Radiotron Type AV25
Demonstration Triode

The AV25 uses an oxide coated filament.
The glow pics are with the tube operating with ~250V on the anode.
The grid is negatively biased. The largest illumination is with low [close to 0] bias, the smallest glow is ~100V more bias than this. Very large grid voltage variations are needed to control the tube. The tube came in the 'suitcase' demonstration box - this has bias adjustment controls and allows monitoring of anode current etc. An interesting feature is that this device also allows AC signals to be input - the meter on the bottom right is "modulation %".

These images provided by Danial Stocks