"Normal" temperature for power tubes?

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

  1. gadget73

    gadget73 junk junkie Subscriber

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    Just adjust it to 0.34v with the Sylvania. No math needed.

    The Tung-Sol will most likely draw too much current if you do not re-adjust it.

    Honestly you run that risk any time you change tubes, even between sets of modern or sets of vintage.
     
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  2. Pio1980

    Pio1980 AK Member Subscriber

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    I would like to see readily accessable front test and adjustment points on all non-self biased tube amps.
    Wouldn't we all.
     
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  3. N8Nagel

    N8Nagel AK Subscriber Subscriber

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    Once upon a time I took these same measurements on my Pilot 654. I should have written them down so I could post them here, but I did not. ISTR them being hotter than what you've posted, something like 200F for the power tubes, but this is going strictly off memory. This is also with a case installed.
     
  4. Pio1980

    Pio1980 AK Member Subscriber

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    If they are operating at an envelope temperature of less than boiling point, that afaik is exceptional.
     
  5. N8Nagel

    N8Nagel AK Subscriber Subscriber

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    I may be off in my memory. They certainly aren't cool, even with my fingertips being pretty callused from years of working on old cars, I can only touch the tops of the tubes very briefly without badness happening.
     
  6. Pio1980

    Pio1980 AK Member Subscriber

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    I'd expect at least 2nd degree burns with lingering contact. I don't recall what I've measured off the 6CA7s in the Marantz or the 5881s in the now decommissioned Fishwin.
     

     

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  7. gadget73

    gadget73 junk junkie Subscriber

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    Its kind of hard to get accurate numbers on these things. I expect if you used a contact thermometer you'd get pretty different readings compared to the IR.

    and yeah, most tubes will burn the hell out of you with even brief contact. Over about 150F gets uncomfortable quickly for skin contact.
     
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  8. sKiZo

    sKiZo Hates received: 8642 Subscriber

    Got the jacks from Weber, but I also picked up a couple of their copper cap rectifiers. Not sure what the shipping would be, but hey, browse the site - they've got some neat stuff.

    https://www.tedweber.com/catalogsearch/result/?q=PNJ-P-

    OR ... I'm sure there's other sources if you want to do some digging ...

    PS ... another issue is iron temp if the tubes and such are located too close. Rule of thumb there, if a transformer is running too hot to touch for a second or so, you may have a problem, if only having the wire insulation go brittle from the heat, especially as the years pile up. Another advantage to my build, as the iron's exposed and there's additional room between the iron and the tubes. And ya ... goes without saying, you want to be extremely careful moving or bending those wires when doing repairs.
     
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  9. sKiZo

    sKiZo Hates received: 8642 Subscriber

    Did some digging and found this showing the standard temps for a a stock ST70. Note the cap temps on the driver board and the quad cap - that's just from proximity to the tubes. All that good stuff is under the chassis plate on my build.

    [​IMG]

    Test conditions were about the same as when I did mine. Also worth noting, the ST120 tends to run hotter than the ST70 due to higher current draw, so hey ... I done good ...
     
  10. thornev

    thornev Well-Known Member

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    390 ?! I hope that's Fahrenheit !
     
  11. thornev

    thornev Well-Known Member

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    I read that this procedure is the best way to try tubes that have vastly different specs (mainly plate dissipation):

    First install your bias probe on one of the old tubes, and set it’s level to the lowest possible milliamp draw. You may also just take your meter on the DC scale (another reason for a separate meter), and set this to the HIGHEST negative voltage. This means that –52 volts is the more preferred value than –49 volts as an example. Now, you can remove the old tubes with the amp off, install the new ones, and work up to the proper idle dissipation target.

    Makes sense to me. The articles states that "Tossing a set of these E34LS tubes into an amp that is not PROPERLY biased, is asking for trouble, and almost guaranteed. When I say PROPERLY biased, I am not only speaking of the current draw, but the method used to set up the amp initially for a tube change. Just plugging, for example, the E34LS in and “chasing” bias is a mistake. In the time it takes to get the bias in the proper range, you may blow fuses, ruin the tubes, or take many hours off their life".
     
    Last edited: Dec 26, 2018
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  12. gadget73

    gadget73 junk junkie Subscriber

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    thats how I go about changing tubes. Set the adjustments for minimum current, let them warm up, adjust as needed.

    If you have individual adjustments, just be aware that you may need to go back and tweak them after setting things initially.
     
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  13. sgmlaw

    sgmlaw Well-Known Member

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    That’s spot on. Over the years, I’ve recalibrated the bias on the receivers to run an old production output on these a little cooler than the historical norm to extend tube life. And it has, and without any noticable alteration in sonics. And it has gratifyingly turned out over time to have proven ‘just right’ with later foreign production outputs, such that instruments I’ve reconditioned in the 80s should never need further bias adjustments, other than for drifting, aging parts, regardless of output used. Last I checked, I have a handful of late 90s jobs still out there running on the original EHs I sent them out the door with.

    So to the OP, I would have no worries plugging in a properly functioning Sylvania into a bias circuit peaked for new production. It will run an acceptable idle and function ok, if not at optimum, and you should still mainly hear what it can do sonically.
     
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  14. thornev

    thornev Well-Known Member

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    Thanks, sgm. I'm curious about the math you used when recalibrating the bias. Did you just simply reduce the bias so that the tubes run at say 50% of their rated maximum output? Thorne
     
  15. thornev

    thornev Well-Known Member

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    Using a laser gun to measure, one of my four 7591A Tung-Sol power tubes in my 500-C is significantly cooler than the other 3 by 100*F. Should I be concerned? I'll try switching tubes to see if it's the tube or the circuit.
     
  16. Pio1980

    Pio1980 AK Member Subscriber

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    The average temp of the EH 6CA7s in my Marantz 8 seems ca 355 F after playing, biased as specified.
     

     

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  17. baconbadge

    baconbadge New Member

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    Are laser guns really that accurate when they can pass through the glass of the tube, or reflect all over the place? I've got an infrared gun too, but I've never put much creedence in what they read on tubes. Transformers, yes.
     
  18. thornev

    thornev Well-Known Member

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    Maybe not super accurate, but at least I would think they are consistent. And since they do measure the very hot temps of the "bottles", I would say they are close.
     
  19. thornev

    thornev Well-Known Member

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    OK, it's not the tube. It's the socket in which the tube is placed. This must mean something is wrong with the circuit on that socket? Anybody venture a guess as to what? I'll have to do some DMM probing.
     
  20. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    If you have 10Ω cathode resistors installed, first that the the resistor for that socket is good.

    Dave
     
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