> 200 amps Something I’ve been curious about: is the current actually required for the thermionic effect, or just the heat? Could you lower the current requirement by thermally insulating the tubes?
[deleted]
A modern 8 bit design, built using 1950s thermionic valves
21–30 of 87 posts
Re: A modern 8 bit design, built using 1950s thermionic valves
#22> 200 amps Something I’ve been curious about: is the current actually required for the thermionic effect, or just the heat? Could you lower the current requirement by thermally insulating the tubes?
I did a quick search for the specs, and, if I am reading it right, Wikipedia [1] gives the filament current as being 350mA at 6.3V, a dissipation of 2.2W. As that current sums to 196A for the 560 tubes, I suspect this is what the 200A figure refers to.
As the tubes are at high vacuum, they are already well-insulated, so I imagine that most of the heat loss is via infra-red radiation. I have a very vague recollection that, in thermionic tubes, the anode has to be kept reasonably cool so that it is not emitting electrons itself. I would be surprised if there are any low-hanging fruit to be plucked here, especially given that vacuum tubes were important technology for a half-century.
Re: A modern 8 bit design, built using 1950s thermionic valves
#23> Every one agrees that the Turner Prize is much more than just a display of virtue signalling by the cultural elite, and I have decided to enter the Valve.Computer for the prize.
Uh, what? Where did this sudden "virtue signaling" by the "cultural elite" stuff come from?
Re: A modern 8 bit design, built using 1950s thermionic valves
#24"But most important of all, is to have a lovely wife, who knows you're daft as a brush, and that life together is brilliant."
Hold out for the partner that cheers you on when you're doing what you love.
Re: A modern 8 bit design, built using 1950s thermionic valves
#25[deleted]
Re: A modern 8 bit design, built using 1950s thermionic valves
#26> 200 amps Something I’ve been curious about: is the current actually required for the thermionic effect, or just the heat? Could you lower the current requirement by thermally insulating the tubes?
Current into a low-resistance "heater" element is used to produce the heat required for Thermionic Emission [0] in a vacuum tube. You only need the heater/emitter to be hot, and insulating the tubes would just spread the heat around to everything inside of it — at some extreme, making everything into an emitter, instead of elements that control the emission. [0] https://en.wikipedia.org/wiki/Thermionic_emission
This is not really accurate. To get meaningful emissions from normal electrodes, you need to heat them up to about 2000 °C. Vacuum tubes operate at 700 °C or something like that. The trick is that one electrode is doped with special rare-earth additives that greatly increase electron emissions. The same treatment isn't applied to the rest of the device. So, even if all internal components have the same temperature, a vacuum tube can still work (to some extent).
Re: A modern 8 bit design, built using 1950s thermionic valves
#27> 200 amps Something I’ve been curious about: is the current actually required for the thermionic effect, or just the heat? Could you lower the current requirement by thermally insulating the tubes?
The high voltage from my understanding is to be able to handle the high inrush current to the heaters on power up, I believe after that the requirements are lower (parroting what I've read written by Eric Barbour on his metasonix vacuum tube synthesizers)
Re: A modern 8 bit design, built using 1950s thermionic valves
#28Earlier quoted context omitted.
Current into a low-resistance "heater" element is used to produce the heat required for Thermionic Emission [0] in a vacuum tube. You only need the heater/emitter to be hot, and insulating the tubes would just spread the heat around to everything inside of it — at some extreme, making everything into an emitter, instead of elements that control the emission. [0] https://en.wikipedia.org/wiki/Thermionic_emission
This is not really accurate. To get meaningful emissions from normal electrodes, you need to heat them up to about 2000 °C. Vacuum tubes operate at 700 °C or something like that. The trick is that one electrode is doped with special rare-earth additives that greatly increase electron emissions. The same treatment isn't applied to the rest of the device. So, even if all internal components have the same temperature, a…
100% correct and I appreciate the additional details! I couldn't come up with a good analogy to explain you want the emitter as a separate and unique element from everything else involved in a tube — oversimplified in the process.
Re: A modern 8 bit design, built using 1950s thermionic valves
#29"But most important of all, is to have a lovely wife, who knows you're daft as a brush, and that life together is brilliant." Hold out for the partner that cheers you on when you're doing what you love.
> Hold out for the partner that cheers you on when you're doing what you love.
If only it were that easy. Those kinds of people are few and far between. I’m almost 50 and I don’t think I’ve ever even met a woman who has actively encouraged me about a single thing. I’ve been married twice and have 4 daughters.
Re: A modern 8 bit design, built using 1950s thermionic valves
#30> 200 amps Something I’ve been curious about: is the current actually required for the thermionic effect, or just the heat? Could you lower the current requirement by thermally insulating the tubes?
Really it's both ways in terms of required current. The thermionic effect requires filament heat, which since being a filament, take some amount of current, often around half an amp, some more some less. A filament could, in theory, be run at any current though so long as the power through the filament stays the same and the voltage is kept low enough not to arc to adjacent parts. There is likely also a minimum current requirement (since you need a source for those free electrons), which often then implies some non-linearity at the low end.
As others have mentioned, the 200amps of this case could be reduced substantially by running filaments in series (can be done with 6.3v heaters as the error from a common 5v or 9v supply is more than if you pair them up and use a 12V supply) though this introduces the failure mode of old Christmas tree lights.
Source: I make vacuum tubes.