Unfortunately Michael, To the average Joe, A turbine works on exactly that principle.
Just like tubes, at 3000F EGT, the turbine fails in short order just like George's tubes.
Fuel burns, heats the combustion air resulting in an increase in pressure. The high pressure, HOT, gasses are expanded across the turbine releasing its energy, that energy is split, some is used to compress fresh combustion air and the rest is used drive propellers etc. In effect the Hot goes to the cold....
It is kinda like a thermionic diode with a built in HV supply..😉
Now when I see a tube with a little whirrly jig spinning in it I am going to find a new hobby.😀
Just like tubes, at 3000F EGT, the turbine fails in short order just like George's tubes.
Fuel burns, heats the combustion air resulting in an increase in pressure. The high pressure, HOT, gasses are expanded across the turbine releasing its energy, that energy is split, some is used to compress fresh combustion air and the rest is used drive propellers etc. In effect the Hot goes to the cold....
It is kinda like a thermionic diode with a built in HV supply..😉
Now when I see a tube with a little whirrly jig spinning in it I am going to find a new hobby.😀
Well unless our friend under the bridge has anything new to add I guess we're off track enough. Everybody here knows how a tube works. Pointing out that it won't work if you don't heat the cathode is not some new insight. It won't work if you remove the vacuum either. Nor will it do anything interesting without changing voltages. In general in electronics if nothing is changing, nothing interesting is happening. Unless you consider staring at an led interesting. The heater stays hot, the vacuum stays vacuumed, but a voltage on a grid controls a current flowing past it and transconductance makes a variety of interesting things possible. In a system with multiple essential parts there is no "one reason" the system works. That turbine or car engine isn't going to work real well without air is it? Is air the whole reason it works, or fuel, or heat? none of the above.
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Thank you, Michael
As for the rest, it's rubbish. A turbine operates because of the difference in energy contained by dry steam at 100 degrees celcius and wet steam at 100 degrees.
w
OK. I tried to edit that last but the time limit had expired.
I didn't set out here to create any offence, and if I have caused any I apologise. I can at times be abrasive, impatient and pedantic but I never sought or imagined that this argument would arise because I used what I thought of as a compact and illuminating expression in an off-the-cuff reply made casually to an enquiry about fans. Which at that time was not receiving a great deal of attention.
Evidently some people think that I was deliberately trying to misrepresent the science underlying the operation of tubes. Others seem to think that I deliberately focussed on something too obvious to mention, and at the same time neglected other features such as the vacuum in order to somehow entrap unwary correspondents into a fruitless and pointless exchange of disinformation or at the very least misdirection.
Neither of these is the case.
It is true that I enjoy a forthright and robust exchange of views; a proposition must be defended. I would never, however, deliberately seek to act against correspondents interests or try to mislead them or waste their time other than momentarily for the sake of humour.
I am sure there is little true disagreement between us as to how tubes work, other than a difference in the way in which we express ourselves, and little point in a further series of provocations.
w.
I didn't set out here to create any offence, and if I have caused any I apologise. I can at times be abrasive, impatient and pedantic but I never sought or imagined that this argument would arise because I used what I thought of as a compact and illuminating expression in an off-the-cuff reply made casually to an enquiry about fans. Which at that time was not receiving a great deal of attention.
Evidently some people think that I was deliberately trying to misrepresent the science underlying the operation of tubes. Others seem to think that I deliberately focussed on something too obvious to mention, and at the same time neglected other features such as the vacuum in order to somehow entrap unwary correspondents into a fruitless and pointless exchange of disinformation or at the very least misdirection.
Neither of these is the case.
It is true that I enjoy a forthright and robust exchange of views; a proposition must be defended. I would never, however, deliberately seek to act against correspondents interests or try to mislead them or waste their time other than momentarily for the sake of humour.
I am sure there is little true disagreement between us as to how tubes work, other than a difference in the way in which we express ourselves, and little point in a further series of provocations.
w.
wakibaki, let's just forget the whole thing. Call it occupational hazard, but I was always taught to express myself in a way that is true to the laws of physics governing the subject at hand. You take a more intuitive angle, taking shortcuts perhaps w.r.t. what science says, and (I admit) that can easily irritate me. No harm done, our disagreement is moot in the circumstances where it matters for audio amplifiers, anyway. I guess we all agree on that 🙂
Kenneth
Kenneth
Hi Tubelab,
The question was concerning thermionic operation of a diode. Your test has proven that a tube will continue to function in forward bias, however it contains no data concerning reverse bias, which is the crux of the argument here.
Tim
The question originally posted was concerning the how the operation of vacuum tubes would be affected by overcooling them. I merely attempted to offer evidence that water cooling or extreme overheating did not materially affect the basic tube operation, and suggest that under more reasonable conditions would not cause any problems at all.
My extreme experiments were done a considerable time ago, and for little other reason than to find out what would happen. In the case of the glowing 6BQ6GA the plate glow was provided by the high current being passed in the forward direction. I did not attempt to measure reverse current since it did not matter to me at the time. This was a working SE amplifier cranking out about 20 watts. Attempting to rewire things for reverse current would have allowed the tube to cool off.
I did discover in a previously discussed experiment that two glowing plates in a 6AS7G can pass considerable current between them in both directions. The source in this case was a variac feeding a 240 volt step up transformer with two 120 volt 100 watt light bulbs in series, wired from plate to plate. The bulbs remained lit and both plates continued to glow after heater and grid voltages were removed. Eventually the "gas" in the tube ionized which increased the current and lowered the voltage across the tube resulting in less plate glow, but more blue glow. I shut down when the glass started to melt. I may repeat this experiment some day, because I lost the cool looking semi melted tube. Video would be cool too.
In another experiment I was exploring some extreme conditions using screen drive on a pair of 6BQ6's. The screen grid's were idling in the 80 volt region with the plates at 550 volts. With extreme drive applied to the screen grids I was extracting over 100 watts from the P-P pair when I noticed that as the screen grid began to glow the current being drawn by the screen grid would go negative. Yes, a glowing grid can become a cathode. I got carried away with the knobs resulting in a meltdown and some blown mosfets.
I will continue to look for creative (and sometimes destructive) ways to extract big power from little tubes, but I have no desire to explore reverse bias current under glowing conditions, unless I have reason to believe that it is the cause for some problem I am chasing. My recent experiments are testing using both G1 and G2 for driven elements since I have found a problem with extreme G2 only drive. There is a method to my madness, I just don't know what it is!
Wakibaki, fair enough. I apologize for calling you a troll then.
Back on the original question of cooling tube envelopes, can anyone think of an example of a tube operating in an extremely cold environment? My first thought was at least a few must have made it onto a satellite but I don't know how you'd go about finding out.
Back on the original question of cooling tube envelopes, can anyone think of an example of a tube operating in an extremely cold environment? My first thought was at least a few must have made it onto a satellite but I don't know how you'd go about finding out.
Wonder if anything turned up with those uses-not saying it indicates anything, more wondering what I'm overlooking, need to consider.
Meanwhile it sounds like there's no temp based 'swing' or change in audio quality within usual tube temp operating ranges (just for my fan question, all other info fine interesting helpful).
I mean if you toss a large speed adjustable fan on an amp, there'd be no noticed change in sound. No dialing in for a certain optimum temp range needed.
(If the amp was speaker cabled from the next room to get past fan noise.)
Though actually I expected, if anything, it'd be minor inaudible-e.g. everyone's not tossing fans on their amps, tweaking them to certain favored temp ranges.
But the discussion helped to understand more and stuff like this helps get me through reading up and learning about things.
Meanwhile it sounds like there's no temp based 'swing' or change in audio quality within usual tube temp operating ranges (just for my fan question, all other info fine interesting helpful).
I mean if you toss a large speed adjustable fan on an amp, there'd be no noticed change in sound. No dialing in for a certain optimum temp range needed.
(If the amp was speaker cabled from the next room to get past fan noise.)
Though actually I expected, if anything, it'd be minor inaudible-e.g. everyone's not tossing fans on their amps, tweaking them to certain favored temp ranges.
But the discussion helped to understand more and stuff like this helps get me through reading up and learning about things.
Thats the method, I'm not sure about the madness though.
Slapping a fan on an amp will have an effect, a big enough effect that I used to install underpowered computer fans in my guitar amps, its just not an audible effect. The effect.....the amps last longer. It's no secret that capacitors (especially electrolytics) have a shorter lifetime at high temperatures. Resistors tend to change value if subjected to repeated temperature extremes. PC boards can become brittle, and switch contacts oxidize. Virtually all electronic components work better and last longer at lower temperatures, except tubes. They generate a considerable amount of heat in normal operation.
I added a small comuter fan designed to run on 12 volts to my guitar amps. I connected it to the rectified filament voltage (about 8 volts) which ran it slow enough so that you couldn't hear it. It sucks air in through the bottom of the chassis and out through holes surrounding the power tubes. This keeps the parts under the chassis cool and helps blow the heat up and out of the box.
In my last amps I use perforated aluminum for chassis. A first, better convection. And second, better acoustic damping.
In the latest one I put transformers under big hot tubes, so tubes work as very quiet fans cooling transformers and PS PCB with heatsinks on it.
You may use this know-how for free. 😉
In the latest one I put transformers under big hot tubes, so tubes work as very quiet fans cooling transformers and PS PCB with heatsinks on it.
You may use this know-how for free. 😉
The term is 'grid blocking' and there is a fascinating graph and paragraph describing negative and positive grid currents as grids go positive on page 21 of Radiotron Design Handbook vol. 4 for anyone who has not heard of this.
Illustration from ARRL handbook showing squirrel-cage fan-cooling of 3CX series transmitting tube with exposed anode in chimney socket.
Quote:-
'One extremely important consideration often overlooked by power-amplifier designers and users alike: a tube's plate dissipation rating is only it's maximum potential capability'
'For tubes with greater plate dissipation ratings, and even for very small tubes operated close to maximum rated dissipation, forced-air cooling with a fan or blower is required.'
Tubes got exposed to plenty cold in airplanes when they were in use, but I think the only consideration there might have been thermal stresses at switch-on or -off leading to premature failure of the gas-tight seal. Fan-driven room-temperature air is certainly not going to be a problem.
w
An externally hosted image should be here but it was not working when we last tested it.
Quote:-
'One extremely important consideration often overlooked by power-amplifier designers and users alike: a tube's plate dissipation rating is only it's maximum potential capability'
'For tubes with greater plate dissipation ratings, and even for very small tubes operated close to maximum rated dissipation, forced-air cooling with a fan or blower is required.'
Tubes got exposed to plenty cold in airplanes when they were in use, but I think the only consideration there might have been thermal stresses at switch-on or -off leading to premature failure of the gas-tight seal. Fan-driven room-temperature air is certainly not going to be a problem.
w
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