[Amps] Tetrodes

[Ian White GM3SEK]

Larry Carman wrote:

>What causes the gas release that produces an internal arc?
>
Here's the story (it's a bit long, because I didn't have time to write a 
short one)...

A so-called "vacuum" tube is actually a very complex piece of chemistry.

For a start, the "vacuum" space is far from empty - typically it 
contains something between a million and a billion free gas atoms 
(depending on the quality of the vacuum and the size of the tube). This 
seems like a lot of atoms, but it's actually close enough to a true 
vacuum to ensure that the main current carriers are the negative 
electrons coming from the heated cathode.

That can change dramatically if more gas atoms are injected into the 
"vacuum" space. Then the main current carriers will be positive ions 
which travel the opposite way to the electrons, and have a lot more mass 
and energy. This is a so-called "vacuum arc".

All the metal, glass and ceramic materials from which tubes are made 
contain small quantities of trapped gases, mostly atmospheric nitrogen 
and oxygen. Even though the manufacturers try to select high-purity 
materials, and de-gas the tubes during manufacture by heating them way 
above normal operating temperatures while still pumping on them, 
de-gassing can never be complete.  At any time later in the tube's 
operational life, small quantities of gas can diffuse to an inside 
surface and be released into the "vacuum" space. This applies 
particularly during the early part of the tube's operational life.

When B+ is present, a sudden gas release may cause an arc, especially if 
the metal surface has grown "whiskers" that increase the potential 
gradient and encourage the arc to strike. If an arc begins, it can also 
tear off metal ions that will then sustain the arc... until you do 
something to stop it.

There are examples of arc marks on a GS35B triode at:
www.ifwtech.co.uk/g3sek/misc/gs35a.jpg (anode)
www.ifwtech.co.uk/g3sek/misc/gs35g.jpg (grid/cathode)
This particular tube had suffered multiple severe arcs because the owner 
had tried to run it at too high a voltage, about 4.5-5kV.
(The photos are by PA3CSG, who had supplied the tube to its 
over-ambitious user.)

A minor gas release problem will usually clear itself automatically, 
because the gas atoms are very quickly collected by the "getter". This 
is a deposit of chemically active metal, put there during manufacture to 
keep the vacuum clean in later life by combining with any free gas 
atoms.

In a receiving tube, the getter is that easily visible film of silvery 
metal (barium). This functions at room temperature, but barium has too 
low a melting point to be usable in any but the very smallest 
transmitting tubes (eg 807, 6146).  In larger transmitting tubes the 
getter is usually something like zirconium, which needs a higher 
temperature to function correctly. In glass/metal tubes that operate 
with a very hot anode, the getter is on the anode itself. In 
ceramic/metal tubes that operate with a relatively cool anode, the 
getter is usually on the cathode because that is the hottest place.

Section 3.9 of Eimac's 'Care and Feeding of Power Grid Tubes' gives more 
information about arcs, and how tubes must be protected from sustained 
arcing:
http://www.cpii.com/eimac/cfcontnt.htm

That section is largely based on Eimac Bulletin 17:
http://www.cpii.com/eimac/eiapps.htm
The references in Bulletin 17 give a lot more background about the 
arcing problem, especially the book by Lafferty on 'Vacuum Arcs' (if you 
can get hold of a copy).



-- 
73 from Ian G/GM3SEK