In the bi-grid type of tetrode, both grids are intended to carry electrical signals, so both are control grids. The first example to appear in Britain was the Marconi-Osram FE1, which was designed by H. J. Round, and became available in 1920. The tube was intended to be used in a reflex circuit (for example the single-valve ship receiver Type 91) where the same valve performed the multiple functions of RF amplifier, AF amplifier, and diode detector. The RF signal was applied to one control grid, and the AF signal to the other. This type of tetrode was used in many imaginative ways in the period before the appearance of the screen-grid valve revolutionised receiver design.