Types of Valves or Vacuum Tubes There are a number of different type of vacuum tube or valve, with complicated sounding names like 'Triode'. This article provides a brief introduction to the various types of tube that you may commonly encounter. The tube naming convention is actually very simple - the name refers to the electrode configuration found in the valve or tube - hence 'ode' in the name. The first part of the name refers to the number of electrodes used - Di (two), Tri(Three) and so on. Diodes Diodes have two electrodes - a cathode and an anode, often referred to as the 'plate'. They allow the flow of current in one direction only. They can be used as rectifiers to 'rectify' or convert AC (alternating) current to DC (direct current). Triodes A triode has three electrodes (tri - three) due to the addition of another 'screen' electrode known as the grid between the cathode and anode. As the charge on the grid is varied from positive to negative, it affects the flow of electrons from anode to cathode controlling the flow. This makes the triode an amplifying device as the current flows in proportion to the signal applied. To make the triode 'linear' in operation (it is significantly non-linear in its behaviour at low volumes, giving rise to harmonic distortion) a standing current is applied to the grid to keep the valve working in the range where it operates linearly. This is called 'grid bias'. Tetrodes A potential problem was discovered when Triodes were used in the first radio sets - there was a tendency for them to oscillate due to capacitance effects. The cure for this was yet another electrode (the secondary grid) - and the four electrode tube or tetrode (tetra - four) was born. This had the welcome side effect of also increasing the tube's gain (amplification) at high frequencies. The beam tetrode is a further refinement of the tetrode design, using special design techniques and shaped electrodes (plates) that focus the electrons emitted into a beam, focusing this beam onto special spots on the anode. This improves the efficiency of the design and hence power output capabilities. This was first seen in the 6L6 tube and the KT66, KT77 and KT88 series of valves which are still widely used today in guitar amplifiers and Hi-Fi amplifiers. Pentodes The pentode evolved to overcome problems with the Tetrode. In this case, electrons released from the anode (caused by by the anode being energetically struck by electrons from the cathode) are captured by the secondary grid, reducing the current flow and hence amplification. This is not an issue in the triode as these electrons cannot reach the cathode or grid as they have insufficient energy to do so. The answer as always was another electrode called the suppressor grid - is biased negatively with respect to the positively charged anode and as such repels the electrons that have escaped from the anode back towards the anode, allowing them to be recaptured. Other tube types exist including the heptode and even octode, but in the context of audio valve amplifiers, the tubes types described already are what you are likely to find in any tube amplifier, modern or vintage. To reduce costs, it became common practice to include more than one set of elements in the single glass envelope (tube) - a common example of this being the 12AX7 (or ECC83) valves which combine two triodes in a single tube. These are still extensively used today (despite being a 1940s vintage design) in Hi-Fi audio preamplifiers, the preamp stages of integrated amplifiers, output stages of tube CD players and guitar amplifiers.