[Amps] Outgassing tubes.

[Ian White G/GM3SEK]

Will Matney wrote:
>
>Your correct about the differences! I've seen the innards of several 
>types of transmitting tubes though and never seen a true getter like 
>the ones used in receiving tubes. Where are they in them as I've not 
>seen them. Generally you have a pin which connects directly to the 
>getter where they use at the factory to flash it. They used to call 
>these pins out on tube specs as NC or do not use. I imagine you could 
>attach one to one of the elements but I've never seen that.
>
The basic principle of a getter is to form a chemical bond with stray 
gas molecules. This is a two-way process because the chemical bonds are 
often quite weak, so trapped molecules can also be released. Even so, at 
equilibrium the getter holds the pressure down to an acceptably low 
level.

What the getter cannot do is instantly absorb a sudden release of gas. 
Such releases can happen if impurities trapped within the structural 
materials eventually diffuse to the surface. Even a small release can 
cause a pressure spike if it happens quickly enough, and it only takes a 
few microseconds to strike an arc. However, once the arc is over, the 
chemical reaction will generally mop up the gas over a timescale of 
several seconds, which is why these small releases can cause an arc but 
still be completely recoverable.

The "silvery" getter used in receiving tubes and small transmitting 
tubes (807, 6146) is usually barium metal, which is very effective but 
has too low a melting point for use in high-power transmitting tubes.

In many glass transmitting tubes, the getter is the grey zirconium 
finish on the outside of the anode. That material works best at high 
temperatures, so the anode is a good location.

Graphite anodes probably act as their own getter.

In ceramic/metal tubes the anode runs quite cold, so the getter is 
usually in the warmest available place, deposited somewhere on the 
cathode structure.



-- 
73 from Ian G/GM3SEK