That's an amusing story, but to pin you down, are you saying that the temperature of the anode swings above and below the temperature of the cathode to make these variations in current happen or not? Yes or no.
It's my contention that your everyday basic signal amplifying tube sits there with constant temperatures for all of it's internal parts. At the very least the time constant of heating and cooling on any part is drastically slower than any audio frequency. The temperature of any part is not changing, but the current through the tube is. How could that be? Could it be voltage? Why is the current through the tube changing to follow the grid voltage without a "temperature gradient" change? Answer that or be proven a troll by resorting to irrelevant stories.
It's my contention that your everyday basic signal amplifying tube sits there with constant temperatures for all of it's internal parts. At the very least the time constant of heating and cooling on any part is drastically slower than any audio frequency. The temperature of any part is not changing, but the current through the tube is. How could that be? Could it be voltage? Why is the current through the tube changing to follow the grid voltage without a "temperature gradient" change? Answer that or be proven a troll by resorting to irrelevant stories.
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I agree with Wakibaki
We are jumping on the guy because he is thinking in a fundamental way we are not. Ok, maybe he hasn't articulated it to our liking either, but he is correct, as is everyone else.
To work as a diode correctly, there must be a heated cathode, or at least a cathode that has been primed somehow to emit electrons. Then and only then will a diode conduct in only one direction (mostly) in a usable fashion.
If there were no heater, the tube could still conduct, but it wouldn't be a "valve". I could conduct in either direction with enough potential to overcome the vacuum gap. But what makes a "valve" special after all? It conducts better in one direction and resists conduction in the other.
Wakibaki's point is well taken by this forum member, so I apologize to him and thank him for encouraging more thought on an interesting fundamental matter.
We are jumping on the guy because he is thinking in a fundamental way we are not. Ok, maybe he hasn't articulated it to our liking either, but he is correct, as is everyone else.
To work as a diode correctly, there must be a heated cathode, or at least a cathode that has been primed somehow to emit electrons. Then and only then will a diode conduct in only one direction (mostly) in a usable fashion.
If there were no heater, the tube could still conduct, but it wouldn't be a "valve". I could conduct in either direction with enough potential to overcome the vacuum gap. But what makes a "valve" special after all? It conducts better in one direction and resists conduction in the other.
Wakibaki's point is well taken by this forum member, so I apologize to him and thank him for encouraging more thought on an interesting fundamental matter.
He realised that thermionic emission is one way of overcoming the work function (the easiest one, it would seem); something that anybody posting in this thread very well aware of. Great.
But then he also claims that temperature difference is the sole reason why vacuum tubes operate, that there is no other difference between the elements apart from temperature (check posts 3, 7, 17 etc. where he amde those claims), and that everybody who thinks there is something else at work that affects the current flow from one electrode to another once the working function was overcome (say potential difference between said electrodes) has the wrong grasp of things ?! Not so great.
Precisely. Yet there was an attempt to ridicule some members for pointing this out as early as page 1 of this thread.
Trolling or not, that's for forum moderators to decide. Misrepresenting how things work by omitting half of the story for some reason unbeknowst to those of us who have tried to point out the missing half, which might lead to somebody coming across his posts at a later time to either take what was written at face value, or - even worse - realize that those statements are incorrect and make a generalization about this entire forum based on them (= implying that all of its contents are dreck).
Will do too and apologies.
Okay the only cooling I may try out is a slow queit fan. The type or degree of cooling doesn't matter, and I'm curious; just giving some idea of what I was thinking of trying.
But anyhow am I right in thinking there'd be no audible change to the sound when a fan is added or taken away ?
The topics, fine, sounds like we're still in the tube and not yet along the chain sitting before the speakers yet. Just tossing a sonics question in, pardon please continue.
Okay the only cooling I may try out is a slow queit fan. The type or degree of cooling doesn't matter, and I'm curious; just giving some idea of what I was thinking of trying.
But anyhow am I right in thinking there'd be no audible change to the sound when a fan is added or taken away ?
The topics, fine, sounds like we're still in the tube and not yet along the chain sitting before the speakers yet. Just tossing a sonics question in, pardon please continue.
No. I just don't think that 6W is moving through that little wire. I'm guessing that they may have invested more money in the manufacture of the tube. Maybe more time baking it to better out-gas the metals so that the plate can run at a higher temperature? I don't know, and I don't have the necessary background to calculate this out. I could easily be wrong. I just am skeptical that that tiny wire is moving an extra six watts out the top cap.
Assuming they were made from the same material, were at the same temperature, and experienced the same electrostatic potential, that's correct.
The cathode coating optimizes the cathode for electron emission. For a directly heated tube it is desirable to have the cathode running as cool as possible as it extends the life of the cathode. The cathode is the filament in a DHT...
I think we fundamentally agree but have vastly different communication styles. Were someone to ask me how a tube works, I would have given an explanation from an electrostatic point of view as this is what controls the current flow and "makes the tube work" (i.e. function as an electronic device). Of course, the electrostatics wouldn't work if the cathode didn't emit electrons -- and the easiest way to make that happen at safe&sane voltages anyway is to heat it up. So I see your point that heat is what makes the tube work. So you can't have a space charge cloud without heat. But you also can't have any amplification or current flow without electrostatic fields.
~Tom
My version is, bulbs are different. However, bigger volume may accept more gases, also form of a bulb helps to cool it down.
Your fan only cools the glass envelope, not the stuff inside. Between the glass and the "stuff" is nature's best insulator, vacuum.
You can find out what would happen if you were to truely cool the whole tube easy enough, just turn off the heater filaments and see. But you fan can't cool the heaters
I understood there were two main differences between these tubes.
1. the top cap allowed for higher plate voltage than the octal socket allowed without breaking down. This was supposed to allow class B operation and help in class AB1/2.
2. the physical tube design was such that less heat was re-radiated back in and less heat was reflected back in to the plate from the envelope, and this is what allowed higher plate dissipation.
I was thinking about wakibaki's assertion that it is the temperature difference that makes a tube function.
Question: Assuming we had hollow leads and could pump some coolant through the anode structure to cool it way down, could we run the cathode at a much lower temperature? Or does the cathode have to cross a certain temperature threshold for the material to emit electrons well?
Question: Assuming we had hollow leads and could pump some coolant through the anode structure to cool it way down, could we run the cathode at a much lower temperature? Or does the cathode have to cross a certain temperature threshold for the material to emit electrons well?
The number of electrons that can be emitted from the cathode per unit of time is determined solely on the absolute temperature of the cathode, the work function of the cathode material, and the active surface area of the cathode.
Note the absence of 'anode', 'temperature difference', or 'relative temperature' in all this. No matter what temperature the anode is at. It's the absolute temperature of the cathode surface that matters.
This is all pretty old and established science. Remember, intuition is just intuition, nothing more.
Kenneth
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ChrisA, I don't think you understood what I was trying to say. I want to create a temperature difference without a hot cathode. If you don't cool the anode in my absurd hypothetical, you don't get a temperature difference.
If what Kenneth is saying is true, then this is indeed more complicated than a simple temperature difference.
Next question: If we choose our plate and cathode materials wisely can we have unidirectional current flow at reasonable voltages if the plate and cathode temperatures are equal?
If what Kenneth is saying is true, then this is indeed more complicated than a simple temperature difference.
Next question: If we choose our plate and cathode materials wisely can we have unidirectional current flow at reasonable voltages if the plate and cathode temperatures are equal?
The absolute temperature of a heated oxide coated cathode is around 1100K - compared to room temperature that's about 4x higher. The dependence of current emission vs. absolute temperature is T^2 * exp(-1/T) so it drops quickly for lower temperatures.
DHT typically run much hotter, so the difference will be even greater then.
DHT typically run much hotter, so the difference will be even greater then.
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The electrodes in a vacuum tube are defined by their temperatures. If you were to build a tube with 2 identical electrodes, both capable of being heated, then when considering the passage of current, if first one were heated alone then it would be the cathode, but if allowed to cool and the other heated, it would become the anode. If both are heated, the device ceases to function in any meaningful sense in this context.
Why it should be that merely obliquely referring to this in passing should excite such great shouting and tumult is a mystery to me.
The fundamental cause which is responsible for the working (when I say works in this context I mean passes current in one direction) of a thermionic diode (all the other vacuum electronics are just modified diodes) is the difference in temperature between the electrodes, IOW that is why they work. If you remove this difference they don't work. That is ALL I have said or implied or intended to suggest or imply.
Granted the absolute temperatures are of consideration, these are dependent on the work function of the material involved, and a secondary consideration in the light of the fact that we are talking about a device which is provided with a heater. Do I have to explain every little thing? All I said was: it needs a cooler too, and in fact it's got one, whether you are aware of it or not.
Remember that when this device was discovered, nobody had ever heard of work function.
All the other reasons quoted such as work function, space charge or geometry are secondary causes, details or enhancements of the basic effect and the suggestions that I said there could be no reverse leakage current or that I am...
'saying that the temperature of the anode swings above and below the temperature of the cathode to make these variations in current happen or not? Yes or no.'
...are inventions.
When current flows through the heater the temperature of the wire rises then equilibriates. The equilibrium point is where the energy flowing into the heater is equal to the energy flowing out. The energy flowing out is radiated or conducted. Ultimately this energy will end up in the outside world. You can see radiated energy escaping through the glass envelope of some valves, particularly directly heated types.
In the case of indirectly heated cathodes the energy produced by the heater can be conducted to a tubular cathode which encloses the whole filament, but it will, in turn, be radiated outwards.
The simplest anode to use with a cylindrical cathode is a surrounding cylindrical anode. Such an anode now becomes the recipient of the heat from the heater, received by conduction and radiated by the cathode. It has the benefit of a larger surface area, but if the anode cannot shed its heat into the environment sufficiently fast, then its rising temperature may compromise its function or the functioning of other structural components.
Valves do not necessarily have the geometry I have described, but you can see that any radiant (or conducted) heat not being usefully employed by the cathode can be counter productive if left sloshing around in there and that in fact the anode will always be the recipient of some unwanted heat, since it always has visibility of the cathode and it is connected to the cathode by the valve structure.
Heat is also generated in the anode by the impact of high-speed electrons.
"The anode dissipates heat by radiation, and in order to maximize radiating area, anodes often have fins" - Morgan Jones, Valve Amplifiers, P276
Obviously they're not for convection...
"A hot envelope implies a hot anode" - Morgan Jones, Valve Amplifiers, P284
The envelope radiates both inward and outward.
I don't recommend grabbing a tube, they can get quite hot. Now some of that heat is conducted, but although temperatures of 1600 degrees are typical of an indirect heater filament the heater is distant to an extent from the pin, the contact patch between the pin and the valve envelope is small, and the temperatures encountered far from the pins suggest that tube glass is not entirely transparent to the IR it encounters, else it would not be being heated by the anode radiation.
Hence; if you cool the envelope, you cool the anode. Thus, envelope temperature is the most significant single predictor of tube life.
The management of heat flow is a critical factor in the design of valves. You may find the description of the situation as a 'temperature gradient' uncomfortable, perhaps because of the discontinuities presented by the vacuum but
1. This neglects the other structure and
2. A curve with discontinuities is still a curve.
w
Why it should be that merely obliquely referring to this in passing should excite such great shouting and tumult is a mystery to me.
The fundamental cause which is responsible for the working (when I say works in this context I mean passes current in one direction) of a thermionic diode (all the other vacuum electronics are just modified diodes) is the difference in temperature between the electrodes, IOW that is why they work. If you remove this difference they don't work. That is ALL I have said or implied or intended to suggest or imply.
Granted the absolute temperatures are of consideration, these are dependent on the work function of the material involved, and a secondary consideration in the light of the fact that we are talking about a device which is provided with a heater. Do I have to explain every little thing? All I said was: it needs a cooler too, and in fact it's got one, whether you are aware of it or not.
Remember that when this device was discovered, nobody had ever heard of work function.
All the other reasons quoted such as work function, space charge or geometry are secondary causes, details or enhancements of the basic effect and the suggestions that I said there could be no reverse leakage current or that I am...
'saying that the temperature of the anode swings above and below the temperature of the cathode to make these variations in current happen or not? Yes or no.'
...are inventions.
When current flows through the heater the temperature of the wire rises then equilibriates. The equilibrium point is where the energy flowing into the heater is equal to the energy flowing out. The energy flowing out is radiated or conducted. Ultimately this energy will end up in the outside world. You can see radiated energy escaping through the glass envelope of some valves, particularly directly heated types.
In the case of indirectly heated cathodes the energy produced by the heater can be conducted to a tubular cathode which encloses the whole filament, but it will, in turn, be radiated outwards.
The simplest anode to use with a cylindrical cathode is a surrounding cylindrical anode. Such an anode now becomes the recipient of the heat from the heater, received by conduction and radiated by the cathode. It has the benefit of a larger surface area, but if the anode cannot shed its heat into the environment sufficiently fast, then its rising temperature may compromise its function or the functioning of other structural components.
Valves do not necessarily have the geometry I have described, but you can see that any radiant (or conducted) heat not being usefully employed by the cathode can be counter productive if left sloshing around in there and that in fact the anode will always be the recipient of some unwanted heat, since it always has visibility of the cathode and it is connected to the cathode by the valve structure.
Heat is also generated in the anode by the impact of high-speed electrons.
"The anode dissipates heat by radiation, and in order to maximize radiating area, anodes often have fins" - Morgan Jones, Valve Amplifiers, P276
Obviously they're not for convection...
"A hot envelope implies a hot anode" - Morgan Jones, Valve Amplifiers, P284
The envelope radiates both inward and outward.
I don't recommend grabbing a tube, they can get quite hot. Now some of that heat is conducted, but although temperatures of 1600 degrees are typical of an indirect heater filament the heater is distant to an extent from the pin, the contact patch between the pin and the valve envelope is small, and the temperatures encountered far from the pins suggest that tube glass is not entirely transparent to the IR it encounters, else it would not be being heated by the anode radiation.
Hence; if you cool the envelope, you cool the anode. Thus, envelope temperature is the most significant single predictor of tube life.
The management of heat flow is a critical factor in the design of valves. You may find the description of the situation as a 'temperature gradient' uncomfortable, perhaps because of the discontinuities presented by the vacuum but
1. This neglects the other structure and
2. A curve with discontinuities is still a curve.
w
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