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Author Topic: Tube heat dissipation  (Read 27671 times)
VA1DER
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Posts: 9




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« on: August 12, 2013, 06:19:06 AM »

I have an SB-200 with two 572b tubes.  I have read of people who paint the inside of this unit black to better absorb the IR (black-body radiation) from their tubes - I presume to prevent its reflection back into the tubes and improve convective heat dissipation.  I'm wondering if it would harm the tubes to go a step further and paint them directly matte black with a good high-heat paint.  It seems to me this would be the best way to make use of the fan in this unit, which blows directly on the tubes.  Or maybe a combination, painting the underside of the tubes black where the fan blows on them and leaving the top half clear to radiate upwards and out.

Has anyone tried this?
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KE3WD
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« Reply #1 on: August 12, 2013, 06:57:36 AM »

What makes black paint black? 

The answer is, "Carbon" my friend. 

Carbon is conductive to electricity. 

The high potential applied to the cap of a transmitting tube shouldn't have a conductive path painted onto the outside of the tube. 

As for painting the interior of the cage black, I personally don't think here is enough to gain there to bother doing that, recommend the Halliday and Resnick physics book for a good starting place on finding out the reasons why. 


73
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W8GP
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« Reply #2 on: August 12, 2013, 07:50:43 AM »

Besides the conductive properties of the paint, it will also act as a heat insulator if applied to the tube envelope. A THIN coat of flat black paint on the surrounding metal surfaces may or may not be beneficial, but it won't hurt anything, either.
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AD4U
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« Reply #3 on: August 12, 2013, 07:57:34 AM »

My SB 200 is 40 years old and is still running the original Cetron 572B tubes providing 600 watts output.  The cooling is "factory stock".  Many have suggested that the original cooling in the SB 200 was marginal at best and there have been numerous articles on how to better cool the tubes.  Whether or not additional cooling is necessary will be debated as long as SB 200 amps are still in use.

Way back when I built amps, there was a saying that the only time you had too much cooling air was if the air blast lifted the tubes out of the sockets and smashed them on the ceiling.   Wink

Dick  AD4U
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VA1DER
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« Reply #4 on: August 12, 2013, 09:57:56 AM »

Way back when I built amps, there was a saying that the only time you had too much cooling air was if the air blast lifted the tubes out of the sockets and smashed them on the ceiling.   Wink

I can't really speak toward whether the cooling in the unit is adequate.  I'll leave that to the engineers.  I just figure something along your lines - if there is an easy way to make it a little better, why not - since too much certainly isn't going to hurt it.
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WB2WIK
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« Reply #5 on: August 12, 2013, 10:39:50 AM »

I wouldn't paint the tubes themselves.  Even if the paint were a great dielectric (black paint likely isn't), this would prevent heat escaping via radiation, and they're designed for radiation.

Painting interior surfaces of the enclosure black prevents the radiation from being reflected (by mirror-like surfaces) back into the tubes.  Whether an SB-200 would benefit from that, I can't say (never tried it), but I don't think that amp needs such a modification.

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W8JX
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« Reply #6 on: August 12, 2013, 11:50:18 AM »

I wouldn't paint the tubes themselves.  Even if the paint were a great dielectric (black paint likely isn't), this would prevent heat escaping via radiation, and they're designed for radiation.

Painting interior surfaces of the enclosure black prevents the radiation from being reflected (by mirror-like surfaces) back into the tubes.  Whether an SB-200 would benefit from that, I can't say (never tried it), but I don't think that amp needs such a modification.



I fully agree on paint side of this but a little extra air cooling is not bad either.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
KD0REQ
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« Reply #7 on: August 12, 2013, 12:05:20 PM »

considering that the amps have been out there for 40 years, pounding along with a good reputation, why fool with it?  a lot of posters have the original tubes in theirs.  it's not a contest amp, and a few strokes of paint will not help the transformers to CCS service, either. 
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G3RZP
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« Reply #8 on: August 12, 2013, 12:10:18 PM »

I believe that there is an old New England saying of 'if it ain't bust, don't try to fix it.'

These guys were right.
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KB1LKR
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Posts: 1897




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« Reply #9 on: August 12, 2013, 12:44:36 PM »

You'd like the tube envelope transparent to IR; painting them any opaque would be a BAD thing. Also note that the color would have no impact on the thermal emissivity of the surface, whereas a father color should increase thermal absorption, hence the rational behind painting the enclosure black. Practically speaking painting the enclosure flat black shouldn't hurt.
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KE3WD
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« Reply #10 on: August 12, 2013, 01:12:42 PM »

"Paint" is not really an engineering term. 

"Coating" is the preferred. 

Those wishing to coat the interior of their amp should understand the ramifications of attempting to get common coatings, or "paint" to adhere to aluminum before undertaking the proposition. 

And, I am fairly certain that there is little to be gained from doing so. 

Your money and time would be better spent on moving air properly through the thing and around the tube(s). 


73
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AD4U
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« Reply #11 on: August 12, 2013, 01:58:36 PM »

Last weekend I picked up a SB 200 at a yard sale.  I am in the process of returning it to proper and original operation on the HAM bands.  It is easy to spend more on a restoration than the item is worth.  Since I already have two, I will probably offer it for sale. 

It still has Cetron tubes and it puts out around 600 watts on all bands, 500 watts on 10M but 80M output is only 250 watts.  This will be fixed along with a few other items.

Someone had cut about a 3 inch diameter hole in the left side of the outer case and through the aluminum inner case into the tube compartment.  I repaired it as best I could with matching preferated aluminum and then painted the outer case Heathkit green.  It appears there was some kind of squirrel cage blower mounted there for additional cooling - AM on CB??

Through it all this little SB 200's transformer, tubes, and other components apparently survived.   IMO this is a testament to the SB 200.  It never ceases to amaze me what some people will do to a perfectly good amp.

Dick  AD4U

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W7VO
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« Reply #12 on: August 12, 2013, 03:01:09 PM »

considering that the amps have been out there for 40 years, pounding along with a good reputation, why fool with it?  a lot of posters have the original tubes in theirs.  it's not a contest amp, and a few strokes of paint will not help the transformers to CCS service, either. 

Amen!

Mike, W7VO Wink
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KE3WD
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« Reply #13 on: August 12, 2013, 04:24:47 PM »

Amen +2
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W8JX
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« Reply #14 on: August 12, 2013, 07:04:10 PM »

I believe that there is an old New England saying of 'if it ain't bust, don't try to fix it.'

These guys were right.

Well I can say from using a SB200 in past that I do feel that power supply compartment could use a little more cooling.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
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