There's a good web page (among many) on getter material at
http://www.thevalvepage.com/valvetek/getter/getter.htm
The 3-500Z does use zirconium as a getter, and zirconium needs to be
at about 1000 C to work well. Titanium is also apparently used in the 3-500ZG.
This page is fascinating as it goes into some detail and the various
getter materials that are used, their pros and cons, and the
difference between getters in receiving and transmitting tubes.
It so happens that thorium also acts as a getter, but at rather lower
temperatures (200 - 500 C). Some getters reverse (release adsorbed or
absorbed gases) at high temperatures. It looks like thorium might be
among those types, but it isn't explicitly stated in this write-up.
For 3-500Z (tantalum anode coated with zirconium) and ZG tubes
(graphite anode coated with titanium), it appears that the getter on
the anode must be run at some color for it to work. On the RF Parts
web site, I read that thoriated tungsten filaments need to be run for
a while to reach full emission, but it didn't say that the filaments
act as getters.
Kim Elmore N5OP
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