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  • #31
    Originally posted by soundmasterg View Post
    I could hear the gears grinding and creaking.

    Greg
    Well I'd hoped to come across better than that. But it's alright I guess
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

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    • #32
      Good reference information on opacity of valve envelope glass to plate thermal radiation is in this application note from a temperature sensor company:
      http://support.fluke.com/ircon-sales...62_ENG_A_W.PDF

      A pictorial of the emission spectrum of a blackbody down at plate temperature is shown at:
      https://www.hgh-infrared.com/FAQ/Blackbody

      Glass is pretty much opaque at wavelengths longer than 5 micron, and a 230C (440F) blackbody has a peak response at 6 micron. So I'd be estimating at least 70-80% of the plates net energy transfer that goes via radiation to the glass envelope gets absorbed as heat.

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      • #33
        Originally posted by Chuck H View Post
        I'm no physics major, but...

        It's my understanding that heat moves in the infrared spectrum of wavelengths (300GHz to 400THz). I'm open to learn about what the opacity of glass is at wavelengths in that range. What really caught my attention though was the implication that if more spectrum moves right past the glass that heat transfer would be improved. I would think that if a fan is in use, and the glass is subject to cooling via the convection of circulated air AND the glass is absorbing a maximum amount of heat from the plate then that would be the most efficient means of removing heat from the plate.?. Any spectrum outside of infrared that the tube is emitting may have consequences, but I don't know that heat is one of them. Further, as far as I know convection is a more effective heat transfer than radiation alone. So I would have thought that heating the glass, which is in close proximity to the plate, and keeping it cool so it can absorb MORE heat from the plate would be about as efficient as we could want. The implication is that any heat that might be radiating through the glass is dissipating more slowly that that which the the glass can absorb and then be relieved of by a cooling fan.

        Like I said, not a physics major. Just thinking out loud.
        Well, I'm a physicist, but...

        I have a language barrier and don't seem to get your point.

        Considering envelope material properties and physical laws involved, I have come to the conclusion that envelope temperature has little effect on plate temperature. There is some re-emission from envelope to plate but as the plate is typically much hotter, this effect will be rather small. Heat radiation is governed by the Stefan-Boltzmann law of heat radiation, which shows that radiated heat power drops with absolute temperature to the fourth power. So if we manage to lower envelope temperature by say 50°C, the cooling effect on the plate will be MUCH less.

        There has been a lot of discussion about plate temperature and how to bring it down. I think this misses the point. How many tube failures are actually caused by a somewhat increased average plate temperature as the primary cause?
        The plate is the most rugged and massive part inside a tube and can take a lot. It seems much more important to avoid out-gassing and thermal run-away caused by excessive envelope temperatures (and of course to avoid over-dissipation of the delicate screens).

        Forced air cooling helps to reduce envelope temperatures and prolongs the life of electrolytics, so it makes sense in particularly hot amps.
        - Own Opinions Only -

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        • #34
          Originally posted by Dave H View Post
          Why use batteries? Why not run it from the heater supply?
          It was easy to supply clean 9v DC this way, no other reason and it has worked well for a couple of years now. I have used heater 6.3VAC rectified on other amps for various tasks, but I like the way this works.

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          • #35
            Fan will help, I've done it often - you typically don't need to blow massive amounts of air - just put a air pressure bias in to assist convection.

            Rectify the 6.3V heater to get around 7VDC and run a 12V DC fan from that. It runs slower but quiet with enough oomph to make a genuine difference.

            Fans are better at blowing into a chassis (blowing against a higher pressure) than at extracting air from a chassis (sucking from a lower pressure). So blow air in, this is also always the quietest option as the chassis acts as a mechanical noise baffle.

            Cheers,
            Ian

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            • #36
              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?
              The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

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              • #37
                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 only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                Comment


                • #38
                  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.
                  - Own Opinions Only -

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                  • #39
                    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.
                    - Own Opinions Only -

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                    • #40
                      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.
                      "Take two placebos, works twice as well." Enzo

                      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                      "If you're not interested in opinions and the experience of others, why even start a thread?
                      You can't just expect consent." Helmholtz

                      Comment


                      • #41
                        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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                        • #42
                          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.
                          The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                          Comment


                          • #43
                            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%?
                            The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                            Comment


                            • #44
                              "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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                              • #45
                                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.
                                The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                                Comment

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