Talk:Vacuum tube

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Contents

[edit] Sugestion for External Link

To whoever is "managing" this page, may I suggest an external link to the "Heath Robinson Rube Goldberg (HRRG) Mixed Technology Computer" project at http://www.diycalculator.com/sp-hrrgcomp.shtml. The idea is to create a computer out of a mixture of implementation technologies, including relays, vacuum tubes, transistors, simple integrated circuits, pneumatic logic, magnetic logic, and so forth.

[edit] durchgriff means transconductance, not "transparency"

durchgriff had been translated as "transparency."

I don't know German, so I'm making the change tentatively. I'm basing it on these judgements:

  • In context, "transparency" doesn't make sense.
  • In context, "transconductance" makes perfect sense.
  • It's easy to believe that the word durchgriff, which babelfish.altavista.com translates as "reached through," might be the word for transconductance.
  • A Google search on "durchgriff transconductance" turns up:
http://branchenportal-deutschland.aus-stade.de/et13.htm transconductance=Durchgriff=Gegenleitwert,

and

http://webcoordinator.de/englisch/dd10.htm Durchgriff=transconductance=reciprocal of amplification=reciprocal of amplification factor

Those last two are good enough for me.

[edit] why is it a good amplifier?

The following sentences in the article are unclear to the layman (of which i'm _almost_ one):

"As the applied voltage of the grid varied from negative to positive, the current of electrons flowing from the filament to the plate would vary correspondingly. Thus the grid was said to "control" the plate current. The resulting three-electrode device was therefore an excellent amplifier"

I had to go back and read this, and try to reason out why the assertion in the last sentence was true. I eventually worked it out, but I know a bit more about things of this nature than the average joe (but not by much).

It seems only implied that the input signal actually comes in on the grid, which mangles the (much higher) constant waveform current flowing from the filament in such a way as to reproduce the input signal's waveform, but with greater amplitude.

which brings me to the next question: why is the amplitude larger at all? i understand how the varying voltage on the grid controls the output wave, but where/why 's the amplitude multiplication happening?

Thanks for this great article, i've been wondering about this stuff since i was a kid :)

Consider these typical figures for a 6SN7 tube [1]. The input to the grid has an amplitude of about 1 or 2 volts and the input impedance of the grid is of the order of a megohm. This means that the input power (V²/R) is no more than about 2²/106 or 4 microwatts. The output from the plate is a current of a few milliamps (from an external power supply, which is where the added power comes from) with an amplitude of maybe 60 volts. This gives an output power (I×V) of about 2×10-3 × 60 = about 120 milliwatts. The power gain is therefore about 120×10-3 / 4×10-6 = 30 000 times (about 45 decibels). I didn't take much care over this calculation, so I could be out by a factor of 10 or 100, but I hope you get the idea. --Heron 08:19, 6 Sep 2004 (UTC)
Considering power gain is not really the way to consider tubes. I would argue that the input impedance is much higher than your assumption. There are specialist tubes known as electrometers. These are constructed to have extremely high input impedances in the range of 10^15 ohms. These would appear to have power gains measured in the billions, but the real gain (actually a current gain as they rely on positive grid bias) is relatively modest at just a few hundred. The power really comes from the plate supply.
I was always confused by amplifiers (specifically transistors) until I realized that they are not creating any power, they're simply changing their resistance to a source of constant power that already exists, and diverting it or directing it into a load. A transistor or tube by itself is not an amplifier, even though everyone calls them that. It needs to be in a circuit with a power source, etc. and the entire circuit is the amplifier. Actually, I still get confused by them, but that helps.  :-) - Omegatron 14:36, Sep 6, 2004 (UTC)

[edit] Illustration

The first picture the reader sees should be an actual photograph of a real-life tube, not a diagram of internals. Image:6L6tubespair.jpg seems the best of the options, althought the rez is low - I might have a couple I could photograph at a higher resolution. Stan 21:39, 16 Nov 2004 (UTC)

[edit] Number of grids and elements

I think perhaps a little explanation could be helpful of the number of elements verses the number of grids verses the last digit of the common valve numbering. For example, a triode-pentode has 3+5=8 elements, 1+3 grids, and a common example is the 6BL8. But the given example of a pentagrid converter is a heptode (7 elements), yet the valve/tube number (6BE6) ends in a 6. I guess this is because grids 2 and 4 are connected together; this enables it to become a 7-pin valve, which has size advantages over the other common size (9 pins). The supression grid (g5) is internally connected to the cathode, as is common with pentodes and other types with 3 or more grids. But this internal connection doesn't seem to warrant reducing the final digit to 5. I could have a go at this, if it's considered helpful. --Mike Van Emmerik 01:16, 5 Jun 2005 (UTC)


[edit] Question

Can you replace a vacuum tube with a transistor of the same properties in one of those really old TVs? -Litefantastic 01:02, 17 Jun 2005 (UTC)

Yes, but good luck finding a transistor that really does have the same properties. (In short, generally no) --Carnildo 03:26, 17 Jun 2005 (UTC)


Depends very much on what valve! Replacing the full-wave rectifier that produces the HT with a pair of solid-state diodes has been a pretty standard service technique for several decades now. But finding any sort of replacement for the more esoteric tubes is a nightmare, or a good challenge depending on why you're doing it. This is especially true for specialised TV tubes, as I guess you're finding out.
Try your local amateur radio clubs, the memberships of these worldwide are getting older and you may well find some of the old boys have stocks of valves they couldn't bear to throw away. Many things like line output valves that were developed for TV found applications in amateur radio. Andrewa 8 July 2005 20:47 (UTC)

[edit] British vs American usage

I challenge the theory that valve for a vacuum tube is only British English. Every electric guitarist in the world knows what valve tone means. Nobody calls it tube tone, even in American magazines. So it's a bit more complicated than that! Andrewa 8 July 2005 20:47 (UTC)

Well... I own an electric guitar, but have never heard the term "valve tone" before. Heh. But anyway, I would think the term is pretty community-specific. - mako 04:28, 10 July 2005 (UTC)
I stand corrected. Every guitarist I've spoken to before this has used the terms interchangeably, eg describing vintage Marshall amplifiers (British) as being all tube and Fenders (US) as having valve tone. But my sample space isn't all that big, and of course is biased towards Australia, which is probably midway between the two linguistically. Which is where the wiki comes in! Thanks for your contribution. Andrewa 00:55, 12 July 2005 (UTC)

[edit] Nikola Tesla

I removed this passage:

"In April 1887, Nikola Tesla developed a single node vacuum tube (similar to his U.S. Patent 514170 ). The form of the tube is a node of refectatory material which is supported by a conductor entering a highly evacuated globe or receiver. A conducting screen (later called the grid) surrounds the supporting conductor. Tesla later produced x-rays with this tube."

while it may be true it is not relivant to the history of Vacuum tube since Tesla did not exploit this apparatus in any way meaningful to the development of the device. DV8 2XL 00:39, 25 October 2005 (UTC)

Tesla did not exploit a great many device he developed. It is relevant to the history, being one of the 1st. Also, his work known was widely known ... and the likes of Fleming, Marconi, and DeForest knew of Tesla's device (and associated work). J. D. Redding

It would seem surprising if he hadn't tried to make his work known as best he could and less surprising if someone else used his results without crediting him.David R. Ingham 05:57, 26 October 2005 (UTC)

The statement needs a source if it wants to remain. And what does "refectatory" mean? - mako 21:19, 25 January 2006 (UTC)
It's widely known that "Nikola Tesla began to investigate X-rays using high voltages and vacuum tubes of his own design, as well as Crookes tubes". This was stated in the Electrical Review in 1896. See http://www.teslasociety.com/biography.htm. J. D. Redding 08:46, 26 January 2006 (UTC)
Put in "Crookes tubes were 19th century evacuated glass tubes with 3 node elements (one anode and two cathodes) developed by William Crookes and is an evolution of the earlier Geissler tube." to round out the _prehistory_. J. D. Redding

"Refectatory" means heat resistant, which is needed in cathodes. David R. Ingham 05:13, 26 January 2006 (UTC)

Isn't the proper modern spelling "refractory"?
Reddi, an x-ray tube is irrelevant here. What this article is concerned with is not vacuum tubes in general, but tubes like diodes and triodes that saw widespread use. Otherwise we'd have to put all sorts of stuff in here. - mako 20:47, 26 January 2006 (UTC)
I rewrote the intro of the section with some general evacuated tube discussion. X-Ray tube would be an appropriate place for specifics. - mako 22:56, 26 January 2006 (UTC)

[edit] What!?

Does anybody have a reference to the statement that's says: "some ww2 radios had tubes integrated into the chassis"?

I've seen hundreds of WW2 radios and have never seen that particular technology.

I have seen subminiature tubes slipped into holes drilled in a casting, for use in early rocket telemetry, but that was neither WW2 nor "integrated".

That sounds like it would be hard to replace the tube, when it burned out. David R. Ingham 05:06, 1 February 2006 (UTC)

[edit] AES whitepaper is old

I added a note. I hope people don't read the AES whitepaper and think that it still applies after a third of a century, I tried to keep it neutral but if someone can phrase the note better they'd be welcome. A few points of interest:

  • In the early seventies, there were few op amps. The venerable 5534 didn't come out until later that decade. The 741 was around, but it has a low enough slew rate and gain-bandwidth that using it in audio circuits is really a joke.
  • PNP transistors couldn't pull their own weight yet. This means power stages were usually NPN only, which is a pretty nasty design limitation.
  • Early transistor audio circuits were capacitor-coupled, like their tube counterparts. Unfortunately, the capacitors had to be large because transistor circuits were typically lower impedance. This meant electrolytic capacitors, which some consider to have unacceptably poor audio performance.

Klodolph 21:03, 23 February 2006 (UTC)

[edit] "Tubes Don't Wear Out" ???

This line is misleading and not totally true: "(Tube cathodes do not "wear out" as lamp filaments do; this is a common misconception among laymen.)"

This MAY be virtually true for SMALL Signal tubes of very controlled construction. It is NOT TRUE, in my opinion, for small or large Power tubes, such as the 6L6, 5881, 6146 etc. I tracked hundreds of these tubes for thousands of hours during many years as a Broadcast Engineer. There was a predictable loss of electron emission in power tubes that is not related to "poisoning".

Unless someone has recent data to show that the above general statement applies to most tubes in general use today, it should be removed or qualified. Terry King 23:51, 26 February 2006 (UTC)

Is this loss of emission due to filament material evaporating (which is what happens to light bulbs), or is it due to filament poisoning as described in that paragraph? --Carnildo 00:19, 27 February 2006 (UTC)
The failure mechanism is not really too well understood. In early devices with bright emitting filaments, the filament failure was all too obvious. Later, when coated indirectly heated cathodes were the norm, things were much less clear.
In the late 1930's and 40's, Mullard (Philips) produced millions of EF50 devices, initially for television use and later for radar. The reason millions were produced was that they had a very short life, though being a signal pentode, there was no apparent reason for this. If one is encountered today, it is most likely worn out. By contrast, Mazda of the UK produced a range of devices in their AC/ range. These were produced for AC powered radio receivers and thus, like the EF50, were indirectly heated (and featured many pioneering constructional details - much copied, even by Mullard in their EF50). This range of devices were extremely rugged, and any failure of a radio was seldom a tube failure (even the frequency changer (AC/TP - Triode-Pentode), often a common failure, seldom failed).
Indeed many repair shops who had equipped themselves with a range of spares would have had them still on the shelves when the radios became obsolete. Indeed, any encountered today are probably brand new and may even be complete with the original box. There is a documented case of the BBC removing a AC/HL (triode) from a piece of equipment and when tested, it met the original specification. The BBC records, which were scrupulously kept, showed that the tube had notched up 250,000 hours of use. I do not claim that such a life was typical however.

Cesium cathode materials with low work functions migrate especially quickly, even though they run at lower temperatures. The vacuum SAGE computers were use in pairs, so one could operate while tubes were changed in the other. Very sensitive photomultiplier tubes are damaged by exposure to full sunlight. David R. Ingham 01:16, 27 February 2006 (UTC)

My understanding, from the vacuum tube manufacturers publications in the 60's and 70's, is that the Cathode material (We're talking about indirectly-heated cathode type tubes) actually became 'consumed' over time. Cathodes could be "rejuvenated " by temporarily overheating them which migrated material to the surface. Cathode Ray Tubes (AKA TV Picture Tubes) were sometimes "rejuvenated" with a combination of temporary overheating plus drawing a High Voltage Arc from cathode to grids, which tore small (often visible) pits in the cathode material which exposed new emissive material.
I think that there are different modes of failure at work here. In small signal tubes the cathode coating would seem to be consumed. However, in shadow mask colour TV picture tubes, the beam current was very high for the available cathode surface area. This was necessitated because the shadow mask considerably attenuated the beam by masking off most of the beam current. As a result, the cathode developed a non emissive skin. Because the material underneath was largely still able to emit, the 'repair' technique was to pass a large current through the cathode to the the grids and anodes or plates all paralled together. This burnt off the skin (I remember the material glowing as it flew off, but I certainly don't remember arcing (difficult in a vacuum anyway). The repair did not usually last very long, and the best the owner could expect was an extra year or so out of his tube.
I think we need to go in the direction of informing readers rather than arguing some arcane mostly-theoretical property. The bottom line for all typical vacuum tube equipment such as radio and TV receivers and Audio equipment, is that small-signal "Receiving" tubes will suffer a significant loss of emission over several years, and indirectly-heated small power tubes will lose enough emission to change their power output and distortion results in 2 to 5 years. That's what readers who read this section need to know. Comments?? Terry King
Far from impressing this: failure of a tube in the manner you describe did happen, but it was by no means universal. A lot depended on the tube's construction and the way in which it was used. Examples of small signal tubes lasting many tens of years was also not unknown. The tubes that suffered the most in terms of a hard life were series pass tubes in regulated power supplies. The longest lasting tubes were those built specifically for the job, but used as audio power output tubes or even transmitter power output tubes, they could last (apparently at least) forever.

OK!! The current revision of this whole section looks excellent, at least from the perspective of this old guy who went to work in 1956 in a large broadcast station with ZERO transistors in it, and eventually helped design Digital Signal Processors with about a million transistors. Good work by all the contributors! Terry King Terry King 09:35, 28 February 2006 (UTC)

[edit] "http://en.wikipedia.org/w/index.php?title=Vacuum_tube&curid=32496&diff=42514854&oldid=42422334"

I don't understand what is going on. David R. Ingham 06:45, 7 March 2006 (UTC)

Some people are experimenting with a new way of organizing Wikipedia. Some other people say the experiment should be done somewhere other than actual articles. --Carnildo 08:52, 7 March 2006 (UTC)

[edit] Combined vacuum tubes

I'm a little dubious about the recently added section "Combined vacuum tubes":

"Some devices included two or more vacuum tubes in the single bulb. The low frequency tubes were built this way mainly to make the devices more compact. The combination for the high frequency tubes in the sigle vacuum bulb provided better characteristics as connections between components were in the vacuum."

The first part was covered earlier in the article, and the last sentence is something I never heard of. All multi-section tubes I know of had any connections between sections made externally, except for filament. I'd like to see a source. --agr 12:15, 18 April 2006 (UTC)

The superior characteristics of interconnected multiple devices came from the short length of the electrical interconnection. One of the limiting characteristics of many tube constructions was the length of the electrode connections from the structure to the tube base. These connections were long enough to have significant series inductance and hence limit the upper operating frequency. By internally connecting structures within a single envelope, these interconnections could be kept short and hence have a low series inductance.

I understand the concept, but there were typically other components placed between tube stages, e.g. coupling capacitors and resonant circuits. The UHF tubes I am familiar with dealt with lead inductance issues by bringing the plate and grid(s) out to low impedance connections in the envelope (e.g. 2C39). Can you give an example of a multi-unit tube that has internal connections?--agr 23:01, 30 April 2006 (UTC)

An anon contributor took a section from this article into the IC article. In seeking a home for it I placed it in the article. Now that I have found it's original location I am reverting the 'contribution'. --Ancheta Wis 02:38, 7 July 2006 (UTC)
Actually, It probaly would do little harm, and perhaps some good to put a summary of this item (the German tube) in a 2 line and link paratgraph. Hyperlinks are all to the good, if they help locate information. It frequently comes up in discussions and was unique.cmacd 13:17, 7 July 2006 (UTC)

[edit] Soviet aircraft

Shouldn't this mention something about Soviet aircraft using radio tubes instead of transistors to avoid being taken down by EMP? I'm not knowledgeable enough about the subject to add it myself. Joffeloff 19:55, 18 June 2006 (UTC)

Do you mean something like this?:
Vacuum tubes inherently have higher resistance to the electromagnetic pulse effect of nuclear explosions. This property kept them in use for certain military applications long after transistors had replaced them elsewhere.

8-)--Light current 20:44, 18 June 2006 (UTC)

[edit] reliability

I see again someone has cast a slur on the reliability of tubes this time with an unfounded asetation on the lifespan of VFD tubes. I do have one applicance at home with a VFD that has lost brightness after a few years, as well as several with over ten years of continual use, and near new brightness...The mecahnism of an "overdriven" filament does not hold water. cmacd 13:19, 16 August 2006 (UTC)

You must be joking. The gradually deterioration of the display is a well documented phenomenon. I have a house full of examples. Sony Minidisc recorder (5 years old) - Only the areas immediately under the filaments are bright, the remainder has decayed. Panasonic VHS Video recorders (2 off, 8 years old) - Same symptom but fainter than when new. Microwave oven (15 years old) - Display now only visible in darkened room. Several DVD players (similar to Minidisc). I notice that someone stated that they were unreliable. Hmm. That's not really true. They do have an incredibly short life, but they are not unreliable as such - they do keep going - even if you do need an image intensifier to see them after 15 years or so!
Also the filaments are visible. If you look at one in a darkened room, the filaments are only just visible. If the display itself is too bright to see it, disconnect the HT (B+ supply). The filaments are now unmistakeable.
Let's see now. 15 year old display on a microwave oven, probably on 24 hours a day 365 days a year. That means it has been operational for in excess of 130,000 hours. By vacuum tube standards that is extremely good - hardly the symptom of an overdriven filament. The issue of course is that the filaments, like all filaments and cathodes, deteriorate gradually from new. These displays are only required to glow and will do so when the filaments' emission has dropped well below the level where an amplifier or oscillator using the same filament has long stopped working.
Unfortunately, they are often called upon to operate continuously. Although they have a life comparable with what may be expected of the technology, that life is far too short compared with the expected life span of the equipment in which it is installed - and they are not easily replaceable (and often not obtainable as spares). For a long life display, it is and always has been, the wrong technology. Fortunately, Liquid Crystal Displays with LED backlighting are becoming comparable in price terms, so the Vacuum Flourescent Display may no longer trouble us. And by the way - I have a 4 year old display where only the middle of the characters glow making it particularly difficult to read - presumably the filaments run over these areas. 20.133.0.14 07:57, 21 August 2006 (UTC)
So in a decade or so, people will be cursing LCDs for unreliabiliy. A typical LED has a half-life of around 100,000 hours, so after ten years, an always-on display will have dropped to half its original brightness. And since the LED is bonded to the display panel, it won't be possible to replace it. --Carnildo 01:29, 28 August 2006 (UTC)
Although what you say is true: you have overlooked the human eye's perception of brightness. That perception is far from linear. In practice an LED whose output has fallen to half its new output doesn't appear to be perceptably much dimmer than when it was new (unless you compare it to a new one, when you may well just notice). The half life of the VFD is much much shorter than that. The reality is that even though the half life of an LED may well be as you state, the actual useful life of an LCD display lit by LEDs will be several decades (this assumes no deterioration of the LCD itself, which is not actually the case). And just to pour petrol on the fire, I have a Panasonic DVD recorder that is just 3 years old where part of the VFD display is on the top periphery of the display, and is almost unreadable, it has dimmed so much. 86.133.167.143 22:15, 28 August 2006 (UTC)
Even the humble flourescent tube dims considerably in use. A regularly used tube roughly halves its light output in the first year. But do you really notice? It's not until you replace a tube many years old that you notice that the new one is a bit brighter. I B Wright 16:58, 3 September 2006 (UTC)

[edit] Reliability section

THis sect really needs structuring with sub hdgs IMO--Light current 02:15, 4 September 2006 (UTC)

Ive split it and done some rearrangement now.--Light current 02:41, 4 September 2006 (UTC)

[edit] Field Emitter Vacuum Tubes

Er, this is largely puffery, and the DMD "flappers" does not even belong on this page. FEDs have never reached commercial utility, are anything but robust, and remain a lab-domain device. If 80.43.91.40 or anyone else has an objection, please chime in before I edit the content -- and flavor -- of this section. Thanks,

[edit] Armstrong

I find it very odd that the page on Vacuum Tubes doens't mention Armstrong, who was basically responsible for figuring out how they work (see the page on the Audion). The page also doesn't say much over any of the disputes over all the things that De Forrest basically stole/plagerized (alledgedly) including the Audion, the Triode (which I believe Armstrong invented), and more. This is all covered in Empire of the Air (both the book and the excellent documentary that was made from it).

It is generally accepted that DeForest invented, what is today, accepted as the triode, even though he didn't fully understand how it worked. It is true that DeForest found himself embroiled in patent disputes, but this was not necessarily over the inclusion of the grid. Although Armstrong succeeded in explaining the operation of the Triode, it is not generally accepted that he invented it. If every patent dispute (succesful and otherwise) was taken into consideration then the article on Edison would read very differently. The article states that he, "... was America's most prolific inventor", when it should really read, "... was America's most prolific invention thief", as certainly his principle inventions were all stolen. The light bulb stolen completely from Joseph Swan, and the Phonograph from Edward Guilliard (though Edison did build a crude working prototype based on German work, most of the features of Edison's patent belong to Guilliard. Edison's battery (The Nickel-Iron) was in fact investigated by the Swedish inventor of the Nickel Cadmium (and found to be wanting).
The patents are in his name, so that's where the credit goes.


[edit] Tubes working together

could someone add a section explaning simply how a series of vacuum tubes works together in electronics such as a computer. If theres somthing on it already i dident see it.

(Unsigned comment 23:06, 2006 November 29 Naramak)

Are you thinking of how a pair of tubes is used in "push pull" in an output stage, I think their is an article on "push Pull" but it describes transistors rather then tubes.
Or are you thinking more about the signal coupling between the stages of a tube device, which is normaly done with a capacitor/resistor network, or sometimes an interstage transformer.

cmacd 13:50, 30 November 2006 (UTC)

[edit] Supply Nomenclature

Why do the yanks persist in believing that they are the only country in the world. No body else refers to the supplies as 'A', 'B' and 'C'.

Yanks aren't usually educated in what the rest of the world calls things. They persist with what they know—just as do you. It's not a personal attack, or systemic plot. Thank you for amending the article. It helps to educate us! — EncMstr 18:50, 23 December 2006 (UTC)
Unfortuntely, I know exactly where he is coming from. The Americans are not only not educated in what the rest of the world calls things, but have no interest in being so educated. The Americans will not permit any output from the British film industry to be shown in America unless the language used is American and not English. Indeed the Americans insist on completely remaking any British television programmes - in America. It is a source constant irritation to hear British people using American instead of English. Virtually no English person will refer to "a bus 'shed-yule'", but instead the incorrect American pronounciation of 'sked-yule'.
Several times, non American references have been removed from articles, and some kind soul even went through an article of mine replacing all the English with American (I reverted it).
I B Wright 19:01, 28 December 2006 (UTC)
As a Canadian I can assure you that their is great truth to the rumor that Britan and the United States are as the wag said "two countries Divided by a common language" (canadian Usage is a cross between the two. we tend to follow american terms but British spelling) In this case A, B, and C are in Canadian Usage cmacd 19:19, 28 December 2006 (UTC)

[edit] Physics of Tetrodes

The article Vacuum tube#Tetrodes and pentodes states that However, the tetrode had a problem too: the positive voltage on the second grid accelerated the electrons, causing them to strike the anode hard enough to knock out secondary electrons. These could then be captured by the second grid, reducing the plate current and the amplification of the circuit. This effect was sometimes called "tetrode kink".

I don't see how this can be true - the energy of the electrons arriving at the plate should depend only on the plate voltage - whether there is a tetrode grid, or not, should make no difference. I suspect the "tetrode kink", whose existance is not in doubt, is just because there is now an electrode, not so negative with respect to the anode, that can collect these electrons. In a triode, the other surfaces are at much more negative potentials, and did not collect so many electrons. Can someone more knowledgeable about this effect comment? LouScheffer 21:46, 4 January 2007 (UTC)

I'm afraid it is true. If the second grid merely just collected electrons as you hypothesise, then the addition of the third grid in the pentode would not reduce the effect as it would not be in the path of those electrons. The reality is that the grid 2 current comprises of intercepted electrons from the cathode plus secondary electrons knocked out of the anode. The positively charged grid 2 accelerates the electrons and it is this extra energy that givs the electrons enough energy to knock further electrons out. As the number of electrons knocked out becomes larger than the number arriving, this gives rise to the negative resistance region (the grid 2 current rising to match).
If the secondary emission did not occur, then the grid 2 would merely work like a leaky anode and there would be no negative resistance region (though it would still screen the control grid from the anode).
I B Wright 16:08, 6 January 2007 (UTC)
I agree that the secondary electrons are the cause. The problem is with the explanation. The energy with which the electrons smash into the plate is only a function of the anode and cathode voltages (assuming these change little during the electron transit). If a triode, a tetrode, and a pentode all have 200V on the plate, then the electrons all arrive with 200 electron-volts of energy in all three cases, independent of the number of grids or the voltages on them. So the number of secondary electrons should be identical in all three cases. But the secondary electrons matter less in the triode and pentode case. In the triode case, they cannot go 'uphill' back to the cathode (since they must all have energy less than the impacting electron). In the tetrode case, some of these can get to grid 2, causing the kink. In the pentode case, the grid 3 is at a low voltage, and again prevents the electrons from returning. LouScheffer 05:46, 7 January 2007 (UTC)
You have made a wrong assumption. The fact that the anode is at +200 volts does not mean that the electrons have an energy of 200 elctron volts. There are other influencing factors. The cathode itself imparts some thermal energy to start with. The control grid, being negative, removes some of that energy. The screen grid being at (typically) 150 volts gives it more positive energy. And of course if it were a pentode the very negative suppressor grid (relative to the screen grid and the anode) would take a considerable amout of energy away. The position is further complicated because the electrons travel through the device amazingly slowly, and some get repeled back to the cathode from the control grid. 35 years ago, I dealt almost exclusively with valve based electronics. The secondary emission effect is very real and accounts for the changes in the grid 2 current, and the valve world has accepted this for nearly a century. It would be very difficult to discuss the issue further unless you had a reasonable knowledge of electron dynamics.
It is, however, refreshing to see somone take an interest in the old fire-bottles. I, and one or two other contributors who are familiar with the technology are attempting to expand Wikipedia's coverage, but time does not permit as much as I would like.
I B Wright 20:45, 7 January 2007 (UTC)

[edit] Double redirects

I noticed that there are a lot of "double" (that is, broken) redirects leading to the article page. If you'd like to help correct some of these:

  1. Click here: Special:Whatlinkshere/Vacuum_tube
  2. Look for the double-indented links to articles, go to those articles, and fix the Wikilinks


Thanks!

Atlant 17:07, 28 March 2007 (UTC)

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