Keit, please stop misquoting me, this is the second time, next I will complain with moderators. 🙄
Keit, I say it with all due respect, you have no idea about elementary Electrodynamics.
Let's suppose an external electric field D = ε E , the cathode interface defines two mediums, 1 (vacuum) and 2 (cathode), at the interface
(D1 – D2) . n = ε1 E1 – ε2 E2 = 4 π σ
Hence
E2 = ( E1 – 4 π σ ) / ε2
Do you hear me now?
What happened between post # 113 and post # 135? 😕
Ah, silly me, your little gopher turned the switch on.
I thought you have a revolutionary way to make your valves work without any applied voltage between anode and cathode.
Except for some particular cases, e.g. shielding, electric and magnetic fields have infinite range.
Where does it come from your field definition?
Your little gopher perhaps? 😀
Your little gopher is advising you wrong.
Keit, I say it with all due respect, you have no idea about elementary Electrodynamics.
Let's suppose an external electric field D = ε E , the cathode interface defines two mediums, 1 (vacuum) and 2 (cathode), at the interface
(D1 – D2) . n = ε1 E1 – ε2 E2 = 4 π σ
Hence
E2 = ( E1 – 4 π σ ) / ε2
Do you hear me now?
What happened between post # 113 and post # 135? 😕
Ah, silly me, your little gopher turned the switch on.
I thought you have a revolutionary way to make your valves work without any applied voltage between anode and cathode.
Except for some particular cases, e.g. shielding, electric and magnetic fields have infinite range.
Where does it come from your field definition?
Your little gopher perhaps? 😀
Your little gopher is advising you wrong.
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The more I think about this, the more I'm swayed by your Schottky argument. I asked Morgan for a couple more measurements to pin down the geometry so I can get a reasonable estimate for the field at the grid. As a chemist, I think of this as an electrodynamic version of the Le Chatelier principle- the field drives away thermally emitted electrons (there's not a high enough density for a true space charge), which drives further emission.
When I did the calculation for the electric field near the cathode, with experimental evidence, I almost fell backwards, then I checked my calculation several times.
Good point, as usual you make me think a lot.
I was thinking on grid temperature vs cathode impurities temperature and diffusion, but I need to study chemistry, or more condensed matter physics to have an answer.
What do you think about it?
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The ironic thing is that it's easy these days to get a better and more detailed picture of what goes on at the surface of tube electrodes, but of course, there's no compelling commercial reason to do so. I'll play around a bit with some calculations before I start throwing out opinions, but you definitely made me think a lot differently about the problem. I don't know if it's right or wrong, but it's sure plausible.
Hermann/Wagener publishes evaporation/sublimation rates versus temperature for BaO and for Ba metal, in units of ug per cm2per hour. Since this is physically verifiable simply by measuring mass, there's no reason to believe such published figures would be wrong. Although BaO evaporates/sublimates faster than Ba from an activated cathode, seems reasonable that arising vapour comprises much of the original composition of the activated cathode. In any event, it is deposits of such material with low work-function that is at issue when it accumulates on the grid.
It seems that Ba/BaO layer over a gold substrate was a work function of c 1.7-1.9eV, this doesn't shed much light.........
In solid lattices, atoms/molecules aren't free to 'migrate', well not very often very far. I don't think there's any 'of course' about it ! You mean BaO dissociates within the solid lattice and sublimated vapour comprises Ba++ and O-- ions ? Reference ? Why then does a BaO oxide cathode need activation by way of redox ?
Le Châtelier's principle
SY
That must be a clever thought, Le Châtelier's principle, I was thinking the very thought. Perhaps with a different twist.
Speaking of space charge, not so much being saturated or not but fluctuating in a dynamic equilibrium with electrons added by the cathode, electrons being taken away by the anode and the effect of the grid.
Le Châtelier's principle, is the Schottky effect influencing your thinking or are you just open minded?
DT
SY
That must be a clever thought, Le Châtelier's principle, I was thinking the very thought. Perhaps with a different twist.
Speaking of space charge, not so much being saturated or not but fluctuating in a dynamic equilibrium with electrons added by the cathode, electrons being taken away by the anode and the effect of the grid.
Le Châtelier's principle, is the Schottky effect influencing your thinking or are you just open minded?
DT
Hi,
@MelB : Yes, that's correct.
The question however was why do this on current production types that didn't come with such grids before (NOS types etc).
The reason is simply to reduce noise levels due to secondary emission of the grid.
Not sure it was actually posted in this particular thread but SY also mentioned the JJ 12AT7 (In the tube regulator thread I think) which according to his measurements produced 6dB less distortion than the NOS and others he tested.
Why? Again, quite simply because a frame grid type of construction is being used which allows for this when properly executed.
That, combined with a fairly high gm may well explain the popularity of the 6DJ8 in most US designed preamps.
In Europe we had frame grid ECC83 variants which were mostly used in pro gear.
So, there you go.........
Cheers, 😉
@MelB : Yes, that's correct.
The question however was why do this on current production types that didn't come with such grids before (NOS types etc).
The reason is simply to reduce noise levels due to secondary emission of the grid.
Not sure it was actually posted in this particular thread but SY also mentioned the JJ 12AT7 (In the tube regulator thread I think) which according to his measurements produced 6dB less distortion than the NOS and others he tested.
Why? Again, quite simply because a frame grid type of construction is being used which allows for this when properly executed.
That, combined with a fairly high gm may well explain the popularity of the 6DJ8 in most US designed preamps.
In Europe we had frame grid ECC83 variants which were mostly used in pro gear.
So, there you go.........
Cheers, 😉
Go on then. Complain now if you like.
Might be problematic for you. First, you'll have to identify just where you've been mis-quoted, and in what way.
chemical nonreactivity?
oxide coating is quite aggressive mixture of barium,calcium,strontium oxides .
there is reason why cathode is greased with wet paste of these metals - carbonates. It is non reactive form.
during vacuum, cathode is heated, conversion begins to oxides. at same time with vacuum pump on.
Any tube manufacturer guys here to confirm-reject?
Gold is indeed chemically nonreactive, but that doesn't seem to be the driving factor here- platinum is more reactive, silver is very reactive, but people have also used both of those metals to suppress grid emission.
one idea with water bucket- it can boil but on lower temperatures, if you have good vacuum. 🙂
(reducing melting-boiling point)
how that is plausible to gold and oxides
(reducing melting-boiling point)
how that is plausible to gold and oxides
Handbook of Thermionic Properties: Electronic Work Functions and Richardson Constants of Elements and Compounds, Vadim S. Fomenko, 1966 sets out many and various measured work functions for Oxides over various metal substrates.
BaO (activated) over a gold substrate has a work function of c 1.8eV, Richardson constant isn't stated. Compared with other metal substrates listed, this is unremarkable, which occur is a range approximately of 1.5 - 2.0 eV.
Using this work function value in the Richardson equation seems to yield results as to emission which are in 'the zone' as to critical dependence on plausible grid temperatures. Perhaps what appears to be a small change in work function versus metal substrate does critically affect emission, due to the exponential nature of the Richardson equation?
Setting aside any modification due to Schottky effect for a moment.....
BaO (activated) over a gold substrate has a work function of c 1.8eV, Richardson constant isn't stated. Compared with other metal substrates listed, this is unremarkable, which occur is a range approximately of 1.5 - 2.0 eV.
Using this work function value in the Richardson equation seems to yield results as to emission which are in 'the zone' as to critical dependence on plausible grid temperatures. Perhaps what appears to be a small change in work function versus metal substrate does critically affect emission, due to the exponential nature of the Richardson equation?
Setting aside any modification due to Schottky effect for a moment.....
I suspect not. EN really relates more to isolated atoms- in a crystal, the electronic structure is quite different, as I know you know. I think that the band structure might be more relevant.
Substances that have melting/sublimation points and boiling points that change noticeably with pressure are the substances that have a high vapour pressure.
Any given substance has a vapour pressure. If a quantity (solid or liquid) is contained within a vacuum-tight rigid flask containing nothing else but vacuum, part of the substance will sublimate or evaporate until the pressur of the evaporated gas within the flask is equal to the vapour pressure. Upon pressure equalisation, molecules/atoms are continually exchanged between the solid or liquaid fraction and the gas fraction.
If the pressure in the flask is raised above the substance vapour pressure, ay by pumping in more material, or by compressing the flask, all of the substance will be sold or liquid.
At a presure, generally lower than atmospheric, there is a triple point, whre the substance coesists in sold, liquid, and gas phases. For water, the tripple point temperature is 0.15 K below the atmospheric melting pint.
All of the metals used in vacuum tubes have extremely low vapour pressures - otherwise you could not have a vacuum tube. Since vapor pressures are extemely low, melting and sublimation temperatures are not significantly affected.
Vapour pressure depends sharply on temperature, as modelled by the Antoine Equation.
For gold, the melting point at atmospheric pressure is 1337K. Standard tables do not quote a triple point or melting point for gold in vacuum, because they are negligibly different from the atmostpheric melting point. It's vapour pressure at 300 K (~room temperature) is 6.80 x 10^-53 Pa. Air at sea level is about 1 x 10^5 Pa, so you can see why presure makes no sensible difference.
(Note 1 for nitpickers: When looking up Antione Equation constants for estimating vapour pressure, be sure to look up the correct constants for off-liquid or off-solid as applicable. Where only one set of figures is given, and the phase unstated, it is invariably the off-liquid values.)
(Note 2 for nitpickers: The antoine equation accuracy various from very good (1% or better) to poor (~20%) depending on the substance. More accurate equations are available for many substances for those who need it.)
Gold is used to coated grids because it offers a high diffusion rate to Ba atoms, so that the Ba atoms do not stay on the surface and emit electrons. Puuting it another way, Ba dissolves readily in gold. Other metals that can be used to make grids offer very low diffusion rates.
This was covered quite early in the thread and backed up by refrences in later posts:-
Beck A HW, Thermionic Valves: Their Theory and Design 1953; [ pp 322] posted by Rod Coleman post #57: "..the barium evaporated from the cathode diffuses into the gold instead of remaining on the surface."
-and, in my post 92#-
A Manual of Materials for Microwave Tubes, Thornburg D L, Thrall E S, & Brous J, RCA 1961, which says on page 63: "Gold internal surfaces are preferred for tubes with oxide or matrix cathodes; evaporated barium will diffuse away from the surface into the metal".
Unfortunately the thread has been clogged up by others trying to insist it is about work function of gold, or its electronegativity, or just trying to push their own barrow on irrelavencies such as schottky effect.
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It could be that gold is slick a shiny where nothing sticks.
Anyone have access to NMR technology to test the grid of a new vs a tube with 1500 hours Use?
DT
Anyone have access to NMR technology to test the grid of a new vs a tube with 1500 hours Use?
DT
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