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Author Topic: How hot is TOO hot?  (Read 10465 times)
KI4DSC
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Posts: 65




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« on: September 27, 2013, 11:16:46 PM »

Ok I have yet another question about my amp.  Until now I have been running the amp at a low output power of 400-600 watts because of power limitations.  I am finishing up my new ham shack and I now have the amp on 220V power as it should have been before and it is running full power.  This amp is 50 years old and works like a champ.  I have not had experience with tube amps and I don't know how hot they should get.

I am running a Swan using two 3-400Z Triode tubes.  Today after a short rag chew on 40 meter the amp began to smell "hot".  It reminded me of a hot hair dryer.  I didn't smell anything burning just the smell of hot metal.  The top of the amp over the tubes was real hot -- not so hot I could not touch it but it was getting close.

This amp is 2000Watt PEP Input and it was putting out just over 1000 Watts while I was talking on SSB.  Does that mean I was dissipating the difference (800 - 1000 watts) as heat?

The fan in this unit appears inadequate to move enough air to cool 1000 watts of dissipated heat if that is the case. 

If this is normal, I will let it be -- Like I said the amp has been running for 50 years but I just think the heat is excessive.
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WX7G
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« Reply #1 on: September 28, 2013, 02:11:10 AM »

What color are the tube anodes when you're transmitting? Anything cooler than bright orange is fine.

The enclosure is not too hot if you can touch it.

The base seals on the 3-400Z tubes are specified for 200 deg C maximum.

To cool two of these tubes in chimneys, key down CW, requires 26 CFM of air. At 800 watts anode dissipation plus filament power the cooling air temperature will rise 140 deg C, making for an outlet temp of 160 deg C or 320 deg C. If the outlet air is cooler than this the tubes probably have enough cooling.

« Last Edit: September 28, 2013, 02:14:42 AM by WX7G » Logged
K8AXW
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« Reply #2 on: September 28, 2013, 09:03:08 AM »

DSC:  Running 3-400Zs at full throttle will generate a great deal of heat.  If the input power is 2000W and the output is 1000W, then you're dissipating 1000W in heat, not counting the filament heat as 7G points out.

 While 7G pretty well covers the situation, you might want to consider the fan speed.  With the amp being 50 years old it's possible the fan has slowed down from lack of lubrication.  I've encountered this before with other gear other than amplifiers.

The "smell" you mentioned could be from hot dust that might be accumulated in the air path.

I found that a cheap IR heat detector works great for putting numbers to "feel."  Worth considering for peace of mind if nothing else.
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KD0REQ
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« Reply #3 on: September 28, 2013, 11:06:58 AM »

I would clean and lube the fan motor (probably a little shaded-pole job) and if you're really worried, get another from Harbaugh.  if you have a nylon fan blade, and there is any distortion, absolutely get another fan assembly.

you can always put a steel-framed Rotron fan above either the tubes or transformer, whichever feels hottest to you, on the cabinet.  a plastic job might not be up to the heat without deforming and freezing.  if you do, it should be blowing upwards and in a place where it takes the waste heat moved by the internal fan and "amplifies" its effect.
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G3RZP
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« Reply #4 on: September 28, 2013, 01:10:49 PM »

If you haven't done a good cleaning job with vacuum cleaner and paint brush, that could, in addition to lubricating the fan motor bearings, make a very big difference.
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WX7G
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« Reply #5 on: September 28, 2013, 01:46:46 PM »

Someone mentioned pulling hot air through a fan.

Be careful with this approach. The quickest way to wear out a fan is to pull hot air through it. A fan should blow outside air into a box; pressurize the box.
« Last Edit: September 28, 2013, 01:49:21 PM by WX7G » Logged
KI4DSC
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Posts: 65




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« Reply #6 on: September 28, 2013, 03:27:02 PM »

Thank you all for the good information.  This amp (in my opinion) has a poor design when it comes to cooling.  The amp has a fan that sits between the tubes and the power supply.  The hot air from the tubes is pulled through the fan and then pushed across the side of the power supply.  I was thinking of installing a baffle to shield the power supply from this hot air and then adding a second fan to blow cool air on the power supply.  I don't know how hot the power supply is getting on its own because it has a blast fernice blowing across it. 

As it was suggested I will use my infored "laser" thermometer to spot check the amp while I am using it.  Now that I have an idea of the upper limit to look for the numbers will actually mean something.

Thanks so much guys.  I wish I had a local Elmer close by that I knew but all the guys I know who are local just operate VHF/UHF. 
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WX7G
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« Reply #7 on: September 28, 2013, 03:45:13 PM »

The amp has lasted 50 years with the original thermal design. Why alter it now?
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K8AXW
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« Reply #8 on: September 28, 2013, 05:27:58 PM »

Too many times gear is built "on the cheap" and isn't the best design even at the time.  I seriously doubt if the manufacturer expected the amp to last 50 years!   Grin

I feel if a piece of gear can be modified to improve it, without hacking it up too bad, then it's in order.  

DSC didn't mention how old the tubes were, but at any rate, if the tubes can be cooled down some, (meaning the seals) then by all means do it.

Blowing hot air into the power supply compartment has to be the dumbest design I've ever heard of so far!

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K0ZN
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« Reply #9 on: September 29, 2013, 07:50:00 PM »

Again, as WX7G asked:   What color are the plates of the tubes during what you consider normal operation?

Are you tuning the amp for maximum RF output at (or slightly above) the operating level?  Do you operate the amp on or very close to the frequency
you tuned it up at?  Improper tuning can generate a LOT of heat very quickly.

If you have any questions/concerns about correct tuning, check out w8ji.com.  He has some very good info on amplifier tuning.

73,  K0ZN
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KI4DSC
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Posts: 65




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« Reply #10 on: September 29, 2013, 08:57:13 PM »

Again, as WX7G asked:   What color are the plates of the tubes during what you consider normal operation?

Are you tuning the amp for maximum RF output at (or slightly above) the operating level?  Do you operate the amp on or very close to the frequency
you tuned it up at?  Improper tuning can generate a LOT of heat very quickly.

If you have any questions/concerns about correct tuning, check out w8ji.com.  He has some very good info on amplifier tuning.

73,  K0ZN

I normally stay on or quite need the frequency I tune up on. I follow the instruction for the amp when I tune up and I tune for max RF.  I normally drive the amp with less than the 100 watts needed for full output. 
The manual calls for the IP current to be at an average of .35 with voice peaks not to exceed .5.  The color of the tube after several seconds (rag chew time) is a soft orange glow.  The air coming out of the vent from the fan appears to be fairly low volume but it isn't very hot. I put a probe at the fan exhaust point and it was only about 140 degrees ferenheight.

Perhaps I am just paranoid about cooking my amp. As I said before, this is my first.experience with a tube amp and I am not acustom to the amount of heat they make. I guess that's why they call them "foot warmers".
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WB2WIK
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« Reply #11 on: September 30, 2013, 09:52:41 AM »

The Swan Mk-I was undercooled, but that risks the power supply components and not the tubes.

The newer version, the Mk-II, separated the power supply into a separate enclosure (it's two pieces, not one) and added a large cooling fan just for the power supply.  That one lasts longer, but it's really the power supply that is at risk and not the 3-400Zs.

I've had to replace power transformers in Mk-I amps a few times.  Some of them failed pretty early on, back in the late 1960s.

If you want to "boost" cooling, I agree, the right way is to blow cool air into it, not exhaust hot air out of it.  And I'd add the cooling to blow on the transformer and filter capacitors.
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W8JX
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« Reply #12 on: September 30, 2013, 10:12:27 AM »

I would reverse air flow for starters so tube heat is not sent you to power supply
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You can embrace new computer/tablet technology and change with it or cling to old fall far behind....
AA4HA
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« Reply #13 on: October 01, 2013, 03:08:04 AM »

I would reverse air flow for starters so tube heat is not sent you to power supply

That sounds much more difficult; 1) It would mean pulling air through the fan, heating up the fan until it fails. 2) If the amp uses convection (heat rises) you cannot just reverse the air flow across the tubes, the seals need to see the coolest air, first. Reversing airflow will take the air and blow it across the bottles, and down into the socket area. As it was mentioned by others, the pins and tube base need the most cooling. 3) the heat will just roast all of the components below the chassis.

It seems to make more sense to separate the cooling of the tubes from the power supply and to give each system an independent fan, pushing the air upwards and away from the tubes, transformer, rectifiers and caps. Maybe replacing the existing fan to get more CFM, also you might be able to get a quieter fan (blade designs have improved).
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
G3RZP
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Posts: 4965




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« Reply #14 on: October 01, 2013, 04:29:18 AM »

I agree with Tisha - in all those 5 volt 14.1 amp filament tubes, the filament seals need cooling. Plus the problem that if the pins get too hot, the socket clips lose their grip and they get even hotter. Applies to 3-500Z, 3-400Z, 4-250, 4-400 and their European close equivalents.
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