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Cooling fan and tube life

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  • #16
    Originally posted by Chuck H View Post
    In this image I found you can actually see the glass flattened against the plate on the left tube.

    I found a similar treated EL34 in a Matchless Chieftain several years ago, a good olˋ fetish for serious Voudou Biasing...
    Attached Files

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    • #17
      Traynor used a fan on it's YBA-1A, 85 watts clean out of a pair of EL34's.

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      • #18
        Originally posted by mozz View Post
        Traynor used a fan on it's YBA-1A, 85 watts clean out of a pair of EL34's.
        Mesa Boogie also used a fan in there Mark series amps.

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        • #19
          So, if I get the main points right, the airflow around the outside of the glass part of the tube probably won't help much, since the guts of the tube transfer by EM radiation (through the tube glass), but the airflow across the chassis, and so tube sockets and base of the tube, might help some, since some conduction does occur through the bottom of the tube, pins, base, etc?

          Early on, after learning bits and pieces, I was very surprised at the Fender typical design, the chassis is closed, no air holes or vents, and the open side of the chassis is bolted up tight to the head or combo cab. And the tubes are upside down, so hot air rising would tend to heat the chassis rather than cool it.

          Even though we have 35, 40, 50+ year old amps out there, still cranking away, tube construction (as well as anything else) ain't what it used to be. I had thought about having holes cut in the top of the combo cabinet and little vent screens put there, but since the chassis is all closed up, no air could get through anyway.

          But might be worth it to put one or two of those little pancake fans on the sides of the combo and have air move across the bottom of the chassis, transformer, and tubes?
          The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

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          • #20
            Originally posted by Zouto View Post
            I found a similar treated EL34 in a Matchless Chieftain several years ago, a good olˋ fetish for serious Voudou Biasing...
            WOW the tube heated up enough to melt glass?
            The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

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            • #21
              Don't you need like 1600 to 1800 C to melt glass like that? What happened inside the tube to generate heat like that?
              The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

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              • #22
                So, if I get the main points right, the airflow around the outside of the glass part of the tube probably won't help much, since the guts of the tube transfer by EM radiation (through the tube glass),
                Nope, cooling the bulb doesn't cool the plate but has been shown to increase tube life. Please see the link I posted above.
                - Own Opinions Only -

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                • #23
                  I think it was concluded that cooling the glass helps too, just not as much we might hope. But you can use a really small, quiet fan. You only need to move a little new air in and take old air out. Still air is a pretty decent thermal insulator. As long as there's any air flow at all what little heat it can be carried away will be.

                  EDIT: The one time I installed a fan in a project I followed the advice of members here and set it up to suck air out of the space to be cooled so the drawn air across the circuit was a gentler, wider swath. Rather than a concentrated blast of air going into the amp. Further, if dust is a concern you can filter the venting so the air coming in will be dust free. It's much harder to keep effective air flow filtering right at the fan.
                  Last edited by Chuck H; 12-03-2018, 02:04 AM.
                  "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

                  "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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                  • #24
                    Originally posted by mikepukmel View Post
                    Don't you need like 1600 to 1800 C to melt glass like that? What happened inside the tube to generate heat like that?
                    My explanation: A hot bulb frees gas molecules absorbed in the getter metal layer on the inner bulb surface. The gas molecules get stripped from some of their outer electrons in collisions with electrons. The remaining positive gas ions get attracted by the negative grid and cause a positive grid leak current, which produces a more positive grid voltage across the grid leak resistor. The result is increasing idle current and (worst case) thermal runaway.
                    - Own Opinions Only -

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                    • #25
                      Awesome, thanks everyone, got it.
                      The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

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                      • #26
                        I am not a big fan of tube down designs (except my 50w JCM800 2204 build which runs really cool) , so I usually put a 12v computer fan in the cab base drawing cool air from under the base right under the PT area(supported on rubber feet of course!). I normally run from a Li-Ion 9v cell, and it will run for many hours, keeping the PT, chassis and to some extent tubes a lot cooler.

                        I have this in my tweaked 6L6 35w 5E3 which runs really hot (30w 250R cathode resistor(s)).

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                        • #27
                          Originally posted by Rog_B View Post
                          I usually put a 12v computer fan in the cab base drawing cool air from under the base right under the PT area(supported on rubber feet of course!). I normally run from a Li-Ion 9v cell, and it will run for many hours, keeping the PT, chassis and to some extent tubes a lot cooler.
                          Why use batteries? Why not run it from the heater supply?

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                          • #28
                            There are a few misconceptions about heat transfer in this thread

                            It is better to think of heat transfer within the valve as just based on the temperature difference between plate structure and whatever it can radiate to - the plate is hotter than glass envelope or anything else nearby and so heat transfers by radiation away from the plate. Trying to think of it as transferring in both directions, but one direction dominating, does not help imho.

                            Most of the plate radiation does get absorbed by the glass, but there is a significant percentage transmitted through the glass and landing on anything nearby - that is because the heat transferred has a wide spread of wavelengths, and glass is transparent at the shorter wavelengths but has a rather sharp transition to opaque (that depends on glass thickness) and so absorbs a high percentage. So keeping the glass clean does have some minor benefit.

                            By far the most heat is transferred away from valve glass by convection to surrounding air - any improvement in air flow, whether by aiding the chimney effect or by movement from a fan, is going to lower glass temp.

                            Probably the main cause of lifetime depletion for tubes operating at or near rated dissipation is outgassing of glass and internal parts, whereby the getter finally gets consumed and can't then maintain a low enough gas pressure for normal tube operation. A bogey tube glass temp level at rated dissipation is the premise for the typical 2,000hr lifetime rating - there will on average be a plus/minus lifetime change for lower/higher glass temp. It's probably a law of diminishing returns for lower glass temp, as other lifetime issues start to come in to play, but certainly worth keeping in mind if your amp has a poor natural ventilation path to and from the output stage tubes, or your room is stinking hot.

                            There was obviously a lot of technical effort spent by large tube makers to confirm bogey lifetime ratings and what had to be done to achieve those levels - probably the best reference for a quiet Sunday afternoons reading is the 1962 RCA book on all matters technical within a valve.

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                            • #29
                              Originally posted by trobbins View Post
                              Most of the plate radiation does get absorbed by the glass, but there is a significant percentage transmitted through the glass and landing on anything nearby - that is because the heat transferred has a wide spread of wavelengths, and glass is transparent at the shorter wavelengths but has a rather sharp transition to opaque (that depends on glass thickness) and so absorbs a high percentage. So keeping the glass clean does have some minor benefit.
                              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.
                              "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

                              "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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                              • #30
                                Originally posted by Chuck H View Post

                                Like I said, not a physics major. Just thinking out loud.
                                I could hear the gears grinding and creaking.

                                Greg

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