Excellent replies from Mlsstl and Atmasphere. Here is a quick and dirty run-down of the different types of tubes.
A tube basically works by boiling off electrons from the cathode (-ve terminal) and accelerating the electrons to the anode (+ve terminal). The cathode may be indirectly heated (have a seperate heater element) or directly heated.
A DIODE is the simplest type of tube. The cathode and the anode are placed relatively close together so current can only flow in one direction.
In a TRIODE, the cathode and anode are placed further apart so no current will flow. A seperate grid, called a "control grid", is placed between the cathode and anode. A weak musical signal is fed to the control grid, which attracts electrons from the boiling cathode and accelerates it towards the anode. In this way, a small musical signal generates a larger current.
However, the amplification that triodes produce is not very much. So the TETRODE was invented. As the name implies, there are four grids. In addition to the three grids in the triode, the tetrode adds a "screen grid" between the control grid and the anode - making the configuration: cathode, control grid, screen grid, anode.
The screen grid is positively charged, and its effect is to accelerate even more electrons towards the anode thereby giving more amplification.
However, the electrons now have so much energy that some of them bounce off the anode, reducing overall efficiency and messing around with the musical system.
So the PENTODE was invented. In addition to the four grids in the tetrode, a pentode has a negatively charged "suppressor grid" between the screen grid and the anode - cathode, control grid, screen grid, suppressor grid, anode. The effect of the suppressor grid is to reduce the energy of the electrons bouncing off the anode, giving greater amplification and less wastage.
A Pentode can be made to function as a triode if you simply disconnect the screen and suppressor grids. Hence some Pentode based amps (like the Cary V-12R and Cary 120S) can be operated in triode or pentode mode.
In MY opinion (and this is a gross generalization), triode amps sound more delicate and have more of that tube "bloom". Pentode amps are more powerful and sound more "solid state" than triodes. Of course, not all amps are like this.
I'll let someone else explain Class A, etc. but hopefully this was of use to you.
A tube basically works by boiling off electrons from the cathode (-ve terminal) and accelerating the electrons to the anode (+ve terminal). The cathode may be indirectly heated (have a seperate heater element) or directly heated.
A DIODE is the simplest type of tube. The cathode and the anode are placed relatively close together so current can only flow in one direction.
In a TRIODE, the cathode and anode are placed further apart so no current will flow. A seperate grid, called a "control grid", is placed between the cathode and anode. A weak musical signal is fed to the control grid, which attracts electrons from the boiling cathode and accelerates it towards the anode. In this way, a small musical signal generates a larger current.
However, the amplification that triodes produce is not very much. So the TETRODE was invented. As the name implies, there are four grids. In addition to the three grids in the triode, the tetrode adds a "screen grid" between the control grid and the anode - making the configuration: cathode, control grid, screen grid, anode.
The screen grid is positively charged, and its effect is to accelerate even more electrons towards the anode thereby giving more amplification.
However, the electrons now have so much energy that some of them bounce off the anode, reducing overall efficiency and messing around with the musical system.
So the PENTODE was invented. In addition to the four grids in the tetrode, a pentode has a negatively charged "suppressor grid" between the screen grid and the anode - cathode, control grid, screen grid, suppressor grid, anode. The effect of the suppressor grid is to reduce the energy of the electrons bouncing off the anode, giving greater amplification and less wastage.
A Pentode can be made to function as a triode if you simply disconnect the screen and suppressor grids. Hence some Pentode based amps (like the Cary V-12R and Cary 120S) can be operated in triode or pentode mode.
In MY opinion (and this is a gross generalization), triode amps sound more delicate and have more of that tube "bloom". Pentode amps are more powerful and sound more "solid state" than triodes. Of course, not all amps are like this.
I'll let someone else explain Class A, etc. but hopefully this was of use to you.