its 1 of 8 tubes from the Hammond Preamp AO-28 (in B3, C3, A-100 console models) Its a rectifier tube. you can read here, what a rectifier is: http://en.wikipedia.org/wiki/Rectifier
the doors use one and the charlatans listen to them and you'll see its the oscillatating organ sound..Jamie65 added:A Hammond Organ is any organ made by the Hammond Organ Company, or it's successors,...
A test tube is a piece of special glass, of different sizes, that can be heated or get cold. You can test small-sized chemical reactions in this tube, by combining chemicals or by heating it to see...