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Author Topic: Tube heat dissipation  (Read 27173 times)
AD4U
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« Reply #60 on: August 20, 2013, 10:49:46 AM »

So....To sum the original post up in 25 words or less:

THE TUBES IN THE SB-200 AMP ARE PROPERLY COOLED IF THE AMP IS OPERATED WITHIN ITS PUBLISHED RATINGS

This fact is verified by the huge number of SB-200 amps still in daily use after being run (and abused) for almost 50 years.

Dick  AD4U   Wink
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NJ1K
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« Reply #61 on: August 20, 2013, 12:13:57 PM »

It ain't all I got, duh, but it may be all I'll share. 

Uh huh... Well then, be that way..
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NJ1K
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« Reply #62 on: August 20, 2013, 12:21:23 PM »

So....To sum the original post up in 25 words or less:

THE TUBES IN THE SB-200 AMP ARE PROPERLY COOLED IF THE AMP IS OPERATED WITHIN ITS PUBLISHED RATINGS

This fact is verified by the huge number of SB-200 amps still in daily use after being run (and abused) for almost 50 years.

Dick  AD4U   Wink

I think this applies to most if not all manufactured tube amps however, "WITHIN ITS PUBLISHED RATINGS" is the responsiblity of the operator.  I have to assume that during the heat of the contest battle, all bets are off.  Thus the desire to "improve" robustness.
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W0BTU
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« Reply #63 on: August 20, 2013, 12:45:10 PM »

THE TUBES IN THE SB-200 AMP ARE PROPERLY COOLED IF THE AMP IS OPERATED WITHIN ITS PUBLISHED RATINGS
This fact is verified by the huge number of SB-200 amps still in daily use after being run (and abused) for almost 50 years.

That's hard to argue with. However, I personally wouldn't run my SB-200 with the outer lid closed AND without a small fan on top drawing air up and out of the PA compartment. Running CW, the metal case gets very hot without doing that.
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KE3WD
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« Reply #64 on: August 20, 2013, 03:14:02 PM »



Aren't emissivity and absorption reciprocal as stated by Kirchhoff's law of thermal radiation? That is, if we paint a surface black and its absorption is increased, its emissivity is increased by the same amount. 

http://en.wikipedia.org/wiki/Kirchhoff%27s_law_of_thermal_radiation

FWIW, I agree with that, am recalling studying long ago that one of the parameters that must be proven before the scientific world deems such to be a Physical Law is that it must pass the reciprocity test. 


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NJ1K
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« Reply #65 on: August 20, 2013, 03:48:42 PM »

For those who would like to better understand thermal emissivity, here are a couple of interesting articles.

http://www.molalla.net/members/leeper/coatbar.htm

https://en.wikipedia.org/wiki/Black_body_radiation

As always, take everything you read on the interwebs with the usual grain of salt.

Bonjour
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W6EM
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« Reply #66 on: August 20, 2013, 07:14:28 PM »

I have a couple of questions that perhaps some of you could address.

Years ago, several glass-envelope tubes had what appeared to be a chrome-like coating on their envelopes.  Since the emissivity of shiny metal surfaces is very low, was this an oversite or of some purpose to reduce heat transferred away by radiation?

Along the same lines, most older tube shields, especially on military gear, had what looked like tin-plated, semi-shiny surfaces.  Quite a few later-version tube shields on commercial gear, though, were flat black.  I understand why those were flat black (higher emissivity), but why were the others tin-plated with much lower emissivities?
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W0BTU
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« Reply #67 on: August 20, 2013, 07:26:35 PM »

glass-envelope tubes had what appeared to be a chrome-like coating on their envelopes.

That's what is commonly known as a barium getter. Its purpose is to collect ("get") air molecules that were not pumped out during manufacturing and those that might leak in later through the seals, etc.

There were other materials used, but barium metal was the most common. If you see it turn from shiny opaque to pale white, that means enough air has leaked in to render the tube useless.
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G3RZP
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« Reply #68 on: August 21, 2013, 01:56:53 AM »

A plain aluminium shield is quite permissible for a tube that is dissipating very little power.
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W6EM
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« Reply #69 on: August 21, 2013, 06:05:31 AM »

I guess, then, that would have made "getting" more important than "radiating."  Thinking about Dewars, and the purpose of the shiny metallic coating of the glass to prevent radiation gain or loss provoked that question.

I guess, then, if one were to reside in a place of temperature extremes, the roof over the head should be white tiles or some such.  However, not of polymeric origin.  I've seen a few of those in south Florida.  The sun's ultraviolet spectra tends to bust up polymers that don't contain a lot of carbon black.  So, plastics of any color but black outdoors are not the best choice.  Including the oft-seen white plastic antenna insulators and unpainted, gray-colored pvc junction boxes containing baluns...

So much for donning either my black Razorback or Cincinnati Reds caps on hot or cold hamfest days.  But, at least they keep cranial sunburn to a minimum Cool

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AD4U
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« Reply #70 on: August 21, 2013, 06:10:28 AM »

Many tubes were made with a small strip of magnesium inside the glass envelope.  After the "innards" of the tube were installed in the glass envelope and the area vacuumed as much as possible by the technology available at the time and the tube was sealed, the magnesium strip was ignited by induction and / or RF.  When the magnesium strip burned, any trace amount of oxygen remaining inside the glass envelope was consumed.  This insured a complete oxygen-free tube.  

Of course over time the tube sometimes leaked, especially at the envelope to base or at the base to pin seals.

Many old tubes were offered in glass and metal enclosures.  Their function was basically identical.  The only practical difference was in the envelope material.  In every case I am aware of, metal incased tubes were always black.  I assume this offered better heat transfer from inside the tube to the outside air than other colors.

Dick  AD4U
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KH6AQ
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« Reply #71 on: August 21, 2013, 08:18:12 AM »

Here's short article on Getters.

http://en.wikipedia.org/wiki/Getter

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G3RZP
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« Reply #72 on: August 21, 2013, 08:34:28 AM »

The original EF50 (a Philips design) manufactured by Mullard in the UK and Sylvania in the US, was red, as was the EF54 VHF pentode. These were meant for TV sets in the prewar days (it was introduced in original form in 1939), but got very widely used in WW2 radars and other VHF applications. They were a glass tube inside a metal can: other manufacturers made them too, under various numbers, and some were, in the case of military ones, plain aluminium cans. The video pentode that went with them in the series had a 10 watt dissipation and its outer casing was black.

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