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HOW DO TUBES NORMALLY GO?

Discussion in 'General CB Services Discussion' started by Sonar, Oct 11, 2017.

  1. Sonar

    Sonar
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    I'm referring to amplifier tubes. I'm sure it works the same way whether the amplifier is the best legally produced amplifier or your readily available 1970s mid 80s 6LF6, 6LQ6 ect ect driver/final tube type.
    I consistently hear qso's that go something like this "I'm running an amplifier that has one 8950 driving three 8950's, and I'm getting 600 watts out of it".
    I'm assuming there's a few reasons why this person is getting that type of output or is believing he's getting it out of four 8950's.
    The operator is driving the amp properly with the recommended drive wattage (or close), and has a very very very very loose/friendly watt meter.
    The other option is that the operator is driving it with more than the recommended output.
    If the latter is the case how would the tubes go?
    Would the tube go up like a firecracker?
    Or would they just quickly stop working?
    If they stopped working because of overdriving, how does that exactly go? Could there be a bang, followed by smoke? Similar to a capacitor? Or will it slowly/quickly drop in output power until there's nothing left?
    I've never had nor heard of that happening, and I am curious
    .
    There was a time I myself thought ever watt made for a stronger signal on the other end. I was 5 years old, and belived in the tooth fairy also.. I get and believe it to be word that 4x one's current output will get a 1 s-unit gain on the other end.
    Even as a newbie I have never over driven an amplifier.
    I never owned rigs that had more than 4 Watts DK or ever thought of using a smaller amp to drive another larger amp. I've seen YouTube clips of people driving 4 watt 6-10 tube recommend drive amps with 50&100 watts. And I'm not talking about the amps that have an option low/hi drive switch. If there's a single thing I've learned it's how to properly drive an amplifier.
    So. Back to my questions. Can tubes actually exsplode? Or do they just get soft until whatever it is (gas?) that makes them work disepates? PS I've given up trying to exsplain about a particular tubes rated output.
    Those every watt types either don't care or believe differently, and refuse to listen.

    You'd think the rediculous cost of the most commonly used sweep tubes would be enough to have one happy getting within 10 watts of a particular tubes rated output/dissapation. Go figure! 73


     

  2. Tallman

    Tallman
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    When a tube dies some times it just a whimper, or it can be a big showy arcs and sparks show. Some times HIGH VOLTAGE smoke gets out and then you smell fried fish burning. At least you hope it's fish burning.

    There are two ways to make radio power, VERY HIGH VOLTAGE and low current, or low voltage and VERY HIGH CURRENT.
     
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  3. Road Squawker

    Road Squawker
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    Heck, I usually just melt the solder outta the sockets.
     
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  4. 543_Dallas

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    I guess fast or slow depends on who you ask. I know a guy that burns through a set of sweep tubes every year or two. He really thinks he's getting his money out of them and has bragged about how long they last. o_O
     
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  5. Captain Kilowatt

    Captain Kilowatt
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    Usually tubes go slowly over time as the emissions go down.......that is the available electrons emitted from the cathode are no longer available. In this case you will have normal power output for a long time and then slowly see a decrease in power. Sometimes you will see the power meter swing up high and slowly drop a bit when the tubes start to get weak. This assumes normal operation. If you drive the piss out of them you may see the plates glow orange or even red hot and this usually leaves a brownish residue on the glass envelope from electron bombardment. If run this way the tube elements can get soft from heat and actually bend a bit. It does not take much movement to short out a tube and then you have fireworks happen. FWIW I once had an 833C tube fail from loss of vacuum caused by extreme heat. It was in a 1 Kw broadcast TX and I just forget what caused the tube to heat up so badly that the glass actually softened and there was a pinhole in the glass envelope pointing INSIDE the tube. As the glass softened from the extreme heat given off from the graphite plate element the vacumm caused the glass to suck inwards and fail. When the vacuum was lost the filament and grid pretty much vaporised and coated the inside of the glass envelope making a mirror finish on it. Pretty neat to see actually but a bit disconcerting when you walk into a transmitter site and see it sitting there in the socket with grossly distorted audio on the air. LOL
     
  6. Road Squawker

    Road Squawker
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    Thats' pretty much the process I used when I operated the "glass room" at General Dynamics.We hand made Mil Spec tubes... BIG, expensive tubes.

    To coat the inside of the CRT, we used an Al ingot about the size of your thumb and used a machine called an "Aluminzor", it ran on 24 Kv that "floated" on top of 75 kV.

    When the voltage was passed thru the Al ingot, it just vaporized and evenly coated the negatively charged phosphor on the tube interior.
     
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  7. nomadradio

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    Tubes go weak, and just won't deliver current any more. The slang term for this is a "soft" tube. Age will do this sooner or later. Running the tube too hard can speed it up.

    Elements get damaged from heat. Sometimes vaporizing slowly, leaving tiny "mirror" spots that are NOT the shiny getter, but instead seen only on the glass opposite holes in the anode. A 3-500Z that has been mistreated with excessive grid current may acquire that "mirror" surface inside, usually along the bottom of the glass.

    The glass can fail. Air gets in and those shiny "getter" patches on the inside of the glass will turn white and fluffy.

    The glass can soften from excessive anode temperature and "dimple" inwards. Got some pics of that somewhere.

    Solder can melt and drip out of the hollow-brass outer pin on 3-500Z and other similar tubes. Won't happen until you overheat the tube. Kinda like in my avatar.

    [​IMG]

    A tube can become "gassy", with the vacuum just slightly contaminated with gas. Tends to cause a purple glow from inside, between the elements. Not to be confused with a BLUE glow that can appear on the surface of the glass itself. That is caused by contamination of the glass recipe with phosphorus compounds Doesn't indicate a fault when you see it.

    The white-oxide coating on the cathode surface of an indirectly-heated tube can flake off and stick to the grid wires. This produces what a tester will show as a "gassy" tube, but it's really being caused by grid contamination with that oxide.

    Elements can come loose inside, touch each other and cause short-circuit faults. A loose element with a broken spot weld that isn't touching another one can become an accidental microphone. Tubes meant for use with tiny audio signals, like in the phono preamp of a stereo amplifier, or guitar amplifier are sold as "low microphonic", with structures inside designed to be rigid, and not vibrate. A microphonic audio tube that is too close to the loudspeaker can pick up accoustic feedback from the speaker and "howl" if the volume is turned too high.

    The insulation between the heater and cathode of an indirectly-heated tube can break down. The "leakage" test on a tube tester checks for this fault. When it happens, a 60-Hz "hum" gets added to the signal the tube is amplifying. Easy to tell, since "hum" from bad filter capacitors is nearly always 120 Hz, not 60.

    And last but not least, the filament can fail to light up. This accounts for less than one of every hundred tubes I have replaced over the last four or more decades. Happens, but not all that often. A customer who advises that all the tubes in his device are good because they all "light up" gets told that this alone gives any tube a one-in-a-hundred chance of being good, on that fact alone.

    73
     
    #7 nomadradio, Oct 11, 2017
    Last edited: Oct 11, 2017
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  8. Sonar

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    If 1-2 year's of life out of a sweep tube has someone happy, and bragging about it as if it's a good thing I'd have to assume that person is either clueless or needs to begin looking for a new supplier.
    Hell the nine original d&a stamped 6lq6's in my triple stage phantom still show 650 watts with 1/2 dk, and 15 pep.
    And their 35+ years old.
    I don't think getting one or two years out of any type of tube is something to be happy about.
    Those tubes he's happy about are either no good from the get-go or they're having the snot driven out of them. That's just my guess. 73
     
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  9. Sonar

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    What type of heat is needed to actually soften glass?
    I'm assuming it's enough to quickly fry a steak.
    Or Worse, have someone looking like Fire Marshall Bill. (Or was it Bob?)
     
  10. Captain Kilowatt

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    No idea but generally tube glass is Pyrex type and good for WELL over a thousand degrees and probably closer to two thousand. The tube I had that failed did noit actually melt the glass but rather had the glass soften to the point that the vacuum sucked it inwards and created a small hole.
     
  11. nomadradio

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    The more I think about it, the more kinds of failure come to mind that I forgot to include.

    Holes melted in the anode, just for starters. This was a 4-400 tube that got a little too hot.

    The 811A is famous for growing a dime-sized hole right in the center of its anode structure's flat face.

    [​IMG]

    I still can't find the other pic that I remember having. It shows what I call the "blown fuse" failure. If you look at the cathode pin at the base of a sweep tube, and follow that pin up into the tube, you will see a tiny strip of sheet metal spot-welded to it. The other end of this thin strip of (I think) aluminum is the cathode of the tube. More than once we have seen 8950 and 6LF6 tubes with a tiny gap in this thin strip, and a tiny ball at each side of the gap. Looks just like a blown element in a glass fuse. Naturally, a tube with an open circuit between the tube's cathode and the pin on the base is dead, Jim.

    Got a decent pic of this around 15 years ago. I'll track it down, but not today.

    73
     
  12. 543_Dallas

    543_Dallas
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    He hits a 12 tube phantom with a 100+ watt driver he calls a modulator and a trashed out radio. You should see it on a panadapter.
     
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  13. 357

    357
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    sweep tubes its usually quick with the cathode strap going pop or slow over the years.
    Or if you dont burn off gas in tubes that have been sitting a long time, fast with a cool light show
     
  14. Mustang 131

    Mustang 131
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    I had a 3000 start smoking once, but still put out full power. Just popped in a new one and sent the burnt one to Enconco.
     
  15. Road Squawker

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    Correct, the melting temp, where glass can be "molded" is lower than the temp where it actually "liquifies" and can be poured like molten steel.

    It also varies with the Pb (lead) content percentage.
    the less lead, the higer the melting temp.

    The 100% lead free glass We used had a melting temp of 1600 C (2900 F).
     
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