In article ,
AndyW wrote:
I have picked up an old radio that I plan on restoring but it is valve
based and I am a child of the transistor era.
I understand the physics of the basic thermionic valve and the grid but
once they start to get multiple grids and screens in there I am lost.

Can anyone recommend a good website primer on valves / vacuum tubes?

It's not a website, but... the folks at the Tuner Information Center
(www.fmtunerinfo.com) have a suggested-reading list of books:

http://www.fmtunerinfo.com/library.html

On their recommendation I found a copy of "The Theory and Servicing of
AM, FM, and FM Stereo Receivers" via an online book-seller. Having
read it, I agree with the plaudits - it's a very good intro to how
firebottles are actually used in radio receivers.

The Radiotron Designer's Handbook by Langford-Smith is a good
resource, and versions of various editions of this are available
on-line. http://www.tubebooks.org/Books/RDH4.pdf is one such.

To address your specific point briefly: the triode is the basic
amplifying "valve" that you understand.

Adding a second grid (the "screen") and giving it a positive voltage
bias, shields the main (control) grid from "seeing" the changes in
voltage at the anode as the circuit conditions change... this reduces
the effective capacitance between grid and anode, thus increasing the
tube's gain and allowing it to operate at higher frequencies
(e.g. RF). This is similar to the way that "cascoding" two
transistors (or two triode tubes), shields the "lower" device from
seeing voltage variations at the load, eliminates the Miller effect,
and increases frequency response by making the amplifier "easier to
drive".

Adding a third grid (the thin "suppressor" grid) between the "screen"
grid and the anode, and grounding it, suppresses the flow of
"secondary" electrons that are knocked off of the anode by the
electrons arriving from the cathode. Without a suppressor, the
secondary-emission electrons can flow to the screen grid if the anode
voltage is low enough (e.g. during times of high current flow from the
cathode) and this causes nonlinearity. The suppressor is held at
ground potential, and thus tends to "repel" the secondary-emission
electrons and send them back to the anode. I don't think there's an
exact equivalent to this situation in transistor circuits.

See the Radiotron manual for more pictures and explanations - the
first few pages go over what I've summarized here in more detail.