I guessed that I lucked out and found a "KoolTron" blower that the motor and
squirel fan itself would fit in the same case. It was a slower motor 2800
RPM instead of the 3400 rpm motor that was in there.. Then I was able to
slow it down some.. Here is a good site for slowing down some of these
One of the blowers that I have found that is quiet is German made and
sometimes be found on Ebay, by a company called "Ebrn" The one I have
here is a 85 CFM and is only 41/4" across, with the motor intergrated into
the blower. Very compact.
A muffin fan is what Drake used on the L7 with a pair of 3-500s and it has
been working for years.. It's a 120 cfm and it is pulling the air thru the
amp and across the tubes.
The way you measure the pressure is detailed in the Handbook under "Amp
Construction" but here is how I did it on the Henry.
The ideal is to measure the pressure created by the blower like it is in
operation. The amp doesn't really need to have HV or even the filiments
on, just the blower and the tube (s) in the sockets. With a clear tube
with water in it, measure the difference between the static state and with
the blower operational. I did this with the amp on its side and measured
from the bottom. Just so happens I had a one inch hole that was plugged
up on the bottom side of the RF deck, that allowed me to connect the tube.
On the clear tube, I marked the static level of the water, turned on the
blower only (with the RF housing sealed as usual) and marked how high the
blower could push the water up the tube, just like in the ARRL Handbook.
Measured the distance. In my case the slower motor still pushed the water
up .6 inches. The 8877 data sheet says .41 for continuous use, so I am
still above the required. The important part on the ceramic tubes is the
seal temperature at the mating of the ceramic and metal. Eimac says that
the seal temp should not get above 250 degrees C. Thats what, 450 -500
degrees F?? Dentron proved you could kool an 8877 with a muffin fan for
Ham use in the DTR2000 amp, which I have one here on my desk now, and it
Here is the parts that I used:
1 - 3 foot piece of neoprem hose, 3/4" ID.
1 - PVC waterproof ebow connector, 3/4"
Mking the pressure test just helped to make me feel that the tube was
getting enough air to surive and so far it has..
Sure hope this helps,
Thanks and 73's,