>Has anyone experimented with using a furnace blower for cooling their
>tubes? I was curious if anyone had actually measured available back
>pressure for any particular size wheels. The handbooks list ratings for
>different size Dayton blowers, and wondered if the furnace type would be
>worth pursuing for cooling some of the larger triodes.
> 73 Jay W7CW
For finned air cooled tubes, you need to supply the recommended CFM of air
volume through the anode fins which will give you a back pressure. Your
blower needs to be capable of supplying that CFM, into that pressure. And
if you live at high elevation or use 50 Hz power, remember to derate things
to accomidate these reductions in air volume. If you overload a blower, it
will fall way off on performance (CFM), and be hard on the motor.
Once you have selected a blower, you really should do a little homework to
prevent meltdown in the future, when you key down into a tough load, and it
is a hot day, and there's dead bugs and dirt on the airfilter blocking it a
bit.
If you play with a furnace blower, you should either know the curves on the
blower/motor combination, or do your own tests to prove this. You will need
a long piece of duct connected to the blower, some Dwyer airflow measuring
pitot tubes (the red fluid in them), and a flap you can shut on the far end
to load up the blower. I can refer you to a paper on the subject of testing
blowers when the Broadcast Electronics model FM-30A 30 KW FM transmitter
was developed. It was written by Jeff Steinkamp of Quincy, Illinois, and
can be gotten by contacting BE at (217) 224-9600 and asking for a reprint.
I believe it was presented at a National Assoc of Broadcasting convention
technical presentation, but cannot remember the year (sometime around
1984).
The furnace type may be big volume movers, but not such high pressure
devices. Duct work does present back pressure, as do filters and floor
grates for heating. I suspect you will find one that will work, but it may
be serious overkill. If noise, power consumption, and size are no concern,
then try it!
Another tip, use a tiny spot of Omegalaq or Tempilaq paint on the tube,
right below the anode seal on the ceramic, or above the seal on the anode
metal. For glass tubes, you have to be very careful due to making a hot
spot with your temperature paint on the glass and causing a suck in.
Besides, if your glass tube is that hot, it will break! The Tempilaq range
I use is from 150 -250 deg C, in increments of about 25 deg C. If your
paint melts and changes color all the way up to 250, at any spot around the
seal, then you can expect that the tube life may be short in your Amp. Oh,
one more thing, on the bigger tubes with handles one has to also cool the
stem, the lower contact of the filament in the socket. Same drill as above.
Perhaps these tips are a bit overkill, but then again, you were talking
about furnace blowers, so I assume that your tubes have handles at least.
John
K5PRO
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