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Old 21st September 2006, 05:24 AM   #1
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Default Tubes and minority carriers

I know that tubes have electrons as their majority (and only) carrier; they flow from the cathode to the anode.

When a high velocity electron flows to the anode and strikes it so hard that it bounces off, the electron flows "against the grain", so I guess I can see that it would have a minority carrier-like effect (it flows against the traditional current, so it would decrease the overall net effective current flow), but does this count as the deflected electron being an actual minority carrier?

Do tubes really have minority carriers?
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Old 21st September 2006, 05:35 AM   #2
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Default Re: Tubes and minority carriers

Quote:
Originally posted by rtarbell
I know that tubes have electrons as their majority (and only) carrier; they flow from the cathode to the anode.

When a high velocity electron flows to the anode and strikes it so hard that it bounces off, the electron flows "against the grain", so I guess I can see that it would have a minority carrier-like effect (it flows against the traditional current, so it would decrease the overall net effective current flow), but does this count as the deflected electron being an actual minority carrier?

Do tubes really have minority carriers?
No. Secondary emission, what you're describing, does have the effect of causing greatly increased screen currents at low plate voltages. This causes nasty nonlinearities, and if carried far enough, can lead to a negative resistance characteristic that causes oscillation. Of course, this has been exploited where oscillators are needed.

Considerable research, resulting in the pentode and "kinkless tetrode" (a.k.a. beam tetrode -- done so as to get around the pentode patents) was undertaken to stop that, or at least greatly minimize the cause.

Minority carriers are strictly a solid state phenomonon resulting from intrinsic conduction electron/hole formation independently of the extrinsic "doping" to form N and P type semiconductor. Without a crystalline lattice within which a "hole" may form, there are no minority carriers. About the cloest thing in a vacuum tube would be a positive ion, but those, too, are undesireable, and of no use for making "P tubes".
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Old 21st September 2006, 05:45 AM   #3
poobah is offline poobah  United States
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Only when an arctic penguin delivers your tubes.

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Old 21st September 2006, 06:18 AM   #4
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Quote:
Originally posted by poobah
Only when an arctic penguin delivers your tubes.

Click the image to open in full size.

Whaddya think of that one, Miles?
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Old 22nd September 2006, 03:37 AM   #5
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Quote:
Originally posted by Geek


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Whaddya think of that one, Miles?
Sounds like an inside joke to me.
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Old 22nd September 2006, 04:53 AM   #6
poobah is offline poobah  United States
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Sorry Miles,

The arctic penguin was the only minority I could come up with that couldn't get angry...



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Old 22nd September 2006, 03:50 PM   #7
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Ionization gauges for high vacuum systems routinely use ion currents. Just need a gassy tube (preferably hydrogen gas) and reverse the voltages. Too much gas and too high a neg. plate voltage though, and the ions will cause additional gas ionization and produce a plasma arc over, like a thyratron.

Another wild idea: Use one of those bar room plasma thingies with two plate electrodes inserted. Then modulate the length of the plasma arc via a magnetic field. Might vary the plasma end to end resistance, so could use it as a resistance modulator or "amplifier". (not too energy efficient though, you'll need a power amp to drive the electromagnets)

Of course, you can always just electronically emulate a P-triode:
http://www.diyaudio.com/forums/showt...ht=#post967206

Don
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