Today, I want to discuss INTERNAL CARBONISATION sometimes also referred to as screeding or colloquially as grey glass. From preceding blog articles, I have already explained that electron emissions are drawn from the cathode to other positive electrodes within the cage and sure enough, the majority of emissions do travel within the electrode cage, however, like recalcitrant small animals and wilful children, a small proportion don't do what they should and escape from the electrode cage striking the glass envelope walls.
These 'loose' secondary electrons can build up a potential charge on the glass which can cause audible distortion and sometimes a striking blue glow on the envelope especially where the electrode cage is closest to the envelope or if the envelope internal surfaces were not scrupulously clean at the time of manufacture. Many customers have sent me valves for testing and diagnosis, often recent manufacture Chinese devices showing this glowing phenomemon which they have mistakenly suspected is a 'gassy' valve whereas in actual fact they are seeing Jablonski shift glow caused by secondary emission due to poor internal envelope cleanliness.
I have here at Mullard Magic HQ, a pair of Chinese globe shaped 300B triodes that exhibit this effect, interestingly, I have also recently tested for my pal Andy Smith of Head-Fi fame, a modern recreation of a CV181 twin triode that showed ridiculously high anode current which also had INTERNAL CARBURISATION that I suspect performed a dual role in both hiding the apalling electrode cage support construction and stopping it's resultant secondary emission from causing the valve to perform as a nightlight!!!!!!!! .
In order to minimise this secondary emission, an envelope inner surface is coated with colloidal graphite which shows very low tertiary emission characteristics. So now you all know why your KT66, KT63, KT61, 6J5, 6SN7 et alia have grey glass.