Figure 1. This diagram of a 6L6GC illustrates the main parts found in the power tubes we’re discussing.
Tech Talk: Anatomy of a Power TubeA power tube is essentially a control valve used to regulate the flow of electrons. (This is why guitarists across the Atlantic refer to our little glass friends as “valves” rather than “tubes.”) The electrons flow from a part of the tube called the cathode to the tube’s plate, as shown in Figure 1. While all power tubes have a cathode and a plate, much of each model’s sonic character is due to its specific components and construction.
The power tubes we’re discussing here fall into three categories: tetrode, pentode, and beam power. Tetrode tubes have four electrodes—the aforementioned cathode and plate, as well as a control grid and screen grid. When you turn on your tube amp, the tube filament at the center of the bottle receives a voltage (usually 6.3V) that heats the cathode to free up electrons in preparation for current flow. The plate and screen are given large positive DC voltages to attract the electrons from the cathode to the plate. When your amp is in standby mode, the plate and screen voltages are removed to keep the electrons from flowing, while keeping the cathode warm for instant current flow at any moment.
The control grid is given a negative DC voltage that restricts the flow of electrons from the cathode to the screen and plate. When the control-grid voltage is made more negative, electron flow from cathode to plate is reduced. When the control grid is made less negative, electron flow increases.When electrons from the cathode hit the plate, other electrons from the plate may become dislodged and flow to the screen grid, a phenomenon called secondary emission that reduces the efficiency of the tube
Pentode and beam power tubes add an additional electrode between the screen grid and plate to reduce the occurrence of secondary emissions. Pentodes add a grid called the suppressor grid, while beam power tubes usually add a beam-confining electrode instead.