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Old 19th March 2012, 12:11 AM   #41
AJT is offline AJT  Philippines
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Are you saying that the dissipation limit curve is outside the linear zone??????
not a question of linearity, the first rule afaik working with tubes, never operate a tube outside those curves(plate dissipation limits), it is a guaranteed way to kill your tube...
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Old 19th March 2012, 02:42 AM   #42
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Hi Tony
Oh my God !!!, You're right, I'm a fool !!!
Possibly because of what you say curves are called dissipation limit curves !!!
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Old 19th March 2012, 07:23 AM   #43
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the next thing to avoid is exceed plate voltage limits stated on the datasheets, they were put there for a purpose......that is if you want your tubes to live a happy longer life in use.....
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Old 22nd March 2012, 09:14 PM   #44
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Originally Posted by DF96 View Post
I'm not sure if by 'electron cloud' you mean space charge. If so, it was not the reason for inventing the tetrode/pentode. The tetrode gives more gain, especially at higher frequencies, by shielding the potential on the anode from the electron stream.
In a triode, the electron cloud/space charge reduces the number of electrons arriving at the anode and thus the current is reduced, then the gain is also reduced.
If we add another grid, the electron cloud extends to a greater space region, the density of the electron cloud is smaller, then its effects are also reduced, increasing the number of electrons reaching the anode, increasing the current, and increasing the gain, we now have a tetrode.
If we now add another grid, the above reasoning is still valid, then again, we increase the gain, we now have a pentode.

Not all electrons reach the anode are absorbed, some are re-emitted to the cathode, others are retained in the cloud, others are rejected by the cloud.
This is called secondary emission.
The addition of a grid (tetrode) or two grids (pentode) may decrease secondary emission, absorbing the electrons re-emitted, further increasing the total current, and then the gain.

Note that it is very difficult to distinguish in the electron cloud, which were there and which was re-emitted by the anode.
I said it in a more crude in post #9 and #13.I know my English sucks, but so much?
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Old 22nd March 2012, 09:18 PM   #45
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Originally Posted by DF96 View Post
He keeps wanting to reduce space charge, because he seems to think it creates. nonlinearity. I keep pointing out that space charge is a necessary part of normal valve operation. Linearity in normal triode operation is created by the grid and anode having the same 3/2 power law, due to geometry.
Not really at all, as I said in post # 36, the Child-Langmuir law or 3/2 power law derived from Poisson's equation, which in turn, derived from Maxwell's equations.
Poisson's equation can be solved in various coordinate systems, then symmetry (Geometry) changes, but remains a 3/2 power law.
Originally, one of the assumptions to derive the Child-Langmuir law was the electrodes are planar, parallel, equipotential surfaces of infinite dimensions, far from triodes we know, and its geometry.
However, we note that triodes manufacturer, used geometry and physics, to improve them.
The cylindrical symmetry allows quieter sleep.
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Old 22nd March 2012, 09:25 PM   #46
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Originally Posted by DF96 View Post
So your personal opinion that space charge reduces valve linearity in a thermionic triode is based on a photoelectric diode experiment to measure Planck's constant? The former is based on voltage change; the latter on light frequency and intensity.
I am aware of the possibility of language problems, which is why I have given popilin the opportunity to explain his ideas so I can see where he gets his strange idea from. I suspect he is confusing emission (whether thermionic or photoelectric) with total current. In the photodiode all the electrons emitted end up at the anode.
As I said in post#32, since electrons have no conscience of their own, they do not know if they were created by the photoelectric effect, or were created by thermionic effect, then the result of Planck's constant experiment can be applied to any electron, including those of a triode.

Because the electron cloud/space charge, in a vacuum photodiode not all the emitted electrons reach the anode, then you who confuses emission with total current.

Last edited by popilin; 22nd March 2012 at 09:52 PM.
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Old 22nd March 2012, 09:30 PM   #47
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Originally Posted by DF96 View Post
You still don't get it do you? If you deliberately run a valve in the region where the space charge is small then you are approaching the temperature-limited region. In this region the triode is not linear: it stops being a triode because the current depends on cathode temperature rather than electrode voltages. This is how a thermionic noise diode works.

Please go away and read up how triode valves really work. There is plenty of good stuff on the internet. I have tried to correct your misunderstanding but there is a limit to how much free tuition I can give to an obstinate student. Maybe someone else can take over. I have had enough.
As explained in post #28 and #36 I consider demonstrated that the electron cloud/space charge affects the linearity of the triode.
Moreover, the curves speak for themselves!!!

Working with triodes, most of us use the information provided by the manufacturer data sheet and work below the dissipation limit curve, which in most cases is in the linear region, as I said in post #36 and #38.

I still have not finished my thesis, you can tell me I'm just a TV repairman if you want, but your PhD degree gives you no right to insult me that way.

Fortunately I am not your student, and I suggest you study a little bit more physics, lest they repent in your university.

Last edited by popilin; 22nd March 2012 at 09:53 PM.
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Old 22nd March 2012, 10:39 PM   #48
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Originally Posted by popilin View Post
Hi Tony
Oh my God !!!, You're right, I'm a fool !!!
Possibly because of what you say curves are called dissipation limit curves !!!
you see these in some of the plate characteristic curves as dashed lines, but where this is not shown, you can always look at the plate dissipation spec.....

actual practice, the plate dissipation spec is much higher than actual operating point chosen, about 25% is typical for small plates.....
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Old 23rd March 2012, 01:23 PM   #49
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Originally Posted by SY View Post
If you look at how a 12AU7 mu varies with plate voltage, the seemingly nutty idea of putting 320V across it starts making sense. I suspect the actual results won't look much like the sim- even with operation on the flatter part of mu vs Vp, there will be a lot of higher order harmonics. But if you're going to extract the most out of a tube with poor linearity, you have to take extreme measures.
Absolutely true and consistent with the physics of the triode, my knowledge is limited and your views always teach me something.

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Originally Posted by SY View Post
Of course, you could get better performance with a different tube and different circuit and not deal with 650V supplies, but that's not a polite thing to say.
It is also true, but what else I can do with a ECC82/12AU7, sell?

Thank you for your contribution, you are one of the few who supported me.
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Old 23rd March 2012, 03:06 PM   #50
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I disagree strongly with your physical interpretation, but the circuit will work. At your chosen voltages and configuration, you're running roughly 3mA through the tubes with 325V on the plate. At that current and voltage, you're in no danger of over-dissipation, but you're still safely in the space charge region. The thing to watch out for is the heater-to-cathode voltage ratings- you may have to use different tubes for top and bottom and bias up the heater supply for the top tube.

And not to be facetious but, yes, if you're designing a hifi amp (as opposed to a guitar amp), you actually would do better selling the 12AU7s and getting another tube which can be used in a simpler circuit and give higher performance. Your circuit does extract better performance than usual for that tube type, but the 12AU7 is not a very linear tube to start with.
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