"Normal" temperature for power tubes?

Discussion in 'Fisher' started by thornev, Dec 23, 2018.

  1. thornev

    thornev Well-Known Member

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    My cat knocked over the magnetic fan that cools my 500-C's four 7591A power tubes. Just for kicks I decided to measure their temperature when not cooled. I use a laser gun. I was shocked that one of them measured 125*C. That's well over boiling. I've got the tubes biased at 34mA. Is my concern warranted at this temperature? What is considered a safe range at which the tubes operate? Is there a circuit adjustment I need to make to lower the temperature? Thorne

    PS - I remember this topic was discussed many months ago, but a search didn't turn up the thread.
     

     

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    At 257˚ F, your tubes are actually somewhat on the cool side. All else equal, the actual installation can have a significant effect on tube temps, with 300˚ F hardly uncommon -- and normal -- for power tubes to operate at.

    Dave
     
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  3. 604man

    604man Well-Known Member

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    Incorrect link and data please disregard.
     
  4. thornev

    thornev Well-Known Member

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    Thanks, Dave G. I do respect that tubes need as much ventilation as possible. My 500-C sits in the wide open air of my office/music studio in its fake wood cabinet with said fan aimed at all 4 power tubes, turning on whenever the 500-C is turned on.
     
  5. Dave451

    Dave451 AK Subscriber Subscriber

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    Yep, output tubes are meant to run hot. I've measured 7591 and EL84 output types running north of 300 degrees F on the glass envelope in typical operation, even if it seems a little scary compared to the world of solid state components. In push-pull AB1 mode, even at idle (no signal) conditions, at 34 mA cathode current your 7591 tube plates are dissipating about 13 watts apiece and some of the stock Fisher gear ran the tubes much hotter that this. That's like having a 60W light bulb under the hood without even considering heat from the screens and other tubes and components, like power resistors.

    Tubes lose most of their heat by radiant heat transfer from the plates through the glass envelopes, so most of the heat will warm up surrounding components and the air around them. As you say, air movement is how everything keeps cool enough to avoid damage to the components. Most often I've read that it's best for the tubes to draw air away from the them instead of blowing air on them to avoid thermal stress (when one side is cooler than the other). From what I've seen, most tube gear has worked well for decades without any forced air flow unless it's housed in a tight cabinet (like some consoles). That's because Fisher, Scott and others cut holes in the chassis (and the individual wooden cabinets), allowed for space around the output tubes, etc. to produce natural convection for sufficient air flow. I suppose, though, that added air flow might extend the life of tubes and components over time and certainly, if space is tight around components, a fan will help.

    The only applications where I've seen designed-in forced air flow consistently applied is around transmitting tubes where operating conditions require constant cooling (even water cooling!) to keep things from melting down.
     
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  6. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Here's what you need Thorne. WLW AM has a 1927 RCA (Westinghouse and GE contracted for the RF section, and control section) transmitter with Water cooled transmitter tubes. http://j-hawkins.com/wlw.shtml Open door view of water-cooled transmitting tubes in one of the chambers. These tubes are 5 feet high. The bottom of the tubes are metal sheaths, through which the cooling water was run and then pumped outdoors to fountains in a cooling pond in front of the plant.
    WLW TRANSMITTING TUBE.jpg

    A little hard to test on yer tube tester. LOL!
     

     

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  7. 4-2-7

    4-2-7 Smart Ass Sponsor Subscriber

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    Do you think people pointed fans at all their tube electronics back in the day? Everything was run on tubes from table top and floor standing radios, TVs, Portable phonographs as well as consoles. And the stereo components we now buy today, they were all stuffed in boxes with backs on them and a few holes for heat to evacuate. And I assume you don't turn the volume all the way up and leave all of 35 watts running for hours.

    The fan is a solution for a issue you don't have.
     
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  8. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Thorne. Concur with the above opinions. Natural convection will dissipate the normal heat of the tubes(output tubes DO RUN NORMALLY @ 300*F or more), transformers, etc. Absolutely no need for Fans. The only unit I put a fan near WAS my Sansui 1000A when it was running EH7591's and had the POWER Resistors (6K30W x2) that ran as hot as the tubes btwn the tubes and the P.T. Once I replaced the resistors with 50w Aluminum finned case units and moved them, and swapped tothe smaller bottle Tung-Sols, the temps came down appreciably. Now it won't boil water on the top. I could literally fry eggs on the metal case before. Now it just gets very warm(and it's now evenly distributed). If you feel the need to fan it, put a 6" oscillator fan off to the side of it and have it direct air about 12" ABOVE the 500c. Otherwise it's not needed.
     
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  9. 4-2-7

    4-2-7 Smart Ass Sponsor Subscriber

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    Operative word here is normal, if the equipment needed a fan for normal operation it would have been built with one.
     
  10. 4-2-7

    4-2-7 Smart Ass Sponsor Subscriber

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    There was a guy using a temp gun to deterring a bad tube in the mac forum on a new MC275 Mk VI. He insisted he had a bad kt88 because it was running cooler than the others. He said the amp sounded fine but the tube was bad or very weak because it's cooler.

    I tried to tell him if the tube was bad the amp would shut down. Mac has a sentry monitoring that will shut the amp off so it doesn't get harmed with a bad power tube. He wouldn't listen, so maybe you got some fake news off the net. Test your tubes with a tube tester not a temp gun.
     
  11. 1rebmem

    1rebmem AK Subscriber Subscriber

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    And consider vacuum tube automotive radio's!
    Those suckers must have run pretty hot stuffed up under the dash in the summer with no AC .
     

     

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  12. thornev

    thornev Well-Known Member

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    OK, everyone, I get it. I just figured the tubes would last longer if they ran a little cooler. Is that illogical thinking? Or do they run more efficiently at non-cooled temperatures?

    A bit off topic... I just replaced V12-V15 (phase inverters and preamp tubes) with used Telefunken smooth plates and wow - more volume, more full sound and whereas I had slightly imbalanced volume in L and R, now the volume is balanced. Before I had new Sovtek 12AX7LPS and new Gold Lions. Go figure. I guess it is also true that old receivers were built with old tubes in mind? Haha. Thorne

    Larry - GREAT picture ! The tubes look like they are sitting on modified toilet bowls !!

    PS - It turned out that the old Telefunkens brought mA down to 31. I re-biased them to 34mA.
     
    Last edited: Dec 24, 2018
  13. sgmlaw

    sgmlaw Well-Known Member

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    They are called indirectly heated cathodes, and all have glowing heaters, for a reason. They are not supposed to run cool.

    As long as you observe the passive ventilation guidelines of the manufacturer, tube life will not be adversely impacted.

    On a 500C, the only time you really need active ventilation is when you mount one vertically in a console cabinet, or perhaps for some in-wall installs.

    Some cooling fans put a lot of noise on the mains, the same mains your nearby audio power supplies then have to drink from.

    If you really want to keep them cool, keep a reasonable bias. The design bias is a little hot on them.

    I finally retired the outputs in my wife's 800B 'music box' this past year. They were the 56 year old originals.
     
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  14. Dave451

    Dave451 AK Subscriber Subscriber

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    Interesting about the old stock tubes vs. new production. Not to add another wrinkle to your 500C activities, but Dave G held forth very convincingly about how the 'noose' resistor and configuration of the driver/phase inverter in the 400 made life difficult for otherwise good new production Russian tubes, while the older types weren't troubled by the circuit. As I recall, this may apply to the 500C as well and I know you've been doing some of the mods. Always another thing to think about! Merry Christmas, Thorne!
     
  15. thornev

    thornev Well-Known Member

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    Thanks Dave451... And a Happy Holidays to you... and everyone else on the Fisher forum ! It's been a wonderful place to hang. Thorne

    It seems there is enough convincing evidence that I don't need any fans on the power tubes. They did work just fine for months without them.

    I did remove the noose resistor months ago.
     
  16. fred soop

    fred soop Super Member

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    Years ago, when I did not or could not spend much for heat sinks, I noted that a 2N3055 (and many other) transistors would be ok with 100˚ C case temperature. So, I dropped it into a glass of water figuring that as long as the water was still liquid, it would be ok.
     

     

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  17. N8Nagel

    N8Nagel AK Subscriber Subscriber

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    Well... For the most part yes. For really high power stuff pushing the envelope, I suspect that sometimes thermal management is on the edge.
     
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  18. Pio1980

    Pio1980 AK Member Subscriber

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    Output tubes "like" open free air convection cooling. Anything that restricts that, including crowded layout, tube cages and dusty dirty tube glass does them no good and should be avoided if possible.
     
  19. thornev

    thornev Well-Known Member

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    Computers require cooling in certain conditions. If one equates them with receivers - they're both comprised of electrical components - it stands to reason that receivers might too. Hey, I just woke up so my logic circuits might not be functioning at normal yet ! Merry Xmas to those who celebrate. Thorne
     
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  20. 604man

    604man Well-Known Member

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    Electrolytic capacitors don't like heat and those above chassis capacitors
    are close to the tubes, a good reason for fan cooling.
     
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