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More Cooling for Your Amp

from Mike Higgins, K6AER on July 30, 2017
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More Cooling for Your Amp
By Mike Higgins – K6AER

Today’s modern tube amplifiers are much better at cooling than the simple fan located in the Heathkit SB-220. Axel fans and centrifugal blowers used on modern Tetrode amplifiers, can move a lot of air with relative lower noise levels than in the past. Most of the current amplifiers have variable speeds that ramp up air flow with anode temperature as well as an extra Axel chassis fan just in case cooling requires a bit more presence. Amplifier manufactures pay attention to airflow dynamics such as power supply cooling, microprocessor, RF deck and the output tube is generally the last heat source in the airflow chain. As a result, most amplifiers run cool except for the tube and the output exhaust. Even with this extra cooling there are situations when still more cooling is needed. Contest operation, RTTY modes and operation in high, hot and humid environments can tax what was designed for sea level operation.

My current Tetrode amplifier (OM2000+) does a fine job of cooling but requires that I leave the amplifier on to bring the critical components (tube and tank circuit) down to room temperature when done transmitting. I could modify the amplifier with a separate power supply and control circuitry but decided to go with a separate fan arrangement. Almost all the current tube amplifiers exhaust the hot air via a vent hole located on the top of the cabinet. For those with two vent holes like a OM-4000 or an 8410 you might have to build a small plenum housing to set the blower on top.

I decided to go with a cage blower, driven with a brushless DC motor. The model I picked is the Orentalmoton ORIX, model MBD8-24 blower. Cost on the unit is about $90 new. You can find them used for under $25.00, which is what I did. This unit moves 51 CFM of air and only pulls .7 amps at 24 VDC. It is all aluminum in construction. It will run fine with as little as 10 VDC but with reduced airflow as a result. The inlet fits inside my amplifier exhaust opening and is very quiet with 17 VDC. In addition, I added a mechanical thermostat from Granger, P/N 6EUD4, which closes at 90 degrees “F” and opens at 80”F”. Cost is under $10.00. I mounted the switch on the exhaust side of the blower cutting a hole to fit the sensor bubble into the airflow. When the blower is not running the air flows through the external blower with little restriction. When you mount the switch make sure in cutting the thermostat hole you don’t nick the squirrell cage for this will affect the balancing. Noise level is very low with no one, on-the-air, hearing the fan.

The blower is powered by a 17 volt /1 Amp wall wart. The before and after application testing is listed below. The amplifier was configured with 1500 watts pulse CW into a dummy load and the anode exhaust temperature were recorded over a 5 minuet period. The amplifier has an anode temperature sensor but during the tests a handheld infrared tester was used to back up the readings. Wattage was confirmed with a Bird 43 and a 2500 watt slug.

No external fan - temperature amplifier test, stock configuration.

Start		28 Deg. “C”
I Min.		38 Deg. “C”
2 Min		53 Deg. “C”
3 Min		62 Deg. “C”
4 Min		66 Deg. “C”
5 Min		71 Deg. “C” 

A second muffin fan on the rear of the amplifier comes on for less than 60 seconds and shuts off when anode temperature goes below 70 Deg. “C”.

Power Temperature test with outboard fan.

Start		28 Deg. “C”
I Min.		38 Deg. “C”
2 Min		48 Deg. “C”
3 Min		55 Deg. “C”
4 Min		59 Deg. “C”
5 Min		62 Deg. “C” 

The second axel fan on amplifier never comes on. An interesting note is the amplifier with the second outboard fan ran about 6-7 degree cooler and the cool down operation is much faster going from 62 Deg. C” to 39 Deg. “C” in 3 minutes. What is great is the fan can easily be moved around as needed for field use and amplifier certification testing.

The last test was a bad boy key down 1900 watts CW into a large Bird 1 KW dummy load with the external fan. Let the smoke began.

Start		28 Deg. “C”
I Min.		40 Deg. “C”
2 Min		47 Deg. “C”
3 Min		57 Deg. “C”
4 Min		62 Deg. “C”
5 Min		67 Deg. “C” 

The anode temperature was a bit higher but only by 5 degrees C. At 67 degrees ” C” or 154 degrees in Fahrenheit, the exhaust was pretty hot. What I noticed is the tube exhaust temperature stated to level off from the initial temperature rise after 3 minutes. After the transmitting stopped, the exhaust temp dropped to 37 degree after just three minutes.

You will also notice that the fan in the picture has a homemade, air deflection shroud, to direct the hot exhaust vertically. This is to keep the hot air from hitting the rear wall and being sucked back into the amplifier cooling intake. Fan shroud is made from Coke cans and duct tape. It would have been better if I used aluminum tape but I was inpatient. My finest tin can origami. The wall wort was obtained at a swap meet for $1.00.

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More Cooling for Your Amp  
by K8AXW on July 30, 2017 Mail this to a friend!
I've read many requests on eHam on ideas for cooling their amps.... especially ones like the SB-220.

Here is one good example of how to do it! The only thing(s) I would change is the voltage the blower requires.... going to 120VAC. It would be less expensive and easier to find the 120V blower than the 24V. As for the temperature thermostat, my philosophy is if you're gonna cool it, cool it!

No doubt I'll get flak for the rapid and frequent tube/element temperature swings using constant flow but I've been doing it for 32 years on the same tubes without a problem.

Thanks OM for the good solution to a frequent problem.
RE: More Cooling for Your Amp  
by N6KP on July 30, 2017 Mail this to a friend!
Just a quick comment on the SB-220. Remember the SB-220 was first produced in 1970. We did not have muffin fans, just plain motor fans that ran slow enough to keep the 3-500's with their big glass surface area cool and the perforated case contributed. We never had to replace customers tubes due to overheating but rather excessive power or wrong band selected. Today my SB-220 has two muffin fans sitting on the lid that blow upward. Still running after 45 years. Did replace caps.
RE: More Cooling for Your Amp  
by W1ITT on July 30, 2017 Mail this to a friend!
Perhaps do you mean to say "axial" fan?
RE: More Cooling for Your Amp  
by K6AER on July 30, 2017 Mail this to a friend!
Must have had "Guns and Roses" on in the beckground.
RE: More Cooling for Your Amp  
by K6AER on July 30, 2017 Mail this to a friend!
Must have had "Guns and Roses" on in the beckground.
More Cooling for Your Amp  
by K4RVN on July 31, 2017 Mail this to a friend!
Hello Mike,
I am thinking that you still have to cut the amplifier fan off and then let the blower you added run until the t'stat cuts it off. Is that how it works? Also did your amp have a hole instead of a grille in the top cover for you to just stick the blower round inlet in without a mechanical connection? I am adding fans to the inlet of my AL 80B to pressurize the inlet grille.
Trying to figure something out for my AL 800 H which runs pressurized 3CX800A7 tubes. Thanks for your Article on cooling.


RE: More Cooling for Your Amp  
by K6AER on July 31, 2017 Mail this to a friend!

The external blower has its own thermostat set at 90 degrees F. The blower sits atop the amplifier screened exhaust vent by gravity. When you shut off the amplifier the external blower continues until it's thermostat drops below it's set temperature.
RE: More Cooling for Your Amp  
by AF6AU on July 31, 2017 Mail this to a friend!
Oh not just Amps, many of our expensive electronic toys in the house would benefit from cooling... I would consider a pair of fans on top of my room warming Henry 2KD, but I hate to do anything to the metalwork. I have thought of adding a time delay relay to the stock fan (or a better fan from Grainger's) to keep the heavy beast cooler overall.

Having repaired and maintained other's gear, the easiest thing to do first, KEEP THOSE SCREENS AND DUST FILTERS CLEAN! All the added on fans will not help plugged up filters from lazy people that do not clean them at all.

I also have replaced cooked power supplies in many home computers because of fan failure and dust/dirt. Many muffin fans can be oiled by peeling off the label, removing the little rubber plug, and adding some sewing machine oil.

One of the worst I had seen was a floor sitting tower computer where the XYL use carper powder frequently... The powder was all over and seized the CPU and power supply fans. The machine would run 3 minutes then quit.

The second worst, a roasty-toasty Dentron amplifier, with the fan screen loaded with cat fur and dust. No flow thanks to the kitties...
More Cooling for Your Amp  
by W0FEN on July 31, 2017 Mail this to a friend!
As a retired Broadcast Engineer, the BEST filament life was in the transmitter that also was the loudest because the manufacturer pushed the air cooling past the requirements. Loud is Good.
RE: More Cooling for Your Amp  
by K9MHZ on July 31, 2017 Mail this to a friend!
"Oh not just Amps, many of our expensive electronic toys in the house would benefit from cooling... I would consider a pair of fans on top of my room warming Henry 2KD, but I hate to do anything to the metalwork."

At work, fans are a huge, huge deal. Lose your avionics cooling, and you're in deep "stuff."

RE: More Cooling for Your Amp  
by K1BXI on August 1, 2017 Mail this to a friend!
A few years ago I built an amp using a pair of 813's. Since I had a healthy supply of 813's (1950's vintage) and always wanted a quiet amp I decided to see how long they would last with no extra cooling other then the ambient air flow. I built the case from expanded aluminum sheets so as to allow the best natural air flow.

To my surprise the first set of tubes I installed are still showing the same output after 3 years of daily use on SSB.

The power supply is separate, so no heat from that. The amp is loaded to 450 mils @ 2500 vdc and shows 500-600 watts out.

Does it get warm? bet. This was started just to see if the old 813 could be used in gg service with natural air flow and still survive. I had made provisions to install a muffin fan if things were to get too warm.

If it were to be used for any mode with a longer duty cycle than SSB then yes....Mike's title of this article would be spot on.

You can see the amp on my QRZ page......John K1BXI

RE: More Cooling for Your Amp  
by KC2MMI on August 1, 2017 Mail this to a friend!
Some years ago, when the internet wasn't public, and computer superstores didn't exist, I needed a better fan. So I called Rotron (they own the "Muffin" fan trademark) and got a paper catalogue. And found FOUR PAGES of all "the same" size fans. Every one of those fans was designed for a different voltage, or a different airflow, or a different decibel level.

Yes, you may be able to get 10x more airflow, or 10x less noise, simply by looking for the fan properties that are important to you, and then getting that specific fan, instead of the "built to a price point" one that comes in even the best equipment.

It really can be that simple. Fans have way more specs than just "it fits". And engineering, building, and testing the actual blade configuration to see how noisy or effective it will be? All costs money. Ante up.
More Cooling for Your Amp  
by W4AMP on August 2, 2017 Mail this to a friend!
Great article Mike.
RE: More Cooling for Your Amp  
by K9MHZ on August 3, 2017 Mail this to a friend!
MMI, you are not kidding! I went through that whole researching drill a couple of years ago to get the right fan for some 33cm gear that's notorious for getting hot when transmitting. Construction, size, bearings, cfm, hours of life, noise, you name it. Ended up with a fan that was way more money than I was expecting. Satisfied, but thinner in the wallet. Glad that won't happen very often.
More Cooling for Your Amp  
by W6SWO on August 22, 2017 Mail this to a friend!
Great article Mike! Wouldn't it be better if you put the thermostat inside the case of whatever you're cooling?
More Cooling for Your Amp  
by WA2ISE on September 2, 2017 Mail this to a friend!
Something I found with fan noise, you can reduce it if you mount the fan grill about 2 inches away from the fan blades. Oh, and use mesh or sheet metal to keep fingers from sneaking from the sides under the grill. This of course assumes you have the room to do this.
RE: More Cooling for Your Amp  
by KA5ROW on September 2, 2017 Mail this to a friend!
I just bought a new ameritron AL-82, the manual said not to add fans for cooling. But thought was/is to add a second A/C which I already have tube it to just blow behind the bench my equipment sits on and not directly to any thing.
RE: More Cooling for Your Amp  
by K6AER on September 17, 2017 Mail this to a friend!
The hottest temperature in the amplifier is at the exhaust opening. This is just above the anode of the tube. I wanted the fan arrangement to be self contained. In that the exhaust air flower through the additional blower I still have a accurate reading of the exhaust temperature.

Using this method I can move the unit from amp to amp.
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