[Amps] "Tubes 201" - How Vacuum Tubes Really Work

[Will Matney]

I forgot to add this reference that Terman used about zirconium getters;

Further information is given by J. D. Fast, Zirconium and its Compounds with a High Melting Point, Phillips Tech Journal, 1939.

*********** REPLY SEPARATOR  ***********

On 7/24/06 at 7:22 PM Will Matney wrote:

>On 7/24/06 at 5:46 PM Tom W8JI wrote:
>
>>>> Rich said:
>>>Are Au and Mo notable emitters?
>>>Can the grid of a tube with a hard vacuum reach high 
>>>temperatures if
>>>no current is passing through it?
>>>If zirconium has to be at 1400ºC (Terman) in order to 
>>>absorb oxygen
>>>and nitrogen, how can a 3-500Z's anode getter air 
>>>molecules?
>>
>>Will said:
>>>One needs to study how tubes are constructed to see what 
>>>may happen. The metals used in them were picked
>>>over their high heat qualities (wouldn't melt). The plating 
>>>had to do with emission of electrons.
>>
>>>Yes, I seen a reference earlier about the 3-500Z having a 
>>>getter of zirconium. For it to work, it's optimum temp is
>>>1400 deg C and is what it's ran at in this application. In 
>>>that case, the anode would be white hot (the welding color 
>>>of steel). There's some other problems with this too, but I 
>>>won't >go into them here (IE Hydrogen gas). If one actually
>>>wanted to make this work, it looks like a tantulum anode 
>>>would have been a better choice.
>>
>>Hi Will,
>>
>>Rather than you and Rich make wild guesses about gettering 
>>in a 3-500Z, why not give Eimac a call and ask them what the 
>>gettering material was in a 3-500Z? If Paul Larson still 
>>works there (last time I talked to him about getters was 
>>about 10 years ago) you might ask him.
>
>
>Althought Terman shows this same thing on page 315 of the 1st edition,
>1943, here is a link to a website below about getters;
>
>(1) http://www.thevalvepage.com/valvetek/getter/getter.htm
>
>It's impossible to run one temperature on zirconium. It's optimum temp
>according to Terman is 1400 deg C to absorb oxygen, nitrogen, carbon
>dioxide, and carbon monoxide in vacuum power tubes. Under this temperature
>it gets poorer and poorer in operation until it doesn't absorb at all
>(around 700 deg C, red anode color) (1). 1000 deg C is closer to 700 deg
>than 1400 deg C. Since the red color of an anode is generally around 525
>deg C to 800 deg C (visible red range), the zirconium is not hot enough.
>Even if it is absorbing some smaller amount when it gets hotter at up to
>1000 deg C (bright cherry red, almost orange), hydrogen gass is being
>released, not absorbed! The optimum temperature is 1400 deg C (white hot
>color) according to Terman, and is what the manufacturers run it at if
>being used for a getter. The problem is that a little over 300 deg C,
>zirconium gives off hydrogen gas, but at or under 300 deg C it absorbs it.
>One has to use two temperatures  in order for zirconium to work correctly.
>That's not "wild guesses", but scientific fact. Actually, acording to
>Terman, large power tubes really don't use "getters". According to Terman,
>either the zirconium is ran as two filaments, or two pieces of zirconium
>has to be heated to two different temperatures if used for a getter. This
>was in 1943 and before that they knew about this, it's not something new
>or dreamed up!
>
>
>>
>>By the way, Terman on page 187 of my Third edition warns 
>>about excessive resistance in grid leads. Terman states, " 
>>If the resistance in the grid circuit is high enough, this 
>>process can become cumulative, resulting in the control grid 
>>potential suddenly becoming positive and causing the 
>>**destruction** of the tube as a result of excessive plate 
>>current."
>
>Who mentioned anything about grid lead resistance? I don't remember this,
>or I never did. Also, your mis-quoting Terman, and talking about gassy
>tubes.
>
>Quote;
>
>If the resistance in a grid circuit is high enough, this process can
>become cumulative , "and in some types of tubes" can easily resulting in
>the destruction of the tube as a result of excessive plate current caused
>from loss of grid bias. 
>
>End quote,
>
>Nowhere in the whole paragraph about gassy tubes (Effect of Gas upon Tube
>Characteristics) which is under part 13, page 316 in this book, says a
>grid became positive! It only says this can cause a grid to become "less
>negative"!
>
>
>Also from Terman, page 286;
>
>4. Space-charge effects.
>
>Most vacuum tubes are designed so that the emitting surface gives off a
>surplus of electrons, with the result that the actual current that flows
>is limited by the mutual repulsion between the electrons and not by the
>emission capabilities of the cathode. Under these conditions the current
>is said to be space-charge limited.
>
>The space-charge limitation of current is brought about by the presence of
>electrons in the space between the electrodes. These electrons introduce a
>negative charge that reduces the potential in the region. The reduction of
>the potential slows down the electrons and thus increases the negative
>charge density, which further reduces the potential. The potential is
>finally reduced to the point at which the potential gradient at the
>emitting surface is zero or even slightly negative. This is a limiting
>equillibrium condition that sets a limit to the current that can flow for
>a given potential difference between electrodes.
>
>>From page 314;
>
>Power tubes are commonly operated so that the grid goes positive during a
>part of the cycle. This results in grid current, and causes power
>dissapation at the grid of the tube, which is sometimes the limiting
>factor in tube operation.(2) As a consequence, the grids of power tubes
>often operate at relatively high temperatures, and such materials as
>molybdenum, tungsten, or tantalum are accordingly used. The fraction of
>the primary electrons intercepted by the grid depends upon the grid
>potential relative to the anode potential, and upon the grid structure. 
>
>The grid heating that takes place is determined by the number of primary
>electrons intercepted by the grid, and by the grid voltage. The actual d-c
>grid current as measured by a meter may differ from the number of primary
>electrons received by the grid as a result of secondary emission causing
>the grid to loose secondary electrons at the same time that it receives
>primary electrons. The amount of current thus lost through the secondary
>emission will be affected by the electrode potentials, by the grid
>temperature, and by the character of the grid surface. In the case of
>thoriated tungsten and oxide-coated cathodes the secondary emissions may,
>under some conditions, become quite large as a result of cathode material
>that has been deposited upon the grid.
>
>(2) Grid Temperature as a Limiting Factor in Vacuum Tube Operation, Vol
>24, Page 447, 1936, By Mouromtseff and Kozanowski.
>
>
>>
>>I can't find a thing in Terman that supports your or Rich's 
>>claims. As a matter of fact everything I do find seems to 
>>disagrees with your theories.
>>I'll scan that page of Terman and put it up on the web page 
>>in a few minutes if you like.
>
>
>Tom, first, didn't I mention all the above, and didn't I say a tube should
>be shut down the moment a grid went open? I think I did in several earlier
>posts. Stll, Terman in this book totally agrees with everything we have
>been stating.
>
>
>
>>
>>73 Tom
>>
>>
>>
>>
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>
>Best,
>
>Will
>
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