boiling off electrons

[John Larkin]

On Wed, 21 Jan 2009 21:57:01 -0800 (PST), RichD
<r_delaney2001@yahoo.com> wrote:

I don't know much about vacuum tubes, but often
I have read statements like "the cathode heats up,
causing electrons to boil off and fly to the anode"..
which makes me scratch my head..

How does an electron boil? Can anyone explain this?

Electrons in a metal or similar are bound by bonding forces. As you
heat things up, some electrons acquire more energy and, if near the
surface, may break away. The energy is called the "work potential",
measured in volts (or electron-volts.) Once free of the surface, any
extra energy past the escape potential gives the electron some
velocity.

Are there degrees of boiling, like a pot of water,
or is there an on/off threshold?

Below some critical temperature, there's insufficient energy to
escape. Above that, electron emission goes up fast as temp rises.

Then there's the grid mask, whatever that is... how
come the electrons don't smash into that? How do
they find the holes?

It's a very sparse grid of wires, so most electrons miss, just drift
through the big holes between the tiny wires. If you apply a high
negative voltage, you can force the electrons further from the wires,
crowd them into the gaps betweem, and eventually shut down the gaps
completely.

Is this like synchronized diving,

they're trained to hit the water at specified spots?

They are random and unsynchronized, although they do repel one
another, which adds a certain sort of order.

John

 

[Tim Wescott]

On Sat, 24 Jan 2009 07:39:45 -0800, John Larkin wrote:

On Thu, 22 Jan 2009 10:19:05 -0800 (PST), Benj <bjacoby@iwaynet.net
wrote:

On Jan 22, 12:57 am, RichD <r_delaney2...@yahoo.com> wrote:
I don't know much about vacuum tubes, but often I have read statements
like "the cathode heats up, causing electrons to boil off and fly to
the anode".. which makes me scratch my head..

Not quite a correct description, because once they "boil off" it is the
electric fields in the tube that cause them to be accelerated and "fly"
to the anode.


Not exactly. Electrons will flow from cathode to anode with zero field,
or even a bit of reverse field. The electrons ate kicked out of the
cathode with a couple ev of energy. There have been thermionic
generators that work that way.

Positive field helps a lot, of course.

John

In fact, one of the methods of biasing small-signal tubes is to drive the
grid through a high (10M-ohm in some cases) grid-leak resistor. This
lets the grid bias itself negative.

It can't be done on just any tube (and no, I don't know enough to know
which ones it can or can't be done on), and if the grid ever gets hot
enough to start emitting electrons on its own it'll go positive and the
tube will run away and (probably) toast itself, so it can't be done _at
all_ on power tubes.

--
http://www.wescottdesign.com

 

[Tim Wescott]

On Wed, 21 Jan 2009 21:57:01 -0800, RichD wrote:

I don't know much about vacuum tubes, but often I have read statements
like "the cathode heats up, causing electrons to boil off and fly to the
anode".. which makes me scratch my head..

How does an electron boil? Can anyone explain this?

Are there degrees of boiling, like a pot of water, or is there an on/off
threshold?

As pointed out, the electrons "boiling" is an analogy of what's really
happening. Do a search on "thermionic emission" and "work function" to
get more of the scoop.

Or go find a tube physics textbook from the late 40's or early 50's, when
they had found out a _lot_ about tubes but hadn't started using
transistors yet. You'll also find older (and, oddly, brand new) ARRL
handbooks helpful -- their explanations are aimed at giving a technician
enough understanding to work with circuits, not giving an engineer the
best understanding possible, so they simplify things to the point of
leaving erroneous impressions, but they're very good for getting you up
and going.

Like this one: http://www.powells.com/partner/30696/biblio/9781882580262

Then there's the grid mask, whatever that is... how come the electrons
don't smash into that? How do they find the holes? Is this like
synchronized diving, they're trained to hit the water at specified
spots?

The electrons don't "find" the holes -- the grid repels the electrons by
being negative, or it attracts the electrons by being positive. If it
attracts electrons, then grid current flows.

Once again, an old tube electronics textbook from "the days", or an old
ARRL handbook will make this clear.

--
http://www.wescottdesign.com

 

[Salmon Egg]

In article <8cdmn4dg24q1lp1he29h6rmnnsar2rbtn4@4ax.com>,
John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

Not exactly. Electrons will flow from cathode to anode with zero
field, or even a bit of reverse field. The electrons ate kicked out of
the cathode with a couple ev of energy. There have been thermionic
generators that work that way.

With the average energy of one electron volt corresponding to 11,600?K
per half degree of freedom, you are not going to get very large currents
of several electron volt electrons.

Bill

--
Private Profit; Public Poop! Avoid collateral windfall!

 

[Salmon Egg]

In article <bidmn45bq27pb5566shjcrcbpg2g4usp8u@4ax.com>,
John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

Then there's the grid mask, whatever that is... how
come the electrons don't smash into that? How do
they find the holes?

It's a very sparse grid of wires, so most electrons miss, just drift
through the big holes between the tiny wires. If you apply a high
negative voltage, you can force the electrons further from the wires,
crowd them into the gaps betweem, and eventually shut down the gaps
completely.

For most receiving tube applications, the control (closest grid to the
cathode) runs with a negative potential with respect to the cathode.
That voltage is a few volts, seldom as high as ten. Minus ten volts
would usually be enough to completely cut of electron flow to the anode.

For power rf amplifiers, there usually is sufficient negative bias on
the control grid to reduce anode current to zero without excitation.
When rf is applied to the grid, the positive portions of the cycle
allows current to flow. Typically, the grid potential is driven positive
through a small portion of the cycle to the extent that there is some
grid current. Anode current flows in pulses. The idea is to use the tube
as a switch to minimize dissipation by trying to keep either the anode
potential or current small. In that respect, transistors do much better.

Bill

--
Private Profit; Public Poop! Avoid collateral windfall!

 

[Salmon Egg]

In article <ToCdnT9rTtO5xObUnZ2dnUVZ_uSdnZ2d@web-ster.com>,
Tim Wescott <tim@seemywebsite.com> wrote:

In fact, one of the methods of biasing small-signal tubes is to drive the
grid through a high (10M-ohm in some cases) grid-leak resistor. This
lets the grid bias itself negative.

It can't be done on just any tube (and no, I don't know enough to know
which ones it can or can't be done on), and if the grid ever gets hot
enough to start emitting electrons on its own it'll go positive and the
tube will run away and (probably) toast itself, so it can't be done _at
all_ on power tubes.

This self-bias will work with almost any tube. Typically, it is used
with triodes. Usually, a capacitor is in parallel with the grid-leak
resistor mentioned above. That allows full rf voltage to be applied to
the grid. This scheme was often used for detectors and oscillators.

Bill

--
Private Profit; Public Poop! Avoid collateral windfall!

 

[Tim Wescott]

On Sat, 24 Jan 2009 11:16:20 -0800, Salmon Egg wrote:

In article <bidmn45bq27pb5566shjcrcbpg2g4usp8u@4ax.com>,
John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

Then there's the grid mask, whatever that is... how come the electrons
don't smash into that? How do they find the holes?

It's a very sparse grid of wires, so most electrons miss, just drift
through the big holes between the tiny wires. If you apply a high
negative voltage, you can force the electrons further from the wires,
crowd them into the gaps betweem, and eventually shut down the gaps
completely.

For most receiving tube applications, the control (closest grid to the
cathode) runs with a negative potential with respect to the cathode.
That voltage is a few volts, seldom as high as ten. Minus ten volts
would usually be enough to completely cut of electron flow to the anode.

For power rf amplifiers, there usually is sufficient negative bias on
the control grid to reduce anode current to zero without excitation.
When rf is applied to the grid, the positive portions of the cycle
allows current to flow. Typically, the grid potential is driven positive
through a small portion of the cycle to the extent that there is some
grid current. Anode current flows in pulses. The idea is to use the tube
as a switch to minimize dissipation by trying to keep either the anode
potential or current small. In that respect, transistors do much better.

Bill

AFAIK, class C transistor amplifiers aren't all that much more efficient
than class C tube stages, because the switching isn't at all crisp. But
tubes have much too much capacitance to make a class E stage, and with
clever design you can make a (transistor) class E stage that has
efficiencies approaching that of a class D (base band switching) stage.

(I haven't actually tried a class E stage, but I know it's been done.
One day...)

--
http://www.wescottdesign.com

 

[Benj]

On Jan 24, 1:08 pm, Tim Wescott <t...@seemywebsite.com> wrote:

In fact, one of the methods of biasing small-signal tubes is to drive the
grid through a high (10M-ohm in some cases) grid-leak resistor. This
lets the grid bias itself negative.

This is clearly an application of the original question about do
electrons hit the grid (mask). Note that the field that the electrons
see has not only to do with the metal structures around (grids,
cathodes, supports etc.) but also with the other electrons that are
close to the given electron (the so-called electron cloud). Not that
we want to go into this in any detail, but since electrons are "boiled
off" with some energy, if the electron "cloud" is minimal, those
electrons striking the metal of the grid will charge it. If that grid
is connected to ground by a VERY high resistance, that charge will
build up on the grid eventually repelling the new electrons coming
off the cathode and biasing the tube. If the tube is built with such
dimensions that an electron cloud can exist in front of the grid, that
negative "cloud" will repel all electrons from the cathode and none
will reach the grid. Hence it will never get negatively charged and
hence will not work with a "grid-leak" bias. OK?

 

[BradGuth]

On Jan 22, 5:45 pm, Salmon Egg <Salmon...@sbcglobal.net> wrote:

In article
2edcf569-c6ce-476b-bb6b-d6ce2eec0...@p2g2000prn.googlegroups.com>,

 BradGuth <bradg...@gmail.com> wrote:
Thorium gives off electrons rather nicely when heated, although radium
(Ra226) gives off electrons when stone cold or even when cryogenic
cold.  Radium is a cold cathode electron emitter, and otherwise
extremely nifty beyond most imaginations.  Radium is also one of the
most secretly horded elements on Earth, with a 1600+ year half life to
boot.

Radium is an ALPHA emitter, not an electron emitter. It is possible that
some daughter isotopes are BETA (electron) emitters. Radium is in the
same column of the periodic chart as is barium and strontium. These are
sometimes used, especially as oxides, for low temperature cathodes,
Thus, radium is also likely to be a relatively low temperature
thermionic emitter, stupid as it may be to use it as such,

Bill

--
Private Profit; Public Poop! Avoid collateral windfall!

Thanks for that Alpha/Beta reminder. Radium, though mostly Alpha can
be made to produce its fair share Beta.

Radium is a whole lot safer than many other elements and known lethal
chemicals, and it's clearly a use it or lose it kind of element.
Unlike the longer lasting thorium or even uranium, it seems radium is
also one of the most privately horded and extremely valued elements on
Earth.

~ BG

 

[BradGuth]

On Jan 23, 7:55 pm, "hhc...@yahoo.com" <hhc...@yahoo.com> wrote:

On Jan 22, 8:45 pm, Salmon Egg <Salmon...@sbcglobal.net> wrote:



In article
2edcf569-c6ce-476b-bb6b-d6ce2eec0...@p2g2000prn.googlegroups.com>,

 BradGuth <bradg...@gmail.com> wrote:
Thorium gives off electrons rather nicely when heated, although radium
(Ra226) gives off electrons when stone cold or even when cryogenic
cold.  Radium is a cold cathode electron emitter, and otherwise
extremely nifty beyond most imaginations.  Radium is also one of the
most secretly horded elements on Earth, with a 1600+ year half life to
boot.

Radium is an ALPHA emitter, not an electron emitter. It is possible that
some daughter isotopes are BETA (electron) emitters. Radium is in the
same column of the periodic chart as is barium and strontium. These are
sometimes used, especially as oxides, for low temperature cathodes,
Thus, radium is also likely to be a relatively low temperature
thermionic emitter, stupid as it may be to use it as such,

Bill

--
Private Profit; Public Poop! Avoid collateral windfall!

ROFL Bill,

This thread is getting more humorous as it proceeds. Let me add my bit
to the chuckle heap.

Radium indeed emits Alpha particles naturally.  Still, when heated to
near incandescent termperature, is capable of emitting electrons. Not
strange, very little information is published on Radium's surface
barrier potential for electron emission.

Perhaps our govenment will find a way to fund such "Important"
research.  :-

Harry C.

It seems a great deal of our public funded research and subsequent
science is not accessible. Makes me kind of wonder where all the
radium is hiding, and why it's getting so privately horded and spendy
as hell.

Perhaps a mix of radium and thorium could make a nifty cold cathode
electron emitter. How about using the radon (Rn222) gas (mostly beta
electrons)?

I'd bet there lots of radium on our Selene/moon, as well as on Venus.

~ BG

 

[BradGuth]

On Jan 23, 9:58 pm, Benj <bjac...@iwaynet.net> wrote:

On Jan 22, 8:45 pm, Salmon Egg <Salmon...@sbcglobal.net> wrote:



In article
2edcf569-c6ce-476b-bb6b-d6ce2eec0...@p2g2000prn.googlegroups.com>,

 BradGuth <bradg...@gmail.com> wrote:
Thorium gives off electrons rather nicely when heated, although radium
(Ra226) gives off electrons when stone cold or even when cryogenic
cold.  Radium is a cold cathode electron emitter, and otherwise
extremely nifty beyond most imaginations.  Radium is also one of the
most secretly horded elements on Earth, with a 1600+ year half life to
boot.

Radium is an ALPHA emitter, not an electron emitter. It is possible that
some daughter isotopes are BETA (electron) emitters. Radium is in the
same column of the periodic chart as is barium and strontium. These are
sometimes used, especially as oxides, for low temperature cathodes,
Thus, radium is also likely to be a relatively low temperature
thermionic emitter, stupid as it may be to use it as such,

This could explain a factoid I know. I do know that right before
transistors put tubes mostly out of business, tube research was making
great strides. I was assured that a cold cathode had been developed
that produced enough current to power a vidicon tube. I was told that
developers were quite confident the current levels could be easily
increased to more typical tube service with some development. (As you
know the big shortcoming of tubes was the energy wasted heating the
cathodes). But all that technology seems to have disappeared along
with the big tube manufacturers of the day.  Today even the standard
tube technology seems to have largely evaporated leaving former
communist countries and third world countries to make some less than
stellar tubes to satisfy tube-loving guitar players.

It is in interesting story that I once applied for a job at MIT and
based on the above information I told the interviewer that I thought
transistors were a "passing fad".  I didn't get the job!  Later my
mistake became obvious. Do you know what it was?  In those days both
tubes and transistors were constructed as 3 dimensional devices. Hence
with no heater losses they were basically equivalent devices with the
tubes having MUCH superior performance in the day.  But it was the
Fairchild "planar" process that made the "integrated circuit" possible
that eventually made solid state the hands-down winner over 3-D wired
and spot welded construction. I doubt that the MIT interviewer knew
this at the time, but doubtless thought I was "wrong" based on the
large power needed to run tube heaters.  It's hell knowing too
much!

Field effect should have taken the lead. Far better technology for
most applications, however wrong place and time when those in charge
elected to go with the transistor.

The cold cathode vacuum tubes could still play an important role for
missions while on the planet Venus, although even a hot cathode
conventional tube (in some cases not even a vacuum required) would
more than do the trick.

~ BG

 

[BradGuth]

On Jan 23, 10:49 pm, Salmon Egg <Salmon...@sbcglobal.net> wrote:

In article
563cf945-51ec-4669-898b-488dafeff...@e1g2000pra.googlegroups.com>,

 Benj <bjac...@iwaynet.net> wrote:
This could explain a factoid I know. I do know that right before
transistors put tubes mostly out of business, tube research was making
great strides. I was assured that a cold cathode had been developed
that produced enough current to power a vidicon tube. I was told that
developers were quite confident the current levels could be easily
increased to more typical tube service with some development. (As you
know the big shortcoming of tubes was the energy wasted heating the
cathodes). But all that technology seems to have disappeared along
with the big tube manufacturers of the day.  Today even the standard
tube technology seems to have largely evaporated leaving former
communist countries and third world countries to make some less than
stellar tubes to satisfy tube-loving guitar players.

I had posted that the extra elements in a vacuum had little effect on
the basic thermionic emission processes. While basically true, some
tubes with field emission cathodes have been produced. In particular,
x-ray tubes requiring relatively little current at high voltage fit that
bill. A high voltage on a nearby anode can produce significant current
from field emission points without preheating the cathode.

Bill

Cathode field emissions from radium shouldn't be excluded, simply
because radium is privately horded and made artificially spendy as
hell.

~ BG

 

[BradGuth]

On Jan 24, 6:45 am, Frnak McKenney
<fr...@far.from.the.madding.crowd.com> wrote:

On Fri, 23 Jan 2009 22:49:21 -0800, Salmon Egg <Salmon...@sbcglobal.net> wrote:
In article
563cf945-51ec-4669-898b-488dafeff...@e1g2000pra.googlegroups.com>,
 Benj <bjac...@iwaynet.net> wrote:

This could explain a factoid I know. I do know that right before
transistors put tubes mostly out of business, tube research was making
great strides. I was assured that a cold cathode had been developed
that produced enough current to power a vidicon tube.
--snip--
I had posted that the extra elements in a vacuum had little effect on
the basic thermionic emission processes. While basically true, some
tubes with field emission cathodes have been produced. In particular,
x-ray tubes requiring relatively little current at high voltage fit that
bill. A high voltage on a nearby anode can produce significant current
from field emission points without preheating the cathode.

A few years back there was a great deal of interest in array-cathode
CRTs called Field Emitter Displays. The basic design involved what
amounted (my view) to two slightly separated plates of glass; the
"front" plate was phosphor-coated, much like the usual CRT face, but
in place of the usual point-source electron gun whose electron stream
had to be bent, twisted, and hosed across the entire face, the FEDs'
back plate was a (dense) XY grid of emission points. Each phosphor dot
was illuminated by multiple cathode points; this redundancy would have
allowed a much larger manufacturing yield than single-point-failure LCD
displays, and much larger screens.

It sounded like a good idea, but I haven't heard FEDs mentioned lately;
I don't know if they ran into technical problems or just got hit with
the "LCDs are close enough" end of the VHS-Beta stick.

Frank McKenney
--
    Fashion is...a search for a new language to discredit the old,
    a way in which each generation can repudiate its immediate
    predecessor and distinguish itself from it.
    -- Fernand Braudel/Civilization & Capitalism, 15th-18th Century
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)

Perhaps we should raid our public funded DARPA archives, and see for
ourselves. If we're lucky we'll stumble over those 700 large and
clearly marked boxes of our Apollo missions, and Jimmy Hoffa to boot.

~ BG

 

[Jon Slaughter]

"RichD" <r_delaney2001@yahoo.com> wrote in message
news:630c2b4c-5174-4ba9-b53d-0ad1977b7426@i24g2000prf.googlegroups.com...

I don't know much about vacuum tubes, but often
I have read statements like "the cathode heats up,
causing electrons to boil off and fly to the anode"..
which makes me scratch my head..

How does an electron boil? Can anyone explain this?

By heating up the atoms you give the electrons energy(the nucleus is tightly
bound). If they get enough energy they are able to escape the nucleus and
eventually the material(this isn't completely true though).

The energy needed to accomplish this is called the work function.

Are there degrees of boiling, like a pot of water,
or is there an on/off threshold?

Depends on a lot of factors. Temperature is the main thing but it also
depends on what surrounds the material. What generally happens is that when
an electron does escape it gets immediately attracted to the
material(remember the material ends up becoming positive). So what you end
up with is a "cloud" of electrons around the material and eventually no
electrons can escape the material. (this is why you need a electric field to
get them to move and a current to replace those lost.)

Then there's the grid mask, whatever that is... how
come the electrons don't smash into that? How do
they find the holes? Is this like synchronized diving,
they're trained to hit the water at specified spots?

No, They do hit the mask. The mask exists to attract those electrons from
the cathode. e.g., you put a voltage across it and electrons will be
attracted to it. Since the grid has holes in it the electrons usually will
go through it. Also if they do hit it then that grid will build up a
charge... those electrons must be removed. Hence the grid usually is
grounded by a large resistor so electrons can flow off the grid.

Also, when they hit the grid they usually can just bounce off because they
have so much momentum. This is called secondary emmissions and usually
occurs when there is another grid such as a tetrode(which causes a pretty
big problem and why the pentode was designed).

It's quite simple:

You heat up the cathode. This causes electrons to gain enough energy to
break away from the material. If any do they are replaced by new electrons
from the power supply. (you can heat up the cathode with the same current
but generally this is a bad idea because you what a steady temperature)

Then the grid supplies an attraction so that any electrons that do escape
will move away from the material and be attacted to it... they are able to
go through the grid(since it is a mesh) and they finally end up on the
anode.

Note that not much current is actually flowing(usually uA's to a few mA) but
high voltages are present.

 

[Salmon Egg]

In article <fJKdnQtARurA6-bUnZ2dnUVZ_rfinZ2d@web-ster.com>,
Tim Wescott <tim@seemywebsite.com> wrote:

For power rf amplifiers, there usually is sufficient negative bias on
the control grid to reduce anode current to zero without excitation.
When rf is applied to the grid, the positive portions of the cycle
allows current to flow. Typically, the grid potential is driven positive
through a small portion of the cycle to the extent that there is some
grid current. Anode current flows in pulses. The idea is to use the tube
as a switch to minimize dissipation by trying to keep either the anode
potential or current small. In that respect, transistors do much better.

Bill

AFAIK, class C transistor amplifiers aren't all that much more efficient
than class C tube stages, because the switching isn't at all crisp. But
tubes have much too much capacitance to make a class E stage, and with
clever design you can make a (transistor) class E stage that has
efficiencies approaching that of a class D (base band switching) stage.

(I haven't actually tried a class E stage, but I know it's been done.
One day...)

One reason for using switching mode, for which class C tube circuits is
merely an approximation, is to reduce plate dissipation as much as for
efficiency. In fact, it is only the fundamental component in the plate
pulse current pulses. For second harmonic generation using a Class C
amplifier, efficiency will be reduced because the second harmonic
component of the plate current will be smaller than the fundamental
component.

Bill

--
Private Profit; Public Poop! Avoid collateral windfall!

 

[Salmon Egg]

In article
<ed70c17b-6435-4945-ac24-dfe5ec853ecd@w39g2000prb.googlegroups.com>,
BradGuth <bradguth@gmail.com> wrote:

Thanks for that Alpha/Beta reminder. Radium, though mostly Alpha can
be made to produce its fair share Beta.

HOW? please be more specific. Thermionic emission is NOT beta emission
even though betas are electrons.

Radium is a whole lot safer than many other elements and known lethal
chemicals, and it's clearly a use it or lose it kind of element.
Unlike the longer lasting thorium or even uranium, it seems radium is
also one of the most privately horded and extremely valued elements on
Earth.

Please explain this as well. While alphas will not penetrate skin,
ingestion of radium has killed many people. Have you heard the story of
women who used to paint radium dials? Alphas emitted internally can be
very damaging because alphas ionize strongly and there is no skin to
protect sensitive tissue. With all the isotopes now available from
nuclear reactors and neutron irradiators, radium is too expensive and
unnecessary for wide application.

Bill

--
Private Profit; Public Poop! Avoid collateral windfall!

 

[Edward Green]

On Jan 24, 9:45 am, Frnak McKenney
<fr...@far.from.the.madding.crowd.com> wrote:

On Fri, 23 Jan 2009 22:49:21 -0800, Salmon Egg <Salmon...@sbcglobal.net> wrote:
In article
563cf945-51ec-4669-898b-488dafeff...@e1g2000pra.googlegroups.com>,
 Benj <bjac...@iwaynet.net> wrote:

This could explain a factoid I know. I do know that right before
transistors put tubes mostly out of business, tube research was making
great strides. I was assured that a cold cathode had been developed
that produced enough current to power a vidicon tube.
--snip--
I had posted that the extra elements in a vacuum had little effect on
the basic thermionic emission processes. While basically true, some
tubes with field emission cathodes have been produced. In particular,
x-ray tubes requiring relatively little current at high voltage fit that
bill. A high voltage on a nearby anode can produce significant current
from field emission points without preheating the cathode.

A few years back there was a great deal of interest in array-cathode
CRTs called Field Emitter Displays. The basic design involved what
amounted (my view) to two slightly separated plates of glass; the
"front" plate was phosphor-coated, much like the usual CRT face, but
in place of the usual point-source electron gun whose electron stream
had to be bent, twisted, and hosed across the entire face, the FEDs'
back plate was a (dense) XY grid of emission points. Each phosphor dot
was illuminated by multiple cathode points; this redundancy would have
allowed a much larger manufacturing yield than single-point-failure LCD
displays, and much larger screens.

It sounded like a good idea, but I haven't heard FEDs mentioned lately;
I don't know if they ran into technical problems or just got hit with
the "LCDs are close enough" end of the VHS-Beta stick.

Gee, if I didn't know this can't be what you're talking about, I would
have thought that "plasma" was an apt description for a tube of this
nature.

 

[Frnak McKenney]

On Sat, 24 Jan 2009 16:47:23 -0800 (PST), Edward Green <spamspamspam3@netzero.com> wrote:

On Jan 24, 9:45 am, Frnak McKenney
fr...@far.from.the.madding.crowd.com> wrote:
--snip--
A few years back there was a great deal of interest in array-cathode
CRTs called Field Emitter Displays. The basic design involved what
amounted (my view) to two slightly separated plates of glass; the
"front" plate was phosphor-coated, much like the usual CRT face, but
in place of the usual point-source electron gun whose electron stream
had to be bent, twisted, and hosed across the entire face, the FEDs'
back plate was a (dense) XY grid of emission points. Each phosphor dot
was illuminated by multiple cathode points; this redundancy would have
allowed a much larger manufacturing yield than single-point-failure LCD
displays, and much larger screens.
--snip--
Gee, if I didn't know this can't be what you're talking about, I would
have thought that "plasma" was an apt description for a tube of this
nature.

Not the same as plasma. Here's a pointer to a discussion of various
display technologies:

http://www.computerpoweruser.com/editorial/article.asp?article=
articles%2Farchive%2Fc0810%2F63c10%2F63c10.asp

I did a bit of searching, and it turns out that FEDs (Field Emitter
Displays, a.k.a. Field Emission Displays) are not as dead as I had
thought. This article, from EE Times Asia, mentions a Sony spin-off
that will be going into limited mass-production (5k/mo) of a 26" FED
display by the end of 2009:

http://www.eetasia.com/ART_8800538618_480200_NT_83f9d5fb.HTM

The article also mentions some difficulties in producing them,
such as the requirement for a very high vacuum level.


Frank
--
Fortunately, man was given a sense of humor to help
compensate for nature's law of gravity.
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)

 

[John Larkin]

On Sat, 24 Jan 2009 18:13:43 -0600, "Jon Slaughter"
<Jon_Slaughter@Hotmail.com> wrote:

"RichD" <r_delaney2001@yahoo.com> wrote in message
news:630c2b4c-5174-4ba9-b53d-0ad1977b7426@i24g2000prf.googlegroups.com...
I don't know much about vacuum tubes, but often
I have read statements like "the cathode heats up,
causing electrons to boil off and fly to the anode"..
which makes me scratch my head..

How does an electron boil? Can anyone explain this?


By heating up the atoms you give the electrons energy(the nucleus is tightly
bound). If they get enough energy they are able to escape the nucleus and
eventually the material(this isn't completely true though).

The energy needed to accomplish this is called the work function.

If the external electric field strength is high enough, you can rip
electrons out of metals even at temperatures near absolute zero. The
required fields are huge, 5e10 v/m or some such. With a very sharp
metal tip, you can reach fields like that with mere kilovolts of
potential, and interesting things can happen, like imaging individual
atoms.

This is actually a field-ion image, but it works with electrons, too.

http://www.aip.org/mgr/png/images/wolkow.jpg

Those are not blueberries, they're atoms.


John

Are there degrees of boiling, like a pot of water,
or is there an on/off threshold?

Depends on a lot of factors. Temperature is the main thing but it also
depends on what surrounds the material. What generally happens is that when
an electron does escape it gets immediately attracted to the
material(remember the material ends up becoming positive). So what you end
up with is a "cloud" of electrons around the material and eventually no
electrons can escape the material. (this is why you need a electric field to
get them to move and a current to replace those lost.)


Then there's the grid mask, whatever that is... how
come the electrons don't smash into that? How do
they find the holes? Is this like synchronized diving,
they're trained to hit the water at specified spots?


No, They do hit the mask. The mask exists to attract those electrons from
the cathode. e.g., you put a voltage across it and electrons will be
attracted to it. Since the grid has holes in it the electrons usually will
go through it. Also if they do hit it then that grid will build up a
charge... those electrons must be removed. Hence the grid usually is
grounded by a large resistor so electrons can flow off the grid.

Also, when they hit the grid they usually can just bounce off because they
have so much momentum. This is called secondary emmissions and usually
occurs when there is another grid such as a tetrode(which causes a pretty
big problem and why the pentode was designed).

It's quite simple:

You heat up the cathode. This causes electrons to gain enough energy to
break away from the material. If any do they are replaced by new electrons
from the power supply. (you can heat up the cathode with the same current
but generally this is a bad idea because you what a steady temperature)

Then the grid supplies an attraction so that any electrons that do escape
will move away from the material and be attacted to it... they are able to
go through the grid(since it is a mesh) and they finally end up on the
anode.

Note that not much current is actually flowing(usually uA's to a few mA) but
high voltages are present.

 

[Don Kelly]

John Larkin wrote:

On Wed, 21 Jan 2009 21:57:01 -0800 (PST), RichD
r_delaney2001@yahoo.com> wrote:


I don't know much about vacuum tubes, but often
I have read statements like "the cathode heats up,
causing electrons to boil off and fly to the anode"..
which makes me scratch my head..

How does an electron boil? Can anyone explain this?


Electrons in a metal or similar are bound by bonding forces. As you
heat things up, some electrons acquire more energy and, if near the
surface, may break away. The energy is called the "work potential",
measured in volts (or electron-volts.) Once free of the surface, any
extra energy past the escape potential gives the electron some
velocity.


Are there degrees of boiling, like a pot of water,
or is there an on/off threshold?


Below some critical temperature, there's insufficient energy to
escape. Above that, electron emission goes up fast as temp rises.


Then there's the grid mask, whatever that is... how
come the electrons don't smash into that? How do
they find the holes?


It's a very sparse grid of wires, so most electrons miss, just drift
through the big holes between the tiny wires. If you apply a high
negative voltage, you can force the electrons further from the wires,
crowd them into the gaps betweem, and eventually shut down the gaps
completely.

Is this like synchronized diving,

they're trained to hit the water at specified spots?


They are random and unsynchronized, although they do repel one
another, which adds a certain sort of order.

John


In other words, a high negative potential at the grid wires produces a

local field that is negative with respect to the cathode- counteracting
the field due to the anode. Do we have "holes" between wires? Not
really- but it is a nice way to visualize (visualise outside the US) it
without messy field analysis-even if it is not true..

--
Don Kelly
dhky@shawcross.ca
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