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Old 31st October 2012, 05:41 AM   #51
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Quote:
But you can't cool the tubes with a fan or vents.
I offer this experiment to the contrary. Light up a tube so that there is visible red glow on the plate. Fire up the biggest fan you can find and point it right at the hot spot on the plate. The glow will diminish slightly. The effect is subtle and best observed when the dissipation is increased until the plate barely shows glow. A fan can reduce the glow to below visibility.

We have a Flir thermal camera at work. Some day I want to do some testing on some of the metal tube coolers on the market.

Quote:
Conducted heat????? Where, out the pins to the socket and wires?
Depending on the internal construction, some older tubes have a glass stem with wires protruding up into the active area. Look at some old 6SN7's. The thick glass will conduct some heat down into the base. Look at some old brown base Tung Sol 5881's. The tan base has turned rather brown from conducted heat, even in well ventillated amps with the tubes mounted right side up.
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Old 31st October 2012, 01:28 PM   #52
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Originally Posted by tubelab.com View Post
I offer this experiment to the contrary. Light up a tube so that there is visible red glow on the plate. Fire up the biggest fan you can find and point it right at the hot spot on the plate. The glow will diminish slightly. The effect is subtle and best observed when the dissipation is increased until the plate barely shows glow. A fan can reduce the glow to below visibility.
Pretty obvious, fans and ventilation have always improved temperatures in valves - it can only help.

While it's pretty well true the filaments are only cooled by radiation, the glass is cooled by conduction and convection as well, and keeping the glass cooler will reduce the filament temperatures as well.
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Old 31st October 2012, 04:34 PM   #53
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ChrisA, Do you mean can't cool it beyond some limit?

It only indicates that the radiation thru the vacuum is the limiting factor so you can't do much to reduce the internal temp further. It doesn't prove you can't cool the tube, it proves there is a limit to how much you can cool the internals. Which is an intentional part a stable design. It says nothing at all about the frequency of the energy outside the envelope and whether radiation or convection or conduction predominates. Can't cool the tube?? No, it proves the opposite, that radiation and conduction and convection DID cool the tube sufficently. I guess you must mean that you can't cool its internals colder than necessary or something like that???? Wouldn't want that, wouldn't want to make the cathode stop emitting. And no, the glass does not work just fine when hot; when the glass is really hot it radiates back into the tube just as much as it radiates out, so convection/conduction is a better way to cool the envelope in that respect, and an abused tube will melt the glass.

But the discussion was about the ratio of radaition thru the glass versus conduction and convection cooling the glass itself. Your water experiment is about what happens inside the glass envelope and how that limits the cooling, and that's interesting but a different discussion. We all agree that inside the envelope it's all radiation, because there's no convection and only a teensie bit of conduction down the pins and supports to the glass and socket.

You're probably right that most of the energy is radiated right thru the glass, but the water experiment says nothing about that.

And of course this discussion is way too broad, as the particular tube is very relevant. Remember that most military models of power tubes don't even have glass envelopes, and the solid metal obviously doesn't allow radiation thru.

Last edited by cyclecamper; 31st October 2012 at 04:39 PM.
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Old 31st October 2012, 04:44 PM   #54
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My immediate question is how to paint the inside of my wooden guitar cabinet. White or black? I'd vote black, with plenty of moving air.
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Old 31st October 2012, 05:00 PM   #55
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Tubelab,
Yes, some real testing of those heat-sink collers would be interesting. They'd convert all the radiation to convection. I'd imagine they'd work great with a fan or good chimney design.
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Old 31st October 2012, 05:53 PM   #56
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Fan cooling of glass envelope transmitter tubes, like Eimac'x 3.500Z, 4-1000 etc are very much standard, rather than exceptions...
usually encased in a tubular section to increase air flow.....
nothing new at all....
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Old 31st October 2012, 08:57 PM   #57
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But a lot of high-frequency tubes like microwave tubes have some sort of extension of the anode and a real heat-sink, so there's real conduction thru a solid rather than just radiation thru a vacuum.
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Old 31st October 2012, 10:33 PM   #58
AuroraB is offline AuroraB  Norway
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Those Eimacs I mentioned, and several more from the same family, are all regular glass envelopes.... albeit somewhat larger than an EL34....
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Old 1st November 2012, 07:06 PM   #59
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White or black? I'd vote black, with plenty of moving air.
It's up to you, but I have been tinkering with guitar amps for about 50 years. I have never seen a white one. They are either natural wood color, or painted flat black inside.

Quote:
Fan cooling of glass envelope transmitter tubes, like Eimac'x 3.500Z, 4-1000 etc are very much standard, rather than exceptions....
Read the data sheet on some of these glass tubes. Forced air cooling is REQUIRED. They mention that the glass to metal seal is the weak point and must be kept below a certain temp with forced air and heat sinks on the pins. This info comes from the 833A data sheet, but I have seen similar info on the 3-500Z, 4-400, and 4-1000. I have built RF amps with all of these tubes. All had blowers.

Quote:
usually encased in a tubular section to increase air flow....
Eimac usually specified a certain socket and chimney for each tube. The warantee was void if it was not used.
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Old 1st November 2012, 08:30 PM   #60
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Uuhhmm -- yes.......
I have quite some hours working with these tubes.......
My only point was really that guitar and Hifi audio is probably the fiels where fan cooling is not used, - with a few exceptions....
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