• 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.

interesting thought

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It may seem a bit radical and far fetched, and perhaps not too practical, but out of curiosity, would it be possible to construct a fantastically powerful audio amp with high power klystrons (like those normally used in huge radar installations)? And have it sound good? If it were, what would it entail? (not that I have the time, money, experience, power source, or space to even think about actually doing it)
Build your own tube!

Its possible to build multi-ampere tubes easily nowadays. Have a look at these journals: 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 easily by just 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)
Since no heater or filament is required with this technology, one can afford to use large cathode areas to make high current tubes. The current density of the cathodes is quite good also. No filaments to ever burn out either. Maybe could even get some tube manufacturer interested, although they probably prefer making tubes that burn out. The microwave tubes for satellites are converting to this technique. Also is of interest to flat panel display makers. Maybe somdeday could just use a flat panel display made this way as one big audio tube.
Kilowatt said:
WOW! How come I've never heard of that? How come they're not in mass production, completely replacing traditional tubes?
<LI>I saw nothing about the linearity of these tubes. If they're not linear, they're of no use in audio.
<LI>The only companies that would be interested in making these for the audio market are those companies that already make tubes for the audio market. This would KILL their profit, as there would be a very low need for replacements.
<LI>This appears to be a completely new process. Figuring out how to build a high power, audio quality, tube will take time, and a lot of experimentation.
<LI>Getting to the point where you could use these as output tubes in a power amp would cost millions. The demand for audio tubes isn't seen as being high enough to give an adequate return on investment.
</OL>This technology will, eventually, spread to the audio market, but it will take time. I would guess that in 5-10 years, these tubes will be available as high-power audio output tubes.

The first step would probably be made by some company buying a case of the current microwave tubes, and using them to build an audio pre-amp.

They do appear to be good for an OTL design :D .
new tubes

The linearity of tubes is mostly determined by the mechanical design geometry of the cathode/grid/plate. Constant Mu tubes usually have all three either circular or elliptical or flat cross sections. The grid essentially has to sit on an equipotential surface between cathode and plate and have constant grid wire spacing. The grid wire spacing must be less than the grid to cathode distance. See Spangenberg's book "Vacuum Tubes" chapters 7 and 8 for tube design formulas. So could just substitute the new cathode surface into an existing design to get similar characteristics.
As to the availability of new design tubes, I think it is essentially just the usual supply and demand thing. Until people demand some supply there won't be any supply. Right now, I don't think anybody even knows about this technology being available. There may be some question of aesthetic appeal too since cold cathodes don't incandesce like traditional tubes. Maybe will need to put some phosphor on the plate surface to make them light up a bit! The tube makers will need to see some significant demand since their sales will soon fall off with no burn-out tubes, but in the long run I think sales of tube designs would grow significantly if they became long term reliable. Just think, no more adjusting bias levels as tubes age. No more change in sound quality with tube age either.
Another interesting area is microlithographic tubes, ie. chip designs with thousands of micro sized tubes. After all, for the same size device, vacuum is 100s of times faster than solid state for electron transit times. Think of tube based computer chips a 100 times faster than todays or microwave power devices a 100 times faster than current solid state devices. Or maybe a microlithographic v. tube Op. Amp. chip!
Hmmm, I just looked at that paper again:

top of page 13,
and it looks like you need around 350K (170 F) to get the high current density with low voltage. So would need a power resistor to warm up the cathode. Maybe some further progress has been made in the last 8 years since that was published. Or wait till global warming brings room temperature up a bit.

Why not just stick to a big tube like this Eimac rated at 17,000v
and 60A or 2,000,000 W wonder what a matched pair would cost.
And it provides a lot of hot water!


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