Quieting The Noisy Blower
Let's fix the amplifiers that

Bob Hutchinson, N5CNN

Quieting the blower - about $18.00



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Ameritron AL-800H tube chamber with
Grainger 2E246 thermal switch in place.

Anyway you 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.

Force, in 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.

Well, 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?

Blow hard 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.

Two main parts used
A 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. Part number 2E246, $8.53. The thermal switch has a much greater current rating than required here and is larger than othe
r 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.

Utilizing 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.

Set high speed
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 speed.

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.

Note: This circuit is for 117 volt blowers. Double the resistances for 234 volt blowers.


Bob Hutchinson, N5CNN
President and Founder
Wireless Industry Association
713 467-0077

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