’t rag chew very much. Even on ssb, it makes some serious heat out of the upper vents when running legal limit. On the gs-35b website, it states 86cfm minimum. The factory blower is supposedly like 25-30 cfm. I haven’t checked the anode temperature but it gets pretty hot to me. I used a Coleman lantern globe for a chimney which is cheap and works I guess. I’ve really never messed with the technical aspects of air flow etc. The factory blower seems to flow plenty of air through the tube and chimney, it’s just really hot. I have been using the dual gi-46b that I put together, and at legal limit on ssb, the air coming out of the tube compartment is substantially cooler even with heavier use. It uses two 12v muffin fans, one mounted under each tube blowing through the tube’s chimney and cooler and another larger 12v muffin fan pulling the hot air out of the tube compartment. I’m just always tinkering and wondered how I might make it run cooler.
  5. KN4CTD

    KN4CTD Ham Member QRZ Page

    Does more back pressure equal more cfm or less?
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Neither, really; backpressure creates "lower" cfm than the blower or fan would have in free air, though.

    Hard to believe such a small tube would require 86 cfm; that sounds like it "might" be a poor translation from Russian, since all the specs were originally in Russian and I have no idea who translated them. An 86 cfm blower in free air would usually create maybe 30 cfm (or something) through the socket and tube. I think the 8877 requires 38 cfm at 0.6 inches pressure (memory -- it's around there). If the Russian tube really requires 86 cfm at similar back pressure, that's an amazing amount of air and would probably require >150 cfm blower (free air rating).

    There are such blowers, of course! But the only way to do it with a small blower would be very high rotational speed (rpm), which in turn makes for a really noisy blower. Easier way is with a much larger cage at lower rpm, which is quieter. Think of a whole-house HVAC system blower: Very large, but pretty quiet since it moves a large volume at low speed.
  7. AG5CK

    AG5CK Premium Subscriber QRZ Page

    The 35b does not create the back pressure that an 8877 does. Maybe half of it. The same blower to cool 1 gs35b will cool a pair of 8877s

    The cooler alone on the 35b is bigger than an 8877
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Brute of a tube, I guess. (Never used one in anything.)

    I think the Russians used these for RADAR systems or something, so they may have been operated at enormous peak power but fairly low average power (if that's true). Since it seems they were mostly used at UHF, at HF they should be very stable (like an 8877, for the same reason). Seems like a great choice, although I have three spare 8877s which will likely last me way beyond my life, so maybe I can will them to somebody.:p
    KN4CTD likes this.
  9. AG5CK

    AG5CK Premium Subscriber QRZ Page

    They aren't as well designed as the 8877 and take more airflow. You can look straight through the fins on the 35b cooler but not the 8877.

    The cooler and anode being two separate pieces doesn't help heat transfer either.

    They are lower mu and not as linear but for the money they're hard to beat. If the copper turns blue you need more airflow. ;)
  10. WA7PRC

    WA7PRC Ham Member QRZ Page

    Flow rate is inverse to back pressure. Back pressure is related to restriction/turbulence in the air path, and in practice, is never zero. You can minimize the restriction-turbulence and/or increase blower output. The GS-35b datasheet sez it wants 2500 l/min (88 cfm). I started accumulating/fabricating parts to use a GS-35b. I went with the W4ZT "socket" because it offers the lowest grid-ground reactance and the lowest air flow restriction. It was a fun project:
    For the blower, I decided that, the same as it is in drag racing, it's easier to turn the "horsepower screw" down than up. I selected a Dayton pn 3FRF1 (link):
    It's rated 151cfm @ 0" back pressure and 110 cfm @ 0.5" back pressure. In any case, key is ensuring the tube doesn't see close to the 200°C anode temperature limit. An IR temperature "gun" makes that test easy.
    KN4CTD likes this.

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