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Thread: Cooling fan and tube life

  1. #36
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    Thanks everyone. Its probably far outside my pay grade, but I spent a a lot of time thinking about this topic, maybe too much given the number of household repairs needed . Heat can be transferred by conduction, convection and radiation. Inside the tube there is a relatively high vacuum, so heat is less likely transferred through convection from the body of the tube (the metal parts) and the envelope. But the metal parts are all connected all the way down to pins through the glass, into the socket. The body of the tube innards conduct to the pins and so, the glass envelope as well, since the pins have to be tightly sealed. which I thought is why the tube glass gets hot all over: mostly conduction from the metal parts in the tube body to the pins and base of the tube.

    So, if we blow a bunch of air over the tube, and chassis, maybe its conduction that is removing heat, and not radiation? If you cool off the whole envelope and the chassis where the socket is bolted to, then aren't you giving the innards of the tube a larger temp differential (between guts of the tube, and where the pins are connected to the envelope, and the envelope itself) so that heat sinks out of the tube innards more efficiently?

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    Look at all those connections, can't they transfer heat down through the pins fairly efficiently?

    Click image for larger version. 

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    The body of the tube innards conduct to the pins and so, the glass envelope as well, since the pins have to be tightly sealed. which I thought is why the tube glass gets hot all over: mostly conduction from the metal parts in the tube body to the pins and base of the tube.
    No, heat conductivity of glass is low and contact areas are very small. The envelope gets heated mostly from partly absorbed heat radiation.

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    So, if we blow a bunch of air over the tube, and chassis, maybe its conduction that is removing heat, and not radiation?
    This would be forced convection and it mainly cools the envelope, which is fine.

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    Bent Member Chuck H's Avatar
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    As a follow up on my earlier post, I didn't have my head wrapped around the issue yet. It occurs to me now that since radiation is the only means of heat dissipation for tube elements that anything radiating through the glass is an advantage. But, since the glass does absorb heat from the tube elements (and radiate some back) keeping the glass cool is the only means we have to mitigate heat. It also occurs to me that any heat radiating beyond the glass has to land somewhere. Like nearby can capacitors, etc. Blowing a fan is definitely a good idea in this regard. It was mentioned that just blowing the fan into the tube heated space seemed to work best. I still don't agree with this method completely because I think that there's too much limitation for positioning most of the time and this usually results in blowing directly onto one tube in a row of tubes and almost not at all onto the last tube. I also like to include a dust filter of some kind and this is easier when you use the fan to suck out hot air and filter the inlets. Putting a filter directly onto a blowing fan seems to limit it's effectiveness more. So there is less overall convection this way, but it's more even and there's less dust being introduced. I don't really think it takes a whole lot of convection anyway. In fact I think the solution only really applies to amps that are designed such that they have almost none. That's really where a fan can help and it doesn't need to do much to help a lot.

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    It appears that heat conduction from the base/pins of the venerable 6L6G was much less than 1W when the rest of the valve was dissipating 18W from anode, plus additional power from heater and screen. You can enjoy the tech assessment of that particular tube from page 261 in the 1962 RCA tube design manual if you are really really keen
    https://archive.org/details/RCA_1962...on_Tube_Design

    There is also a nice graph in Fig.35 showing max bulb temps for different style ST bulbs. The KT88/6550 style bulb is a good example of how to maximise glass surface area in region of max plate radiation, as a larger glass area cooling by convection achieves a lower glass temp for a benchmark plate dissipation level.

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    Quote Originally Posted by Helmholtz View Post
    No, heat conductivity of glass is low and contact areas are very small. The envelope gets heated mostly from partly absorbed heat radiation.
    Ah, well it was nice for that brief instant i thought I figured something out, thanks.

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    Quote Originally Posted by trobbins View Post
    It appears that heat conduction from the base/pins of the venerable 6L6G was much less than 1W when the rest of the valve was dissipating 18W from anode, plus additional power from heater and screen. You can enjoy the tech assessment of that particular tube from page 261 in the 1962 RCA tube design manual if you are really really keen
    https://archive.org/details/RCA_1962...on_Tube_Design

    There is also a nice graph in Fig.35 showing max bulb temps for different style ST bulbs. The KT88/6550 style bulb is a good example of how to maximise glass surface area in region of max plate radiation, as a larger glass area cooling by convection achieves a lower glass temp for a benchmark plate dissipation level.
    Thanks and thanks for the link. so, 1w/18w isn't nothing, right? Its not "Yuuuge" as the presidential candidate said, but around 6%?

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    "much less than 1W". The reference states "in the order of tenths of a watt ... it will be neglected". So that heat flow path through the socket contributes to perhaps about 1% of heat flow from the tube.

    Note that for a 6L6G the plate has an 18W dissipation rating, but there is also nearly 6W dissipating from the heater and at least a watt could be coming from the screen - so about 25W could be being radiated from the plate. Note that the cathode and screen radiate their power to the plate, as they are hotter than the plate - the filament is about 1500C, the cathode sits at around 790C, the screen grid at about 530C, the plate at about 420C, and glass at about 160C.

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    Thanks I think I finally got it. SO, all I need to do, is drill a couple of holes in the glass, say one on each side, then put the fan so it blows *through* the tube, and everything inside will cool down nicely.

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    Yes, exactly. Better still, drill a hole in the tube and fill the insides with automotive coolant.

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    Quote Originally Posted by Enzo View Post
    Yes, exactly. Better still, drill a hole in the tube and fill the insides with automotive coolant.
    Thanks Enzo. After some additional reading, I found that high speed steel drill bits work on glass.
    And I happen to have an extra water pump left over from my dad's '65 Ford Fairlane 500.
    This is going to be the coolest amp in world history.

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    Ever wondered why there needs to be vacuum (= nothing) between the electrodes?

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    Quote Originally Posted by Helmholtz View Post
    Ever wondered why there needs to be vacuum (= nothing) between the electrodes?
    Nothing except for a gazillion electrons but they are just passing through.

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    Quote Originally Posted by Dave H View Post
    Nothing except for a gazillion electrons but they are just passing through.
    Sure, but anything else would cause collisions.


    Also cooling the cathode wouldn't be a good idea.

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    Last edited by Helmholtz; 03-05-2019 at 11:48 PM.
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