833A transmitter triode, unmarked.

This valve is 220mm x 110mm, directly heated, with filament connections at the bottom, and grid and anode at the top (anode to the RHS in the picture). The grid and anode are marked G G and A A. One filament connection is cut and not a full circle. It has 'BM128' marked on the upper grid/filament support.

Its thoriated-tungsten filament rating is 10V/10A. Max anode voltage is 4kV. It is rated at 1250W up to 30MHz with natural cooling, or 1800W up to 20MHz with forced-air cooling.

Thanks to Bob Weiss for identifying this valve.