Hah! what a way to finish this series on the components that make up a valve, yes, we've saved the best till last with episode 14 dealing with..... the GETTER. Having assembled the electrode cage and mounted it into the glass envelope, the last stage in valve manufacture is to remove the air within to form a high Torr vacuum immediately before the sealing-off process.
In order to achieve this vacuum, the valve is connected to a vacuum pump and whilst being evacuated is passd through a chamber where radiant heat and an RF eddy current is applied. This double whammy of heating - radiant & induced - is an attempt to ensure that any occluded or interstitial gas adhering to the glass or trapped within the metal componentry is driven off. Inevitably, some gas molecules will remain, in fact, the mean remaining gas level has been quantified as one part per one-hundred, thousand, million, so as you can appreciate, we are talking about a miniscule amount of residual gas!
In order to remove this residue, a volatile transition metal is submlimated onto the glass envelope, typically barium but sometimes magnesium which can be seen as a bloomed silver mirror on part of the interior of the glass envelope. - this is the GETTER. The GETTER is a sacrificial material which 'mops up' the slowly released residual oxygen. Many of you reading this I am sure, will have seen high power beam tetrodes - KT88 are particularly prone - on their last legs with the getter apearing a pinkish-brown shadow of it's former self as the shiny metallic bloom is consumed by the process of oxidation.
The pre-charging and application of the GETTER is an elegant and beautifully engineered process which is not often discussed, but we will now rectify that! The picture below shows a typical bar getter ring as used by Mullard and other valve manufacturers and these found use either as a pillar mounted trans mica assembly and as a pinch supported assembly.
Many people mistakenly describe the getter ring as the GETTER, many people think that the entire assembly gives up parent metal to become the bloomed GETTER silver mirror, many people believe the getter ring to imbue valves with a particularly favourable sonic property. OK, let's take each of these points in turn: -
The only part of the getter ring that gives up parent metal is the part labelled B in the inset photo above. The getter bar, B, comprises of an hollow iron sheath, C, which is filled with a core, E, of 0.5 to 25mg of metallic Barium. The content of the core is varied dependant upon the size of the glass envelope used. The sheath, C, has a thinner planar side, D, which allows sublimation of the core when an RF eddy pulse is applied.
The nickel induction loop, A, attenuates the RF eddy current applied during evacuation to allow effective sublimation of the core. The shape of the induction loop varies and is governed by the geometry of the getter ringmounting provision and the size is governed by using the least amount of metal to effectively attenuate the RF eddy curent pulse to ensure adequate heating to provide effective sublimation such that a complete GETTER silver mirror is formed.
The shape of the getter ring should have no effect on the sound quality of the valve, however, some valves may in the course of continuous development have used strengthened or lengthened support rods in conjunction with different getter ring shapes which could effect overall cage rigidity which could have a consequent effect on micophonics and perceived sound quality. These subtle changes would then be attributed to getter shape changes giving rise to today's urban myths.