Author Topic: The function of the electron tube (DISCUSSION)  (Read 7221 times)

Offline ghostinthemachine

The function of the electron tube (DISCUSSION)
« on: May 27, 2009, 11:34:45 AM »
The TRIODE Electron Tube Operating As An Amplifier
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To understand the most basic principle of electron tube operation you have to understand how a vacuum tube diode works.

Electrons are "boiled" off the heated (hot) CATHODE - In some cases the CATHODE are directly heated and in other the CATHODE is indirectly heated. The "free" electrons can now travel quite easily through the vacuum inside the tube. If no voltage supply is connected to the tube the electrons will go nowhere.

Letís connect the diode in a basic circuit and see what happens. The heated cathode shall be connected to the ground plane of our experimental circuit. The negative pole of the high tension (HT) direct current supply shall be connected to the ground plane as well. Now an ampere meter can be connected, in series with the positive pole of the HT supply to the ANODE of the tube. The ampere meter will indicate that current is flowing.

So, in forward current operation using the diode tube, the ANODE is positively charged so that it electrostatically attracts the emitted electrons (that is boiled off by the heat). However, electrons are not easily released from the unheated ANODE when the voltage polarity is reversed. 

Now we have established that there can only be electron from the negatively charged, heated CATHODE to the positively charged ANODE.

So how does a triode work? To be continued...

________________________________________________________________

Remeber, this discussion is open to questions and suggestions, so fire away.

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Offline ghostinthemachine

Re: The function of the electron tube (DISCUSSION)
« Reply #1 on: May 27, 2009, 05:38:43 PM »
So what happens next? Take a vacuum tube diode and place a fine metal grid between the ANODE (plate) and CATHODE. Now you have a triode.

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.                   
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Offline ghostinthemachine

Re: The function of the electron tube (DISCUSSION)
« Reply #2 on: May 27, 2009, 05:45:17 PM »
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Offline ghostinthemachine

Re: The function of the electron tube (DISCUSSION)
« Reply #3 on: May 27, 2009, 05:46:24 PM »
Above is a basic diagram for a triode amplifier.
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Offline Viagara

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Re: The function of the electron tube (DISCUSSION)
« Reply #4 on: May 27, 2009, 07:11:09 PM »
Sounds essentially like an electric globe :D
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Offline ludo

Re: The function of the electron tube (DISCUSSION)
« Reply #5 on: May 27, 2009, 07:34:30 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.                   

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...?

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Offline Viagara

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Re: The function of the electron tube (DISCUSSION)
« Reply #6 on: May 27, 2009, 07:53:30 PM »
Forgive me, but how does a pentode work in comparison?
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Offline ludo

Re: The function of the electron tube (DISCUSSION)
« Reply #7 on: May 27, 2009, 11:42:09 PM »
Forgive me, but how does a pentode work in comparison?

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

If it 's going vrot, put it on the buffet ! - Fats

Offline iondb

Re: The function of the electron tube (DISCUSSION)
« Reply #8 on: May 28, 2009, 08:10:47 AM »
MM,, interesting info.  Keep it coming....step by step.....

Offline ghostinthemachine

Re: The function of the electron tube (DISCUSSION)
« Reply #9 on: May 28, 2009, 08:20:26 AM »
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.

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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Offline ghostinthemachine

Re: The function of the electron tube (DISCUSSION)
« Reply #10 on: May 28, 2009, 08:29:52 AM »
Sounds essentially like an electric globe :D

Thats how it all started!

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

Rustig, we are coming to that....

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Offline Ampdog

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Re: The function of the electron tube (DISCUSSION)
« Reply #11 on: May 28, 2009, 10:13:52 AM »
So what happens next? Take a vacuum tube diode and place a fine metal grid between the ANODE (plate) and CATHODE. Now you have a triode.

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.                   

GITM,

I am afraid you will have to reign in a bit. Firstly, you begin by calling the 'emmitter' of electrons the cathode (correctly), but in the following paragraph you say 'plate or CATHODE'. That would be 'plate' or ANODE.

Then there is a problem with the description of the grid's role. It is untrue to say that no electrons can pass through when the grid's potential equals that of the cathode. It is the opposite; maximum electrons then pass through! The electrons have just left a surface, how can they then be blocked by another 'surface' at the same potential? The  anode sits close by at perhaps 100V+, attracting the electrons through the grid (a coil of fine wires quite far apart), and this gives us the condition of maximum current in a triode in amplifier design. In fact in amplifier design one never works with positive grid voltage. The point of zero anode current (stopping most electrons) will be at some -25 grid V for a 12AU7, and -4V for a 12AX7. The signal will swing somewhere between those limits, depending very much on several other factors, which will have to be dealt with before one goes on. How is the anode current changed, how is this converted to a voltage change, why do triodes have specific characteristic [otherwise you will not later be able to point out why tetrodes (pentodes) were designed].

I would further suggest one also begins with the cathode as 0 potential, not negative, otherwise we will jump back and forth between + and - without knowing what it is referred to in the first place. I don't want to interfere too much with your explanations, but as said earlier, unless the first steps are correctly understood and treated in the correct sequence, misconceptions difficult to overcome later will manifest themselves.




Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline ...

Re: The function of the electron tube (DISCUSSION)
« Reply #12 on: May 28, 2009, 10:17:20 AM »
Now this is starting to sound like someone I can take advice from.

Offline Viagara

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Re: The function of the electron tube (DISCUSSION)
« Reply #13 on: May 28, 2009, 07:19:17 PM »
Now this is starting to sound like someone I can take advice from.

Absolutely! Okay, I have to admit that he loses me in 60 seconds flat,  but he(Ampdog) is extremely knowledgable as well as one of the true gentleman of audio ;)
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Offline ghostinthemachine

Re: The function of the electron tube (DISCUSSION)
« Reply #14 on: May 28, 2009, 07:55:17 PM »
Absolutely! Okay, I have to admit that he loses me in 60 seconds flat,  but he(Ampdog) is extremely knowledgable as well as one of the true gentleman of audio ;)

That is why I want him, Ampdog, to C O N T R I B U T E or at least guide the discussion.

Yes I might have made an error calling the bloomin plate the CATHODE but please, give a guy a break. Each one is probably entitled to their quota of mistakes???
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