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Author Topic: Tube heat dissipation  (Read 27664 times)
G3RZP
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« Reply #15 on: August 13, 2013, 02:39:15 AM »

Some years ago, there was a big discussion on this black paint business: someone (I can't remember who) did some measurements. The result was that in fact the black paint increased the temperature in the compartment because it acted as a thermal insulator preventing the heating of the metal work.

Probably what is needed is black anodising, although it is arguable how much difference it would make.
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KE3WD
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« Reply #16 on: August 13, 2013, 08:47:33 AM »

Sounds to me like a recipe that would simply make the surrounding enclosure hotter. 

At some point in time, then, you've got the recipe for an oven.

So I still say the better way to remove heat in this circumstance is to do something to increase airflow through the components. 

Even stirred air within an enclosure can dramatically lengthen the time before overheating can occur, but in the instance of the linear amp in the average room, there is no reason not to affect air exchange with the reservoir of room temperature air available, this is not a sealed situation such as needed when going into a reactor or outer space here. 

My 30-L1's, back when I used such, did pretty good along those lines with nothing more fancy than a 120V muffin fan placed atop the tube cage area...


73
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K6AER
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« Reply #17 on: August 13, 2013, 09:39:14 AM »

If you need more cooling why not increase the fan speed?
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WB2WIK
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« Reply #18 on: August 13, 2013, 10:39:16 AM »

If you need more cooling why not increase the fan speed?

What did you say, Mike?  I can't hear you over my fans... Cheesy
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W4VR
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« Reply #19 on: August 13, 2013, 12:54:15 PM »

Why don't you buy a small muffin fan and Velcro-mount it on top of the cabinet to exhaust the warm air.  I did that with my old Heathkit amp many decades ago and it worked great!
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AC0JX
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« Reply #20 on: August 14, 2013, 10:31:43 AM »

Good question, and an interesting thought experiment.  Two thermodynamic material properties that need to be considered:

Thermal emissivity
Thermal conductivity

The emissivity is largely dependant upon its ability to absorb radiation in the IR spectrum.  In general, black is much better at this, yet there are paints that are designed to be black in the visible light range, or nearly so, and yet absorb relatively less radiation in the IR spectrum.  The amateur’s gold standard for black IR absorbtion is Krylon 1602 flat black.  High emissivity also means that the emittance is high as well.  So a high IR absorbtive paint will not only readily absorb IR, it will readily release it as well.  Coloring the inside of an oven black with a highly emissive color will act as a buffer to maintain stable temperatures within the oven: readily absorbing and releasing heat as ambient temperatures vary.  Painting the inside of the amplifier case black will absorb the IR (heat) yet will also readily re-emit the IR into the environment until a steady state is reached.  So in the amplifier example, the case that is painted inside will get hot more quickly until the metal case reaches a steady state, then the temperature will stabilize unless we give the heat somewhere else to go.  Same with painting the tubes themselves, unless we give the heat somewhere to go, the temperature will remain the same.

That’s where the principle of conductivity comes into play.  We want to take the heat away from the environment, in this example from inside the case to outside the case.   Conductivity is measured in units of (cal/sec)(cm^2 C/cm).  The higher the value, the more conductive the material.  Examples are copper 0.99, aluminum 0.50,  air 0.00057.  Copper is a great thermal and electrical conductor, air is a great insulator.  Steel, the material of the amplifier has a conductivity about 13% of that of copper.  The best combination is a great conductor and a large surface area to the outside environment into which to radiate the heat.  Think a heat sink or radiator.   These thermodynamic properties can be modeled by using an RC circuit, which should excite any active or aspiring ham.

So to really over think the situation: replace the steel case with a copper case, paint the inside to quickly absorb the IR generated by the tubes, and have large surface area copper fins over which circulates a fluid.  One could then consider the properties of the fluid to be used: air, water, etc., taking into consideration the specific heat of the fluid, the properties of the flow with probable unstirred layer effects, the vaporization points of the liquid, the volume of the liquid, and the eventual radiation sink that will cool the cooling liquid if recirculation of the cooling fluid is desired.  Please consult the EPA regulations for release of cooling fluid if water is used and is released into a lake, stream, or the ocean.

On further consideration, cut another hole in the case and install another fan.

Seriously, a great question that made me think.  The challenges and pleasures of this hobby are many.
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W7VO
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« Reply #21 on: August 14, 2013, 11:04:10 AM »



On further consideration, cut another hole in the case and install another fan.



.....or leave it alone and use the amp for another 40 or so years.... Grin

73;

Mike, W7VO
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KE3WD
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« Reply #22 on: August 14, 2013, 11:58:33 AM »

Hams are often pretty good at coming up with solutions in search of problems...


 Grin
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W8JX
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« Reply #23 on: August 14, 2013, 12:36:06 PM »



On further consideration, cut another hole in the case and install another fan.



.....or leave it alone and use the amp for another 40 or so years.... Grin

73;

Mike, W7VO

Extra cooling is never bad.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
G3RZP
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« Reply #24 on: August 14, 2013, 12:55:54 PM »

To my simple mind, any extra air flow is worthwhile....
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VA1DER
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« Reply #25 on: August 14, 2013, 12:59:09 PM »

Some people are confusing radiant IR "heat" (better call it radiation) with real heat.  Having the temperature on the inside of the cage rise isn't necessarily a bad thing, because that indicates success at converting the IR radiation into heat that we can more easily deal with.

As it stands, the fan under the tubes does virtually nothing to cool the tubes, because the parts of the tube that get hot are inside a vacuum with very little to conduct the heat to the tube envelope where the air can cool it.  Making this problem worse is the fact that the wider axis of the plates face each other in the two tubes, so a good third of the IR radiation that comes off each tube goes straight into the neighbouring tube.  The rest of the IR radiation bounces around the nice shiny aluminum sides of the cage. The air blowing around the cage doesn't do anything to reduce any of that IR radiation until it stops reflecting and gets absorbed into something and is converted to real heat.  Until that point, the IR radiation is as likely to hit the tube plates as anything else.  Having that radiation absorb into /anything/ else is better than those plates.  If the temperature of that anything subsequently rises, then great!  Now the fan can actually cool something.

Almost anything should be better than having two radiatively cooled vacuum tubes sitting inside a shiny box where the only method of cooling is air blowing uselessly on the tube envelopes.  The fact these units have lasted 45+ years is less a testament to that great feat of ingenuity than, I think, to the engineering of Cetron tubes.  Alas, we can't get Cetron tubes any more, so I'm quite willing to take steps to improve their cooling.  Even if only marginally.  At the very least, I intend to put a barrier between the two tubes to prevent their radiating into each other.  That alone will improve their cooling by a third.

I think I'll get a thermal imaging camera from work and make an experiment out of this.
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W8JX
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« Reply #26 on: August 14, 2013, 01:18:23 PM »

Some people are confusing radiant IR "heat" (better call it radiation) with real heat.  Having the temperature on the inside of the cage rise isn't necessarily a bad thing, because that indicates success at converting the IR radiation into heat that we can more easily deal with.

As it stands, the fan under the tubes does virtually nothing to cool the tubes, because the parts of the tube that get hot are inside a vacuum with very little to conduct the heat to the tube envelope where the air can cool it.  Making this problem worse is the fact that the wider axis of the plates face each other in the two tubes, so a good third of the IR radiation that comes off each tube goes straight into the neighbouring tube.  The rest of the IR radiation bounces around the nice shiny aluminum sides of the cage. The air blowing around the cage doesn't do anything to reduce any of that IR radiation until it stops reflecting and gets absorbed into something and is converted to real heat.  Until that point, the IR radiation is as likely to hit the tube plates as anything else.  Having that radiation absorb into /anything/ else is better than those plates.  If the temperature of that anything subsequently rises, then great!  Now the fan can actually cool something.

Almost anything should be better than having two radiatively cooled vacuum tubes sitting inside a shiny box where the only method of cooling is air blowing uselessly on the tube envelopes.  The fact these units have lasted 45+ years is less a testament to that great feat of ingenuity than, I think, to the engineering of Cetron tubes.  Alas, we can't get Cetron tubes any more, so I'm quite willing to take steps to improve their cooling.  Even if only marginally.  At the very least, I intend to put a barrier between the two tubes to prevent their radiating into each other.  That alone will improve their cooling by a third.

I think I'll get a thermal imaging camera from work and make an experiment out of this.

This is true and false. Tubes are not operating in a vacuum externally. As air around them heats up, increasing amounts of heat is reflected back to tube plate. Same is true of envelope to some degree. Therefore extra cooling can help but like with radials on a vertical, you will eventual reach a point of diminishing returns.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
G3RZP
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« Reply #27 on: August 14, 2013, 01:30:11 PM »

Exactly, '8JX. If you cool the metal surround with more air, it must cool the tube surroundings. But there is a point of diminishing returns.

Way back when, there was an article in, I think, the RSGB Bulletin about a station in the Artic that used external air at down to -30 deg C or whatever to cool their air cooled PA. They had to slow the blower down at times to stop ice forming on the anode fins! that was when the air wasn't quite  dry enough - i.e. 'warm'!
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KE3WD
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« Reply #28 on: August 14, 2013, 02:33:04 PM »

The device is, after all, a THERMionic valve...


 Grin
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AD4U
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« Reply #29 on: August 14, 2013, 03:24:13 PM »

I built my SB-200 in 1971 on a one night binge on my study desk at college.  It was a Christmas gift from my Dad who died a month later.  I used it exclusively with my SB-102 until around 1980 when I bought a used Drake L4B.  

Granted my SB-200 has been used only occasionally since the 1980's, but it still gets used and it still runs the original Cetron 572 tubes and it still gives an honest 600+ watts PEP output on 80-20M as measured on my Bird 43 with PEP kit.

As far as I know the SB-200 and the Collins 30L1 are the only amps that are 40-50 years old that are still in WIDE SPREAD use.  If not I challenge someone to name another one.

I am sure it could benefit from additional cooling, but given my case and MANY similar situations, I say that Heathkit got it right the first time.

Dick  AD4U
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