Indicator tubes for transistor logic machines.

Doable. The glass envelope for the vacuum takes the form of two concentric cylinders joined at their ends, topologically a torus. The cathode is another concentric cylinder, this time of metal or whatever, just barely larger than the inner glass cylinder, with the grids and anode/plate forming successively larger cylinders. The heater flame passes through the centre of the cylinder (which is outside the glass vacuum envelope), heating the cathode by infrared radiation, much as a conventional heater does. (The glass forming the inner cylinder needs to be infrared-transparent and probably high-melting-point.) /~\ The ASCII der Mouse \ / Ribbon Campaign X Against HTML mouse at rodents.montreal.qc.ca / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B