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

[Will Matney]

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