Marconi 2LO transmitter
I saw this at the new BBC Broadcast Center - they donated it to Science Museum a couple of years ago but have it back on loan. It is the Beeb's very first transmitter and could kick out a whopping 1.5kW of radiated power! On the right-hand side of the picture you can see four triodes (made by Osram - not just light-bulbs for those boys!) which I'm assuming makes up a dual-darlington type output stage. Aside from that I couldn't remember enough transmitter theory and valve configurations to guess at which bits did what. There is another good page on the BBC's history site.
I noticed that the transmitter was housed at Savoy Hill House on The Strand - a building now occupied by the IIE, my institute.