Continuing our journey to look at what's inside a valve, today we are going to look a little further at our EL37 pentode with the envelope broken open. After a little more surgery on the ELECTRODE CAGE we can see in the centre. the white vertical cylinder of the CATHODE, surrounding that are the three concentric grids, the inner being the CONTROL GRID, then comes the SCREN GRID and finally the SUPRESSOR GRID.