[AMPS] getters

[John T. M. Lyles]

W8JI sez:
>
>External anode tubes are different, because obviously the anode
>can't be operated at high enough temperatures to allow a gettering
>agent to be mixed in or coated on the anode. They getter through
>filament heat.


Larger external anode tubes do have special getters that are used 
during the manufacturing, just after pinch off of the vacuum tube. 
RCA/Burle uses a glass enveloped thing with some getter material, 
that is broken open just at the sealing step. In the 4616 tetrode, it 
is reached down through the pinchoff hole, while the tube is being 
pumped. In the 7835 triode, it is broken open when the pinchoff clamp 
is squeezed, as several glass extentions of this internal bulb reach 
up into the pinchoff pipe.

These getters are one time, flash getters. They remain inside the 
tube, with a bit of broken glass. When the tube is cut open 
(autopsied) they are usually covered with a white dust inside. In big 
CPI/Eimac tubes, I have seen getters that attach near the filament, 
to receive heat and be activated that way. More better getter gives 
longer gas free life, if the tube has an internal source of gas while 
operating. A few years ago I ran a 'special' 4CW250,000B tetrode that 
way, and the company put a special getter inside that didn't saturate 
with captured gas molecules for the life of the filament emission. 
Normal hard vacuum tubes shouldn't need that.

K5PRO
John

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