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From: adams@swbatl.sbc.com (Tom Adams - 235-7459)
Newsgroups: sci.electronics,rec.ham-radio
Subject: Re: Has anyone made any homemade valves (tubes), semiconductors ...
Message-ID: <1990Jul27.040826.961@swbatl.sbc.com>
Date: 27 Jul 90 04:08:26 GMT
References: <872@massey.ac.nz> <37291@shemp.CS.UCLA.EDU>
Reply-To: adams@swbatl.UUCP (Tom Adams - 235-7459)
Organization: Southwestern Bell Tele. Co. - Advanced Technology Lab - St. Louis
Lines: 59

In article <37291@shemp.CS.UCLA.EDU> cccph@eel.cs.ucla.edu (Charles Hobbs) writes:
>In article <872@massey.ac.nz> GMoretti@massey.ac.nz (Giovanni Moretti) writes:
>+
>+ (stuff deleted)
>>I've had this dream of making a triode in a peanut butter jar (a
>>small strong one :=) ...
>You could do this successfully if you had some way of getting a  
>good vacuum. (Commercial valves have chemical compounds known as
"getters" which aid in removing oxygen from the valve)

Before I run off about getters, I'd like to mention that light bulb
filaments have low emmissivity, and probally aren't suitable, and that
a test tube, or olive jar would probally be a lot easier to evacuate.
Also, using direct filament emmision means you'll need DC filament
supplies to avoid hum.

I quote from Physics of Electron Tubes, L. Koller, McGraw-Hill, 1937 (2e)
No matter how well a vacuum tube has been exhausted, there always remains a
small residual pressure of gas. This may perhaps be of the order of
0.0001mm of Mercury in commercial tubes. Even at this low pressure,
the number of molecules of air per cubic centimeter reaches the
astounding figure of 10^12.
...

The getters most commonly used at present are phosphorus, calcium,
magnesium, barium, strontium, aluminum and various mixtures of alloys of
these.  Phosphorus is used in the red form.
...
Barium and strontium are very active getters but have the disadvantage of
oxidizing rapidly... For this reason the are often used in the form of
alloys with aluminum, which are quite stable in air and from which the
active metal can be vaporized by heating, after exhausting the tube.

... [Discussion of reduction of gas pressure during getter vaporization
by collision between getter and gas molecles, and later, after getter has 
volatized onto surface of tube wall by formation of monatomic layer of
gas molecules, mentioning 10^2 cm layer could reduce pressure 3x10-3 mm Hg
in 100 cc bulb.]   I believe getters were vaporized by strong RF field,
which also contributed to the vacuum by accellerating gas ions into the
tube walls.

Glass and electrode metals substantially outgas and must be heated for a
considerable time to reduce this effect. 

Getters were actually invented by John T Marshall (1908) to exhaust tungsten
filament lamps, and many others patented improved getters in the next few
decades.

All in all, soft tubes like Brian's grandaddy used to make would be a
whole lot easier.  Reasonable detectors, lousy amplifiers. Still could
awfully interesting. Others have done tubes at home, and I've thought
about it a lot.  Anyone seen documents on Sprengel vacuum pumps
as used by Edison?

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Tom Adams: 314-235-7459: Southwestern Bell Telephone Advanced Technology Lab
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