Vacuum Tube forced air cooling

Discussion in 'General Electronics Chat' started by #12, Jul 8, 2015.

  1. #12

    Thread Starter Expert

    Nov 30, 2010
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    In reference to a thread called, "Tube Creep":
    I have never seen mention of forced air cooling for vacuum tubes in the books I have read. I wonder...does blowing air on a vacuum tube change its power rating much? I know some monster tubes are water cooled, but I'm talking about octal base power amplifiers or rectifiers that any of us might meet tomorrow on a repair job. It seems to me that the vacuum in the tube makes a good insulator so there is no convection. Conduction exists and radiation exists. So, does anybody have a reference or personal knowledge?

    Number Twelve
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I also wonder if uneven/fluctuations in cooling can cause noise in the tube output.
     
  3. #12

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    I don't think so. I have installed fans in more than one guitar amplifier and the audiophools had no complaints.
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Lotsa air-cooled tube gear in old TV stations and video gear. A tube runs much hotter than the ambient by design, so cooling it by 50 degrees doesn't really extend its life much, and can shift its operating point. But air circulation has a huge effect on the reliability of everything around the jars. The partial vacuum insulation is overpowered by a ton of radiation, as anyone who has latched onto a 6L6 can attest.

    ak
     
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  5. #12

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    The reason I installed some slow, quiet, fans was to protect nearby components. A little bit of air movement can do a lot of good. I cooled a filter cap by about 50F with a 4 CFM fan. I just wondered if the extra cooling had much effect on the power dissipation limit of the plates in the tubes. It's amazing how hot the glass gets, because it's transparent to light, but apparently not at all transparent to the infrared! And, yes, I have coated a couple of octal tubes with a thin layer of human skin. :p
     
  6. Ancel UnfetteredOne

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    Jul 3, 2015
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    I love reading about electronics history. I recall my first glimpses of glowing tubes thru the air holes in the hardboard backing of our 1960's B&W Westinghouse TV.

    100 years ago, great things were happening in the world of thermionics.

    http://www.eta-i.org/History of Electronics electricity.pdf
     
  7. Lestraveled

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    May 19, 2014
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    When I was a kid, I was always looking through my father's ham radio stuff. One day I found some things that looked like tubes. They were marked 6L6GC but the shell was made out of metal with an odd flange at the base. I asked my father about them and he told me that they were indeed 6L6GCs with a metal shell over the glass and that they could be mounted upside down in oil and you could get double the power out of them. He was going to build a big ham RF amplifier out of them. I just googled "6L6 metal mount in oil" and sure enough, my father wasn't blowing smoke.
     
  8. Ancel UnfetteredOne

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    Jul 3, 2015
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    At the risk of derailing the thread,i apologize in advance.

    As Les mentioned radio...I got started at age 12 after I bought this for a quarter:
    http://www.mds975.co.uk/Content/george_dobbs_trf_radio.html

    I built it by age 13 from pocket money and scrounging with zero help from anyone. Spent hours listening to it and tweaking it.
    It changed my life.

    The butterfly effect it would seem.

    There's a whole bunch of folks worldwide building these as replicas now:
    http://www.mds975.co.uk/Content/trfradios02.html
     
  9. crutschow

    Expert

    Mar 14, 2008
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    That's the greenhouse effect of course.
    Standard glass is transparent up to about 2.5 microns wavelength which corresponds to a peak blackbody temperature of about 700°C. Thus any longer radiation wavelengths from below the peak of that curve or from surfaces below that temperature are blocked by the glass and either reflected back or absorbed by the glass.
     
  10. alfacliff

    Well-Known Member

    Dec 13, 2013
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    yes, air cooling the glass will help tube life and allows increased power dissapation. also, the wseals where the leads go through the glass get a lot of stress with heat. the only way to get heat out is either through the glass, or conduction through the seals by metal. overheating the seals causes cracking, which has a bad effect on the vacuum. and the metal tubes upside down in oil was a way to increase the dissapation conciderably. getting rid of the heat by radiation heating the glass, then conduction coling the glass in oil is much better than just glass to air. just as real large tubes being water cooled. and by the way, the water had to be very pure to not conduct from the plate to ground on some of those tubes there was around 45 kv on the plate. look up the wiki on WLW sometime for pictures of tubes rated at over 100kw each output.
     
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