10-03-2014, 09:22 AM
That would be the material used to coat the filament so electrons are given off at a lower filament temperature. I have found over the years that tubes that have been abused electrically, ie: over heated due to bad caps, tend to do this. I have also noticed lately that some of my NOS early globes are doing the same thing and they have never seen any service so not sure what to make of it. The biggest problem with this is this material can cause a short in the tube if it gets between a couple elements.
Gregb
Gregb
