saps - 23-10-2005 at 15:37
i want to build an electron gun like one found in a crt. can anybody give me some advice??
Mr. Wizard - 23-10-2005 at 18:11
You could cheat and use the gun from an old CRT monitor. Are you trying to get heating or just a beam to move around? The amount of current is the
big variable as I see it.
12AX7 - 23-10-2005 at 21:36
Simple.
Grab a thermionic cathode -- plain tungsten (as in any lightbulb) will suffice, but less power is needed to heat a thoriated tungsten or rare earth
oxide coated type. Follow this with g1, a capped tube with hole in it, then accellerating and focusing grids g2, g3, g4, etc. as needed (basically,
follow the construction of a gun as seen in a CRT). Then you can add a larger, even higher voltage electrode if you want to accelerate the electrons
to keV levels.
When you've got your apparatus secured, seal it, pump down the vacuum, induction heat the electrodes orange hot to degas them, finish pumping
down to a hard vacuum circa 10^-9 torr, then fire the getter (barium metal works well).
Tim
Mr. Wizard - 23-10-2005 at 22:20
Anybody who uses a vacuum tube number (12AX7) for his handle should be able to guide us on electron guns. 
Was it a double triode or tetrode? Obviously 12 volt filament. I have a half dozen or so of them. BTW if you need
thoriated tungsten for the electron source you can buy it as the electrode at a welding supply. (TIG welding electrodes) I'd get the thinnest
one to keep the power bill down. You can also find them on eBay.
bio2 - 24-10-2005 at 00:19
.....You could cheat and use the gun from an old CRT monitor. Are you trying to get heating .......
I was looking at the business end of a 25inch picture tube the other day that I want to cut the end off of to get the goodies. I collect these
primarily for the power transistors, transformers etc.
Saved this particular tube as it has a more massive assembly in sealed end than any others I have seen of this size. It came from an old color TV so
old in fact there isn't a single MOSFET in the damn thing but lots of nice big ceramic power resistors that are now dinosaurs with the new
switchers.
Knowing very little of vacuum tubes but being electrically competent (EE) I've been thinking there has to be some really cool uses for these high
power tubes and have considered making some sort of heater induction or otherwise.
Any ideas? Even a 200watt rheostat could be very useful compared to a monstrous wirewound.
stygian - 24-10-2005 at 05:29
Lenard rays have always sounded interesitng to me. A crookes tube with a thin foil at the end allows the electrons to pass out of the tube. You can
shoot things (at very close distance) with electrons.
unionised - 30-10-2005 at 07:37
Is it my imagination or is that a good way of zapping yourself with Xrays?
froot - 30-10-2005 at 12:25
A 12AX7 is a low power double triode. The heater runs on 6,3V, used in audio preamlifiers. I was always under the impression that an electron beam
needs a posatively charged target to increase it's efficiency otherwise it just charges up the air and dissipates. Unless you want to use
ludicrous amounts of energy and as unionised said, risk your health.
12AX7 - 30-10-2005 at 13:30
Not necessarily a target but an electric field to accelerate the electrons.
If you have a hole in the "target", you can get electrons passing through by kinetic energy. (Electrons aren't attracted to the
*surface* -- only attracted through the charged *space* between electrodes.) This is how a screen-grid tube works. If you remove the anode
altogether (or rather, provide fields and electrode shapes such that the electrons are never absorbed), you can accelerate a current to a pretty good
velocity (IIRC, the electrons in your monitor are around 0.9c 
).
If, for a final act, you provide a permeable barrier that lets electrons through (without melting or breaking down electrostatically!) but not air (so
as to seal the vacuum), you can get low-energy beta radiation in quantity. 
Too bad it won't get very far, the electrons are probably done ionizing molecules after six inches, if that.
Tim
[Edited on 10-30-2005 by 12AX7]