why vacuum tube require vacuum ? - Page 1
Forum section: electronics / beginners

author: ker2x
author_rank: Regular Contributor
timestamp: November 14, 2017, 06:15:03 am
content: Friendly greetings !Simple question : why vacuum tube require vacuum ?I can understand why some gas could be unsuitable but why vacuum ?I tried googling with no luck and wikipedia is overflowing with information about vacuum tube. The answer might be in there but i couldn't see itI have read that some specialized tube does have gas in it but, again, i don't understand why.Thank you <3

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 14, 2017, 08:40:51 am
content: If the voltage is kept low (below the ionization energy), a gas can be used. This is the basis of some new "vacuum tube" research, using field emission cathodes and sometimes no vacuum (as such) at all.It can also be made smaller than the mean free path at ambient pressure, which is a necessary feature of any vacuum tube -- since the classics are macroscopic, a very good vacuum is needed (mean free path of ~mm). Microscopic ones can operate at higher pressure.And of course, some tubes did use gas, usually for the conductivity enhancement that ionization brings. This ranges from gas rectifiers and thyratrons, to even just phototubes (where the ionization cascade has much the same effect as an avalanche photodiode).Tim

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: November 14, 2017, 09:14:54 am
content: On a physical level, an electron cannot take a straight path through a gas because it is a charged particle, and to an approaching electron a gas molecule seems to be negatively charged because of the electrons on the outside, even though it is net neutral. (near field effect) Hence it gets deflected all over the place.This is why some uncharged particles, eg photons or neutrons, do pass through gases for a considerable distance but charged ones generally do not get far.

author: ker2x
author_rank: Regular Contributor
timestamp: November 15, 2017, 12:46:44 am
content: thank you for the additional information

author: Syntax_Error
author_rank: Regular Contributor
timestamp: November 15, 2017, 02:03:23 am
content: A related question:Why are some spark gap tubes specified to be deuterium filled, as opposed to plain hydrogen (proteum)? Is it related to gas diffusion or thermal performance?

author: Audioguru
author_rank: Super Contributor
timestamp: November 15, 2017, 05:20:46 am
content: Does anybody use vacuum tubes anymore? Why?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 15, 2017, 06:37:15 am
content: Quote from: Syntax_Error on November 15, 2017, 02:03:23 amDunno. The only difference is ionic mobility -- twice the atomic weight.Thermal would be slightly worse, but very slight. AFAIK, the pressure is low enough not to be significant regardless. Diffusion would be slightly slower.The spectrum is very slightly different; I've seen it before, it needs more than the usual diffraction grating setup to resolve accurately. Other than that, the electronic structure is identical, and therefore any property that depends on charge and size.Tim

author: BrianHG
author_rank: Super Contributor
timestamp: November 15, 2017, 07:30:03 am
content: Quote from: blueskull on November 15, 2017, 05:29:59 amDoesn't one of the older (80's/90's) American jets use vacuum tubes in their ignition system since the tubes are immune/survive a EMP pulse from a nuclear detonation?

author: Nerull
author_rank: Frequent Contributor
timestamp: November 15, 2017, 07:33:57 am
content: Deuterium has a higher breakdown voltage than hydrogen and the tubes can operate at a slightly higher gas pressure, which results in lower anode dissipation at high current rise times. Hydrogen has a faster recovery time so is preferable when fast switching is needed.

author: GreyWoolfe
author_rank: Unknown
timestamp: November 15, 2017, 06:22:37 pm
content: Quote from: blueskull on November 15, 2017, 05:29:59 amHam radio amplifiers such as Alpha and Acom use tubes. They are much more forgiving than the solid state amps. The tube amps make serious power. The old Henry 3K could deliver 3,000 watts and the the old Alpha 77SX could deliver 4,000. Not that their owner ever pushed them like that since max allowable power is 1.5K

author: james_s
author_rank: Super Contributor
timestamp: November 15, 2017, 06:54:59 pm
content: I use them because it's fun and I like retro tech. One of my hobbies is vintage arcade games and only a CRT looks right in those. In the case of vector games a CRT is the only display technology available with enough dynamic range. If you ever play a real Asteroids cabinet and see the shots the ship fires you'll see what I mean. Emulated on a PC it is not right at all.Another area they're useful is if you are building Tesla coils and related HV toys. I wouldn't let my "modern" Tek TDS scope anywhere near one, but an old vacuum tube scope can take huge spikes and RF noise in stride that would destroy the input section of a more modern instrument.Then there's audio, I'm not going to argue that tube amps sound better because "better" is subjective, but there is something almost magical about sound coming from glowing glass bottles.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: November 15, 2017, 07:12:29 pm
content: Quote from: james_s on November 15, 2017, 06:54:59 pmLike a coal fire.

author: jmelson
author_rank: Super Contributor
timestamp: November 15, 2017, 08:14:54 pm
content: Quote from: blueskull on November 15, 2017, 05:29:59 amThe Krytrons are gas devices, somewhat similar to the old photoflash strobes, but with a 3rd electrode.Jon

author: jmelson
author_rank: Super Contributor
timestamp: November 15, 2017, 08:19:08 pm
content: Quote from: BrianHG on November 15, 2017, 07:30:03 am80s/90s?? No, I seriously doubt it. Ignition system? That is basically an auto spark coil and a buzzer, VERY unsophisticated. All of the current generation aircraft have been run through one of several EMP simulators the Air Force uses to make sure their fleet could survive EMP. I think this dates back to at LEAST the F-111 and F-4, there are some pics of these aircraft suspended on a GIANT goniometer in the EMP simulator.Jon

author: TimFox
author_rank: Super Contributor
timestamp: November 15, 2017, 08:31:16 pm
content: Why do people still raise horses? Were they not replaced by internal combustion?

author: Mjolinor
author_rank: Frequent Contributor
timestamp: November 15, 2017, 08:40:36 pm
content: Quote from: TimFox on November 15, 2017, 08:31:16 pmTry putting exhaust gas on your garden and see how it helps the plants.

author: Audioguru
author_rank: Super Contributor
timestamp: November 15, 2017, 11:10:07 pm
content: Some expensive audio amplifiers have vacuum tubes glowing on top. But the amplifier circuit is completely solid state, the vacuum tubes are only to see.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: November 15, 2017, 11:18:59 pm
content: Come on, *real* amplifiers and preamplifiers use tubes all the time. I bet you think vinyl is dead, too!Case in point, McIntosh:

author: CJay
author_rank: Super Contributor
timestamp: November 16, 2017, 10:07:58 am
content: I don't personally think valves/tubes have any place in HiFi if the aim is accurate reproduction but they can sound nice and there's a definite feeling of 'magic' to them (but that may be the warm glow of nostalgia).Valves in RF amplifiers are slowly being replaced with LDMOS devices which can be bombproof but I think it's going to be some time yet before MOSFETs replace the high power valves in TV or Broadcast radio transmitters I think (maybe someone else knows better though?)There are new valves coming onto market now, I think Noritake have launched a few new devices recently under the Korg name, they're not particularly cheap and they aren't in a standard 'bottle' form factor but they are interesting and I do fancy having a play with some.