The Noisy Blower
Let's fix the amplifiers that
BLOW TOO HARD.
Bob Hutchinson, N5CNN
Quieting the blower
- about $18.00
any picture larger
Ameritron AL-800H tube
2E246 thermal switch in place.
look at it the centrifugal blower is a necessity for these external anode
triode and tetrode tubes furnished in amplifiers today. Air can only
provide cooling by moving into a plenum chamber, around corners, through
the cutouts under the tubes, up through the anode radial/axial heat
exchanger and out though the exhaust chimneys or chamber.
the form of pressurized air at zippy velocity is required. Muffin
fans can't effect or accomplish this. Legal limit plus amplifier cooling systems
require at least 30 CFM air flow at high power, high duty cycle output
levels. The centrifugal blower must turn at high speeds to create the
necessary plenum chamber pressure to get the high power job done.
wait a minute - I don't operate at high power, high duty cycles. Sure, I
get a little long winded sometimes, but someone smarter than me has
already figured out that even long winded SSB yammer is only 15% duty
cycle to the transmitter and it's associated ancillary power equipment. Do
I have to be subjected to the howl of a centrifugal blower anyway?
reduced to blow soft.
If I can slow
down the rotating machinery to a CFM capacity that will support my 15%
duty cycle yammer, yammer, yammer, which would be 5 or 6 CFM, I know the
howl will be dramatically reduced. Follow me through as I install a simple
circuit costing less than $20.00 to calm down the AL-800H from Ameritron
or any other amplifier utilizing a howling centrifugal blower.
main parts used
200 ohm, 12 watt variable, wire wound resistor or rheostat, available from
new surplus houses or
Nebraska Surplus Sales and a thermal snap switch from W.W. Grainger.
2E246, $8.53. The thermal switch has a much greater current rating
than required here and is larger than other
small units made by the thousands. It is designed for
hot air discharge in attics But it is the only switch readily available
that has the proper close and open differential of 10 degrees.
It closes at 120 degrees and opens at 110 degrees. It has a large sink
area so it reacts quickly to changes in temperature.
this circuit, see diagram, the amplifier blower runs at the default or
slow speed with normal SSB chatter duty cycle. If a
long winded, ratchet-jaw act with the associated heat is engaged, the thermal switch will switch the fan to the
pre-set high speed. When chatter stops it will switch back to the default
slow speed, usually in less than a minute.
The 200 ohm rheostat is in series with the blower motor by default. When
the thermal switch closes at 120 degrees it switches the rheostat
adjustable wiper into the circuit to provide a user selectable high speed.
I use about 50% of full capacity for high.
Two blue blower
wires are found at the AC section of the Ameritron AL-800H main circuit
board. These provide AC power for the blower. I unsoldered one blue wire,
lengthened it with similar wire in a series loop with the rheostat in the
circuit. See diagram.
A pair of the blue
#24 wires are routed from the bottom of the tube chamber to the other side
of the amplifier. They are then soldered to the switch. A #6-32 x 1/4"
machine screw holding the tuned input box cover is removed and replaced
with a 1 1/4" screw. A Nylon standoff, which helps thermally isolate
the switch, is slipped over the screw. After the
1/4" push tabs are straightened, the switch
is secured with a washer and nut. This positions the switch right in the
air flow from the tubes without interfering. In this installation a 510 ohm 3 watt
resistor was soldered in parallel
with the 200 ohm rheostat to provide a default resistance of 150 ohms for the slow
Adjust default and high speed
If you substitute a 1K to 2K rheostat for R2 you can adjust both the
default slow speed and the high speed. Both can be preset.
A 1/4" hole in the
back panel for mounting the rheostat and soldering the blue switch wires
and R2, if used, and this quieting job is done. After chimney installation
and cover re-installation it works just as it is supposed to. No
surprises. This amplifier is especially well suited for this simple
modification because it has an exhaust chamber. Other amplifiers with
Teflon chimneys reaching to the top outlets in the cover are a little more
complex but provide superb results. No more blowing too hard.
circuit is for 117 volt blowers. Double the resistances for 234 volt
Bob Hutchinson, N5CNN
President and Founder
Wireless Industry Association
If you would like to
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