crispyambulance
If we're talking homemade vacuum tubes... I wonder if it
wouldn't be easier to just use metal endplates with
feedthroughs for electrical (like spark-plugs) and with
v-grooves for o-rings or some other gasket material. this
kind of construction can handle vacuum easily, I think?
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tliltocatl
One thing about gallium/galinstan - it would actually make
a descent high vacuum seal as it has lowest vapor pressure
of all elements - so it doesn't evaporate. The problem is
that it sticks to just about everything that isn't
PE/PTFE. Galinstan thermometers use some proprietary
coating to make glass repel it.I was once entertaining the
idea of using gallium for an electrostatically or MHD
boosted Sprengel pump, but figured out sticking would make
it infeasible. And now it's unobitanium too.
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alister
What was the large-scale commercial procedure for making
electrodes that pass through the glass without letting air
in? I assume that electronics manufacturers must have been
making millions of such vacuum tubes in the past. Is the
knowledge lost (or not practical for hobby use)?
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> adrian_b
As mentioned in TFA, the most important factor for
successfully joining a metal and a glass is to match
their thermal expansion coefficients.Most pure metals
have a much greater thermal expansion than any glass,
which will cause cracks.In the nineteenth century, the
first successful joinings of metal with glass were
done using platinum, but that is obviously too
expensive for normal applications.Eventually a special
alloy of iron-nickel-cobalt was developed, which is
named kovar and whose thermal expansion is matched to
that of a certain type of borosilicate glass.The use
of kovar was widespread in electronics, starting with
the vacuum tubes and gas tubes, and then continuing
with the first generations of transistors and
integrated circuits, which used metal packages.All the
old transistors and operational amplifiers that were
packaged in metal cans had pins and package bases made
of kovar.When kovar had to be joined with a different
kind of glass than the type with which it is matched
in thermal expansion, that glass was coated in one or
more layers of different kinds of glasses, with that
matched to kovar in contact with the metal and the
intermediate layers having intermediate thermal
expansion coefficients, interpolating between the bulk
glass and kovar.Kovar is not a good thermal or
electrical conductor, which is why the modern power
transistors that use plastic packages (e.g. TO-247)
and copper bases and pins (which are plated with
nickel or tin, to avoid corrosion) can easily
dissipate much greater powers than the old transistors
in TO-3 metal cans, which had the same size. On the
other hand, the old transistors in metal packages were
pretty much immune of environmental influences.
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> > SoftTalker
Ordinary incandescent bulbs must have similar
sealing requirements, but they probably mostly
rely on using a thin conductor that doesn't
contract much when it cools. Also IIRC modern
incandescent bulbs do not use a high vacuum but
contain a low pressure inert gas so leakage would
be slower if it occurs at all.
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> > > adrian_b
As you say, incandescent bulbs are less
demanding because they do not use high vacuum,
but they have the additional requirement that
the pins that support the tungsten filament
must resist to very high temperatures, because
some heat is conducted through the filament
into its support.This is why the pins that
support the filament are typically made of
molybdenum. Molybdenum has a relatively low
thermal expansion coefficient in comparison
with most metals, so there are certain glass
compositions that can match its TCE. The glass
through which the pins pass is not of the same
type as the bulb, which is made of cheaper
glass, but it is of the type matched in TCE
with molybdenum.
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> > > ssl-3
AFAIK, in in the US: Modern high-efficiency
incandescent bulbs are exactly like the
halogen bulbs of yore, but with the small
quartz envelope wrapped in a familiar
light-bulb-shaped shell, and with a base that
matches. It's like a light bulb within a light
bulb.See this random example of a GE bulb
(which I selected just because it includes the
first picture I could find of a modern bulb
made with clear glass):
https://www.toolboxsupply.com/products/ge-ligh
ting-62616-ene...Except for all the ones that
aren't modern or efficient. Common 40-Watt
appliance bulbs, for instance: Those are still
built using the old methods. They never
changed. This strongly suggests that we never
forgot how to seal metal wires into a glass
bottle full of nothing.But this article isn't
about industrial processes. It's about
rediscovering things at home, and that stands
on its own merits. :)
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> > > tliltocatl
Incandescent bulbs used dumet/platinite, which
is an nickel-iron alloy like kovar except it's
turned for a different CTE. This stuff isn't
that expensive when mass produced - it's just
those who can afford usually borosilicate can
afford paying a premium for kovar.
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> SoftTalker
Vacuum tubes are still made today, so I'm sure the
knowledge is not lost. I'm curious about the answer as
well.
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> > NoMoreNicksLeft
I was under the impression that they were only
made in eastern Europe at this point, former bloc
nations. Even then, the demand must be microscopic
at this point.
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> > > drum55
xray, photomultiplier and laser tubes are
still SOTA. PM tubes in particular have a huge
number of glass feed through for the
intermediate plate voltages.
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> > > > SoftTalker
Musical instrument amplifiers still use
them as well, at least some of them.
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> ludicrousdispla
I'm not sure what specific glass and metal are used in
neon sign electrodes, but their definitely built to
hold a higher vacuum under decades of use. Their
relatively cheap and you can get them with small tubes
on the end for pulling the vacuum.
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> CamperBob2
The article pretty much tells you: "Copper's red oxide
bonds very well to glass. In fact, the bond is
stronger than the bulk glass: when it breaks, there's
always a thin layer of glass left stuck to the metal.
Along with its excellent electrical properties, it
seems like an ideal electrode material." If you look
at how vacuum tubes are constructed that's essentially
what you see.Tubes are evacuated through a hole
created elsewhere, nowhere near any electrical
connections. The getter is then flashed to clean up
any gas molecules left over.
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> > adrian_b
Nope, as also mentioned in TFA, copper has a too
great thermal expansion coefficient in comparison
with glass.If vacuum tubes had pins of copper, the
glass-metal joining would have cracked very soon
during normal usage cycles, and there would have
been no vacuum left in the tube.Real vacuum tubes
and gas tubes had pins made of kovar, which is a
Fe-Ni-Co alloy with a TCE matched to a certain
composition of borosilicate glass.The kovar pins
were normally plated with nickel on their external
parts, to enable soldering, because molten solder
does not wet kovar.
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> > > CamperBob2
Right, point being that metal-glass seals are
very effective given the right materials. This
has nothing to do with how the tube is
evacuated, and there is never a point where
the wires have to be "passed through the glass
without letting air in."
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tyingq
The research here is clearly interesting, but if you just
need to get something like this working, premade neon tube
electrodes are plentiful and inexpensive.
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kazinator
> For the seal, this is a good thing: less metal means
less expansionBut power tubes need to pass some decent
amounts of plate current through some of the pins. Even
small signal tubes have considerable current going through
the heater filaments; you don't want hookup wires for that
which are like metallic spider silk.
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projektfu
I was wondering about the feasibilty of this, but I
thought that useful tubes needed a harder vacuum than
that. Is this really "good enough" for a triode?I figured
the wire-holding/element-holding aspect of a standard tube
was in the base, and the glass-to-base seal is the
important part. You can have a less-hot metal holding the
filament and penetrating through the base. But I haven't
looked carefully. These are my off-the-top-of-my-head
thoughts about it.
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> Animats
> "good enough"Probably not. The classic fix is a
"getter".[1][1] https://en.wikipedia.org/wiki/Getter
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> rigonkulous
> Is this really "good enough" for a triode?Let us not
overlook that its also a lot of energy. Its not a
matter of "good enough", I think in this case - more
"can I?" ..
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LgWoodenBadger
Would you be able to reseal the cracked glass and
regenerate the vacuum through the other end?More glass,
epoxy, or similar?
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> bluGill
Plastic generally isn't an air tight seal. The leaks
may be slow, but generally we hope vacuum tubes last
for years.
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> adrian_b
An accidental crack can be resealed, but if the crack
had appeared because inappropriate materials were
used, e.g. an unsuitable metal-glass pair, resealing
is pointless, because cracks will appear again after
the device is turned on and off several times, causing
expansion-contraction cycles.
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smlacy
Hmmmm. Wonder if you could just induct through the glass
with coils on each side? Seems perfect for high voltage
applications?
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> tliltocatl
If you want a gas discharge tube and not vacuum, you
can even drop the coil on the inside:
https://hackaday.io/project/194683-plasma-toroid-sky-g
uided-...But most hollow-state devices run on either
DC or pulses, so coupled inductors wouldn't work.
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> bluGill
That depends. Often vacuum tubes are used with DC
(that is a rectifier) in some form though, in which
case you can't do this since induction depends on AC.
I'm not sure what purpose the article had for a triode
though, depending on their application this might
work.
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> K0balt
Interesting idea! Wouldn't have to be particularly
high voltage either.
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