Audio and Video Talk > Valves / Vacuum Tubes

The function of the electron tube (DISCUSSION)

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ludo:

--- Quote from: ghostinthemachine on May 27, 2009, 05:38:43 PM ---...
When the grid's potential equals that of the cathode (NEGATIVELY CHARGED) no electrons can pass through to the plate or CATHODE. So we will have no electron flow.

But if we make the grid more positive than the CATHODE electrons will rush towards and then past the GRID. We now have some control over the flow of these billions of tiny electrons.                   

--- End quote ---

Not quite. ;)

Dew, this will possibly sort of work for some triodes, but not all. The grid is usually made a couple of volts more negative than the cathode in order to shut off conduction through the tube. As the grid becomes less negative with respect to the cathode (ie as the grid "moves to the positive side of things" while remaining negativecompared to the cathode), at some point, conduction begins.

There are tubes with which one can make the grid more positive than the cathode, but I believe those are a bit rare in audio. Mars and F_D and Ampdog probably know lists of those off by heart, but I hope they will agree that triode grids are usually at lower voltages than cathodes, at least in our dealings with them.

Ie, triodes are depletion devices. Ie they conduct like there's no tomorrow when grid voltage=cathode voltage and there is (+) voltage on the anode.

If the grid and cathode are at the same voltage, the grid provides no "shielding/blocking" between the cathode and anode. N channel JFets are similar.

No how does one make the grid more negative than the cathode when you have no negative supply rail? Or am I getting ahead of you...?

Viagara:
Forgive me, but how does a pentode work in comparison?

ludo:

--- Quote from: alphabet on May 27, 2009, 07:53:30 PM ---Forgive me, but how does a pentode work in comparison?

--- End quote ---

Similarly, Abc. There are some issues (secondary emission?) relating to when those electrons meet the anode to which they are attracted, that foul up the operation of the triode. These are solved by extra grids. The extra grids then have interesting effects on the operation of the valve too.

But I think the triode basics had better sink in first before we go there. Especially since I know very little about valves and will be out of my depth here soon enough ;D

iondb:
MM,, interesting info.  Keep it coming....step by step.....

ghostinthemachine:

--- Quote from: ludo on May 27, 2009, 07:34:30 PM ---Not quite. ;)
Dew, this will possibly sort of work for some triodes, but not all. The grid is usually made a couple of volts more negative than the cathode in order to shut off conduction through the tube. As the grid becomes less negative with respect to the cathode (ie as the grid "moves to the positive side of things" while remaining negativecompared to the cathode), at some point, conduction begins.

--- End quote ---

Stop the bus, hold it.... I am coming to that... to the BIAS current. First we need to discuss the bare basics.

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