Max Exhaust air temp ?

[N8ETQ]

Good Afternoon,


    I converted an old Dentron MLA-2500B to run
a pair of 4CX400A's from Svetlana. (No easy Task!)

    Anyway it is working quite well however Exhaust
air temp  seems pretty high. I'm reading 65C or
150F. I know these things were hard on Jugs which
is why you can't find 'em anymore. I also know that
with all these little amps the two main drawbacks
are: The air system Sux, and, The air system Sux.

    The temps would go higher if I let it, anyone
have an idea as to how high is safe or reasonable?
Spec sheet on the jugs says Max seal and core
temps to 250C, ouch! almost 500F

    it just has a 4" Boxer fan blowing into an
Air box with the exhaust being forced out through
the fins of the tubes.

    The original conversion article was in Comm.
Quarterly "Summer 1996" by W6KT

http://www.qro-parts.com/images/GS-36B/convert.pdf

     It's obvious that more air would help but the
goal was to retain the original footprint of the amp.
is there a better boxer fan available or would stacking
a couple be better?

Thanks

/Dan

[WQ9E]

Dan,

150F doesn't seem too unreasonable, that is probably an 80 degree rise over ambient.  Fans and external anode tubes aren't a great combination since most of them provide high air resistance as the tradeoff for their small size. 

An external blower in a box connected to your amp with blower hose (shop vac hose will work OK) would be a much better choice for longer tube life especially if you are running AM.  A decent blower in a box with foam sound deadening material on 5 of the inside walls and an air filter on the remaining will be pretty quiet.  This is the setup I use with my homebrew amp running 3 4CX800 tubes.

[Opcom]

The air looks like it is blowing past or around the anodes in the article's picture, but the picture is not very good. You have it going through the fins axially though, right? 

The tube manufacturer usually states a certain amount of air pressure applied under the socket will result in enough air flow for a given dissipation. While that is a good guide, it is even better to have 2x the amount as long as noise does not become troublesome. After all, the life of the tubes will be extended if the anodes are kept cool.

One way to measure the air flow accurately is to measure the pressure with a water manometer you can make. If the value of desired pressure is 2-3 inches, then it's fine to make it as shown in the ARRL and Radio handbooks, with the measurement tube sticking straight up.

If it is only a fraction of an inch pressure as with the 4CX400, then the manometer tube can be put at a shallow angle so that for instance 10" of tubing covers 1" of water column height, making it easy to read such a low pressure accurately. I made a nice one with clear plastic tubing but glass works, anything transparent works OK. An alternative is a 'Magnehelic Gauge', which is a meter that reads the difference in air pressure applied to two ports, one going to the amp and the other left open. They are expensive but very accurate and can be found used cheap.

The datasheet shows 8 CFM for 400W dissipation is accomplished with air pressure of 0.2 inches of water. That is why I mentioned the inclined manometer. With it, each 0.1" of pressure spans 1" if tubing length making it easy to read. The only special part in making one is that the lower loop has to be at least as long as the horizontal span, so there is always enough water for some to remain on the left side of the loop, or the air can bubble through. The whole thing can be made of rubber or plastic tubing but it is best to have the horizontal part made of glass so it is straight.

Doing it with a manometer this should remove any doubts about the cooling of the tube and as to whether the temperature measurements are meaningful.

[N8ETQ]

 Thank you's,

     I forgot about that water in the tube thing. I know
the customer wants to use it with a Ranger so I think
a small blower would be best if he wants to go that way.

     just playing around with it this afternoon I put a piece
of heat shrink on top of the cabinet above one of the tubes
and it does start to shrink! I also tried just putting another
fan right behind the one bolted to the chassis but didn't
see much change in airflow as measured by a piece of
paper in the exhaust stream.

     See the pix. The air box is pretty tight. Fans don't like
back pressure so may have to go with the blower but I will
check him out with the water manometer.

Thanks again,

/Dan

[Opcom]

Stacking fans is best done with some space between them as the turbulence interferes with one to the next if stacked directly. The pressure requirement is fairly low, maybe a fan will do. Can't go wrong doubling the flow if you can, keeping the tubes and that insulating plate cooler. I'm leery of fiberglass PCB material touching external tube anodes after seeing a fiberglass duct from the top of a 4CX5000 looking charred, but it's not easy to guess what might happen after years of service. Do you have one of those infrared thermometer guns? Some have a pretty high range. Those can be a cheap and useful tool, including checking your a/c in the house. Please post about the manometer if you try it.

[WD5JKO]

  I did an 8877 conversion on my MLA-2500. Used a Bud box in back to house the socket and fan. I added an additional (exhaust) fan on the top cover in the vicinity of the orange tank coil. Then I blocked the other exhaust vents on top. Had to play with fan speed resistors in order to be right for two fans.

  The picture was an early one while I was shaking things out. With 240V feed, will do 400 watts AM. With my wimpy 120V service, I don't do more then 200 watts. The exhaust is pretty warm. The 2nd fan eliminated the cheap non RF Current rated caps in the pi-network from drifting as things heated up; a common problem with these amps.

Jim
Wd5JKO

[Opcom]

I thought I'd throw this out there. Lately these little blowers have been available for around $10, and depending on model they make plenty of air for an RF tube of the kinds in the topic. They are much smaller than the usual 120V blower and pretty quiet. These are a pancake style one, 120x32mm.

The Svetlana document says 8CFM @ 0.2" for the 400W tube so I suppose it is a possibility. The graph looks like it is just at 17-18 CFM for 0.2". Mounting might take some ingenuity but what doesn't?

http://www.bgmicro.com/FAN1105.aspx
http://www.nidecpg.com/fanpdfs/gamma32.pdf