[Amps] why a tube fails ?

[Jim Garland]

Hi Charles,
Very intersesting questions!  I can't help you with your question about tube
failure modes, though I know at least one mode with indirectly heated
cathode tubes is that excessive cathode current can cause the filament
coating to evaporate and coat the grid, which then starts emitting electrons
and quickly destroys the tube. .

I can be a bit more informed about your second question. First, a
nomenclature correction. Free electrons are not ions. Ions are atoms which
have temporarily lost or gained one or more electrons, and thus have a net
charge. For example, a hydrogen ion is just a proton, since a neutral
hydrogen atom normally has a proton nucleus and a single bound electron.
When that electron becomes unbound, the hydrogen atom becomes an ion.

The second point has to do with current flow in a tube. Plate current always
flows from the plate (positive) to the cathode (negative). This point often
mystifies people who believe that current in a tube should flow in the
opposite direction (from negative to positive), because the current is
carried by electrons which travel always from the cathode to the plate. The
confusion comes about because of a misunderstanding about current. Electric
current is a defined quantity involving the flow of charge, independent of
the sign of the charge. Current is a statistical quantity that is
meaningless when talking about individual electrons. In that sense it is
like temperature, which is a statistical propery of large numbers of moving
atoms (e.g., air molecules, or atoms in a bar of copper.) Just as it doesn't
make any sense to speak of the temperature of a single moving atom,  it
doesn't make any sense to speak of the currrent of a single moving eletron.
You won't go wrong if you always treat current as flowing from positiie to
negative. The fact that the charge may actually be moving in the opposite
direction doesn't change the definition.

A third point refers to charge neutrality in a vacuum tube.  When a vacuum
tube is operating in a working circuit, it is electrically neutral. As
electons boil off the cathode surface, they expeience a force that radiates
out from the plate (called an electric field) that sucks them toward toward
the plate. The elctrons that leave the cathode are replenshed by the
external circuitry, Similarly, the excess electrons that strike the plate
are siphoned off by the external plate circuitry. The rate at which
electrons are siphoned off the external  circuitry exactly equal the the
rate at which electrons on the cathode are replenished (though these are not
the same electons!).

An interesting effect occurs when a tube cathode is heated, but there is no
exteral circuitry connected to the plate.  In that case electrons are boiled
off the cathode and a "cloud" of electrons builds up inside the tube
envelope. The resulting electric field pushes back on the electrons that
otherwise would leave the cathode (because like charges repel each other),
so the cloud becomes self-limiting and doesn't grow beyond a small amount.
The tube as a whole remains electrically neutral, but a voltmeter would show
a slight potential difference between the plate and the cathode, because the
so-called "space charge" depletes the cathode of electrons. The process is
similar to using a "grid leak" resistor to self-bias a vacuum tube. Charge
builds up on the grid, causing a negative potential between the grid and
cathode.  The value of the grid leak resistor is high enough that it doesn't
allow the voltage difference to climb too high. By the way, the electron
cloud doesn't mean that electrons are just hanging out in the tube enevelpe.
They are moving very fast (thousands of miles a second) and colliding with
each other, just as the air molecules in an ordinary cloud are moving very
fast, even though the cloud appears stationary.

Another question you asked is whether the transfer of electrons to and from
tube elements causes a change in chemical composition of the element?. Of
the chemical composition, the answer is possibly. Surface alloys or coatings
can be changed, but there is never any change in the individual atoms
themselves. A tungsten atom always remains a tungsten atom, and the same for
all the other atoms in the tube. They can be ionized (lose or gain
electrons), but they never form a new isotope or new element. (A particle
accelerator can cause an atome to change into a different atom, and with
radioactive atoms this happens all the time. But it doesn' t happen in a
vacuum tube!)

And I'll now finish with one more interesting tidbit about electrons.
Electons are not permanent creatures. They can be readily destroyed and
created. This happens when an electron absorbs a photon (particle of light),
or disappears, leaving a photon in its place (not really "in its place," of
course, since photons rocket away at the speed of light.).  So electrons
come and go all the time. Sometimes, very strange things happen. For example
an electron can collide with a photon and get knocked backwards in time, by
which I mean it can travel from the present back into the past. An electron
moving backwards in time is called a positron, and it has a positive charge
and spins in the opposite direction from  electrons moving forward in time. 

And here's the most amazing fact: the entire world we experience, the colors
and textures, and whether something is transparent like glass, or conducts
electricity, like a bar of copper, is entirely -- entirely! -- due to
electrons emitting and absorbing photons.  Even the electons in an atom,
like the single electron bound to a  proton in a hydrogen atom, is
constantly emitting and absorbing photons from the proton nucleus.
Physicists call this process the "virtual exchange" of photoss. A century
ago, people thought electrons orbited nuclei like tiny planets, but today we
know that is not the case. Electrons are bound to nucleii by the virtual
(so-called because it happens very fast) emission and absorption of photons.
And every time this happens, the original electron is destroyed and a brand
new one takes its place, zillions of time  a second.  BTW, the theory that
explains all this is called "Quantum Electrodynamics," and is probably the
best physics theory ever. It has been confirmed thousands of times in every
conceivable way and is astonishingy accurate and correct. About the only
thing it doesn't explain is gravity, and the inner workings of atoms at the
nuclear level. All fundamental particles we know about (protons, neutrons,
even electrons) are made up of sub-particles. I won't go into that here,
except to say that there are a lot of Nobel Laureates on the planet who got
their prize by figuring all this out.
73,
Jim Garland W8ZR


> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Charles
Harpole
> Sent: Tuesday, August 27, 2013 7:27 PM
> To: amps at contesting.com
> Subject: [Amps] why a tube fails ?
> 
> Some time ago I asked WHY DOES A TUBE FAIL?
> Or What happens when a tube fails or becomes "soft"?
> 
> I hope to get a discussion about this matter and electric flow generally.
I am asking this
> question on the ATOMIC LEVEL.
> 
> The best answer I have heard is that the vacuum inside the tube is not
perfect and the tiny
> amount of oxygen inside does oxidize or interact with the metals inside
and eventually
> breaks one of the metal parts through deterioration of one spot which
becomes thin and then
> separates via the chemical changes of the metal in the presence of oxygen
(a process often
> called "rusting").
> 
> Then what happens when a tube gives low output but is not dead?  Is it the
case that the
> "rusting" has reduced the area of conductivity (a part is smaller but not
broken) and the
> output drops?   Is this how the tube becomes "soft"?
> 
> Then, the allied question arises about the flow of electrons as it relates
to tubes.  The
> common understanding is that the element in the tube is heated causing
additional
> excitation of the atoms (electrons will leave their atom more easily) and,
that is combined
> with the application of an electric current which forces an exchange of
electrons with a net
> increase of more electrons ending up in one metal part of the tube
compared to the other
> part.  Those extra electrons must be ions (electrons not attached to any
nucleus), yes or no?
> If they are not all ions, then if electrons are actually added to an atom,
then does that
> change the chemical composition of the metal (a change of its
characteristics as given by the
> Periodic Table of Elements)?  That is, with the addition of electrons,
does that make the
> matter a different element?
> 
> So then is the collection of extra electrons on one part inside the tube
then "siphoned off"
> and that becomes the tube's output?  More electrons are "pumped in" by the
flow of current
> into an emitting part?  Then, how is GAIN obtained ?  That is, what
happens inside the tube
> to increase the flow of electrons beyond what is pumped in ?
> 
> Inquiring minds want to know.  Tnx, 73, Charly K4VUD
> 
> 
> 
> Charles Harpole
> k4vud at hotmail.com
> 
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