. . . . . .and it's the abstraction that frees you from the strictures of having to use this or that tube. That's my problem - I've been stuck with the initial idea that the tube is a self contained discrete component, not keeping in mind that once it is in circuit it's better to dissolve the glass envelope and see the larger network of R C L etc. This is an eye opener, and now I understand engineering thought a little better. Thanks