• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Yet another nutty idea - don't say I didn't warn

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Suppose we built a valve inside out...

Cathode on the outside, coated on the inside.
Metal cathode might even be its own envelope.
Maybe inductively heated by a coil wrapped
around, or solar oven, or propane burner...
Would our cathode need to run as hot if the
emissive surface area was greater?

Plate in the middle. A solid metal rod, perhaps
even a heatpipe... Does it need zirconium or
we can figure some other way of gettering
inside a metal envelope?

Any historical precedent, or is this unworkable
for some reason I should already be aware of?
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I'd love to be able to design/make some tubes to our own crazy designs. One way to do this might be to simply buy a vacuum pump and bell jar and build the whole amplifier inside it - I did some things with vacuum pumps and glass chambers in a previous life.

Trouble with anode on inside is heat removal. It's in a vaccum and has to radiate it's heat so you want metal with lots of surface area on the outside of the tube not stuck in the middle.

Another thing that might need to be thought about is electron orbits. The inverted tube would behave like a planetary system, central anode would be the sun and pull electrons towards it which may do some interesting things like orbit around the anode if they had any tangential velocity - which could be imparted to them by the grid(s). Maybe some interesting plate curves.
I have an incredibly vague "teach yourself electronics" book that describes klystrons or whatever they're called, which resonate the the electron level with whirling electrons or something. If you're creating a pipe-shaped cavity, is there danger of becoming an oscillator/resonator?

- keantoken
Inside out tube?
They already make them, they are called Ion Gauge tubes. And they are DIY too, since they have an open glass pipe on the side that you have to connect a vacuum pump to. The plate is a single wire down the center, the grid is a large spiral grid around it, and usually two directly heated filaments are provided outside of that. The extra one is for when you accidentally let the magic vacuum out (the pump quits, a leak, ...) and the filament burns up, a handy spare is sitting ready. Obviously low power dissipation, but interestingly they are real P type tubes, since they operate on ions from the residual gas in the "vacuum". The plate operates at a negative potential. Another interesting feature is that both ends of the "grid" spiral are brought out to terminals, so that high current can be passed thru it to outgas the tube. Sort of a reuseable "getter".
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This was all solved 10 years ago. No one was interested in it apparently.

Physics World, October 2000, page 25 to 26 or Physical Review Letters 2000 Vol. 85 page 864.. "Two scientists, Vu Thien Binh and Christophe Adessi, have solved the problem of a room temperature thermionic emitter 50 years too late. They use a 50 nm film of Titanium oxide on a nickel plate as a cathode. It has a mere 0.1 electron volt thermal barrier which is satisfied by a little above room temperature. The technique is robustly stable in the normal vacuum tube environment against adsorbed surface contaminants as well as ion bombardment. Previous attempts to achieve this using micron scale pointed field emitters were not robust or stable." Two construction techniques for making these new cathodes are suitable. One is vacuum sputtering, the other is sol-gel plating. The latter is similar to electro-plating thru a gel solution. (See references in articles)

One can afford to use large cathode areas to make high current tubes this way. The current density of the cathodes is quite good also. No filaments to ever burn out either. The microwave tubes for satellites are converting to this technique. Also is of interest to flat panel display makers. A TO-264 ceramic package could be made with a planar cathode, laser micro-perforated grid sheet, and a copper plate/heatsink-mounting tab (ceramic insulating layers between). A power resistor on the main heat sink would be powered up initially, at turn on, to get to working temperature for the cathodes. The close planar electrode spacing and high current emission capability would allow operation at SS like plate voltages.
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