Electronics project notes/Vacuum tubes: Difference between revisions
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The basic principle of most vacuum tubes centers around [[thermionic emission]]: | The basic principle of most vacuum tubes centers around [[thermionic emission]], a.k.a. '''thermal electron emission''': | ||
you heat a metal (filament), which makes it easier for electrons to come loose | you heat a metal (filament), which makes it easier for electrons to come loose | ||
This is also used in CRTs, certain lightbulbs - in rather different ways. | |||
In | |||
{{comment|(for context, yes, 'current through a filament' is broadly the same as making an [[incandecent]] lightbulb, but the engineering is a little different - you can get thermionic effect from different filaments, and whether they glow visibly is a side effect from a different goal)}} | |||
In our atmosphere (or other gases at the same pressure) this would do very little, | |||
because these loose electrons would mostly collide with that gas, and slightly heat it, with no other effects. | |||
It's only in a near-vacuum that it starts to be useful. | It's only in a near-vacuum that it starts to be useful. | ||
The reason | |||
Probably didn't hurt that they're easier to inspect and maintain this way. | {{comment|The reason vacuum tubes, as a product, are in glass seems to have been production convenience of the time. | ||
And look pretty cool. | Probably didn't hurt that they're easier to inspect and maintain this way. And look pretty cool.}} | ||
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The first tube was a simple two-plate thing: a heated cathode, and an anode. | The first tube was a simple two-plate thing: a heated cathode, and an anode. | ||
The only thing this does is act as a diode (literally 'two electrodes'), | The only thing this does is act as a diode (literally 'two electrodes'), [https://en.wikipedia.org/wiki/Thermionic_emission#History] | ||
used as an early switch and rectifier. | used as an early switch and rectifier. | ||
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The next logical step, and much more interesting, is a triode. | The next logical step, and much more interesting, is a [https://en.wikipedia.org/wiki/Triode triode]. | ||
A triode takes that diode setup and puts a metal grid in the middle. | A triode takes that previous diode setup and puts a metal grid in the middle. | ||
If that grid is at a negative potential compared to the cathode, it draws away the electrons | If that grid is at a negative potential (as compared to the cathode, the source), | ||
it draws away the electrons into that grid, and means they won't arrive at the anode. | |||
And at some potential difference it stops acting as a remove-everything valve, | |||
and starts allowing through the electrons. | |||
It turns out that a | It turns out that a (=voltage) on the grid has a lot of effect | ||
on the often-larger current of the overall flow in the tube | on the often-larger voltage-and-current of the overall flow in the tube. | ||
As such, a smaller voltage could control a larger current, | |||
and that's a ''really'' useful property, | |||
because (aside from other uses) that basically makes it an amplifier - specifically a voltage-controlled current amplfier. | |||
Sure, a triode's gain is fairly low, often measured in milli-amps per Volt, and there are other limitations, but it's still useful. | |||
There are also tetrodes (two grids) and pentodes (three grids). | There are also tetrodes (two grids) and pentodes (three grids). | ||
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Roughly speaking, | Roughly speaking, | ||
the principle is the same, | the principle is the same, | ||
but their properties work around some limitations. | but their design properties help work around some limitations. | ||
They provide larger amplification, alleviate capacitance issues | They provide larger amplification, alleviate capacitance issues, and a few other things. | ||
Note that triodes exist to this day. For example, the [https://en.wikipedia.org/wiki/12AU7 12AU7 / ECC82] and [https://en.wikipedia.org/wiki/12AX7 12AX7 / ECC83], once common, still made, are two triodes in one tube. | |||
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Are tubes high voltage? | "Are tubes high voltage?" | ||
In most practical uses, yes. | In most practical uses, yes. | ||
While the heater is often a ~6V device (partially just convention), | |||
it's the cathode that you usually energize to over 100V, | |||
in part because it's more predictable and the response more linear that | in part because it's more predictable and the response more linear | ||
(though in most uses that also implies you need similarly higher voltage on the grid, | at that point (though in most uses that also implies you need similarly higher voltage on the grid, | ||
so it's a design-specific tradeoff) | so it's a design-specific tradeoff) | ||
So most tubes are designed to be operated at | So most tubes are designed to be operated at voltages we now consider wallpower {{comment|(though portable tube radios had ~22, 45, ~67, or 90 volts batteries at the time -- they had three batteries, A, B, and C, for three distinct parts of such radios)}} [https://antiqueradio.org/bsupply.htm] | ||