OK for those interested a little tutorial for you.... ;D
The valve I'm making the bases for is what's known as a triode (as in three elements) Simply it's an amplifying device that amplifies the flow of voltage/current which is the flow of electrons. (electricity, voltage=pressure/current=flow)

We have a heater which we typically connect to a low voltage of around 6.3V, though this can vary from valve to valve. This heats the cathode which is sitting at a negative DC voltage potential. The anode is sitting at the positive DC potential. (voltage potential typically 250-500V DC)
Point to note, electrons move from negative to positive and not the other way around as most people believe. This is important.
Our heater heats up the cathode, this makes the electrons become a little excited and thus easier to leave the cathode. Some will hover around and just off the cathode, this is called the "space charge"
Now, opposites attract, and equals repel, remember that girl from school?
Electrons are no different, they want to flow across the vacuum inside the glass envelope and hurtle towards the positive anode, bit like us young men on our first date ;D
We need to control these electrons into some sort of order, Slow them down a bit if you like.
We do this by applying a negative low voltage on the grid, typically 60-100 volts, but can be as low as 1 or 2 volts. If we increase the negative voltage (more negative) the flow of electrons will slow from the cathode to the anode (equals repel), if we make the grid more positive more electrons will flow.
So there we have it, we have a low voltage controlling a much larger voltage,
Now imagine that the low voltage is coming from your CD player output.......music modulates, so we have a negative voltage modulating the grid of the valve which in turn modulates the higher voltage flow and thus amplifies a low signal into a much larger one.
I hope you can follow that? It sounds complicated, but it's really very simple

Darren