CAUTION - TUBE AMPS CAN USE 500+ VOLTS DC POWER WHICH CAN KILL YOU. BE CAREFUL, ASK QUESTIONS FIRST. Useful links for tube and tube amp information are located here. Explanation of how a tube amp works is continued here and here. What are the parts used in a simple triode (three part) vacuum tube? Anode – the piece of metal in a tube, also called the Plate, which is positively charged (B+ DC voltage) and to which the negatively charged electrons flow. The electrons flow out of the tube via the anode (plate) to generate an electric current. In a small signal (preamp) tube, the anode current is fed to the control grid of the power tube. In a power tube, the anode current is fed to the output transformer input winding. Grid – also called the control grid. It is placed between the cathode and anode. The grid uses a negative bias voltage. The AC signal from your source (e.g., CD player, turntable, tuner) is applied to the grid. The grid acts like a valve inside the tube that controls the flow of electrons from the cathode to the anode. This is why vacuum tubes are also sometimes call Valves, or thermionic valves. While we are using the water hose analogy to understand electricity, think of electrical voltage as being the pressure inside the water hose (wire), electrical current being the flow rate of water through the hose (wire) and a ball valve or nozzle on the end of the hose being an electrical component, like a resistor or rheostat in the electrical circuit, that controls the flow rate of the water through the hose (wire or circuit). If a tube didn't have a control grid, the signal leaving the tube would be at the maximum all the time. The control grid regulates the flow of electrons from the cathode to the anode. Higher electron flow equals more amplification. Cathode – the piece of metal in a tube that surrounds the heater, but does not touch it (an indirectly heated cathode), or is located nearest the heater. The cathode emits electrons when heated and DC power (B+ voltage) is applied. These electrons generate a current as they flow from the cathode to the anode. Heater – the filament (piece of wire usually wrapped around a rod) that heats the cathode so the cathode can emit a cloud of electrons. Electrons are negatively charged. The heater is what you see glowing inside a vacuum tube you amp is turned on. The heater is also of course what causes the tube to be hot to the touch. Heaters typically use AC power (but can use DC power), 6.3 volts for 6DJ8 and similar tubes, 12.6 volts for 12AX7 and similar tubes. Getter – usually a halo shaped piece of metal wire near the top of the tube, but it can also be on the side or bottom. Other shapes are also used: squares, “D” shaped, windmill, dishes, etc. The getter is coated with a highly volatile powder before being installed in the glass envelope. The getter has no effect on how a tube operates in a tube amp. After it is flashed (see Getter Flash below), it serves no purpose. Getter flash – after the tube is built, all the air removed and sealed to create a stable vacuum in the tube, the getter is flashed by heating it and the volatile powder explodes and created the getter flash. Getter flash appears as a silvery mirror-like coating on the inside wall of the glass envelope near the getter wire. Getter flash turns powdery white or light brown as a tube ages. This is typically caused by a slight loss of vacuum in the tube and the air that leaks in reacting with the getter flash. Loss of getter flash can indicate that a tube is going bad. If the getter flash is 100% powdery white, the tube is probably bad. Glass envelope – a glass bottle (or sometimes metal) is used so that the air inside the tube can be evacuated to create a vacuum inside the tube. A vacuum is required to keep the components inside the tube from burning up rapidly. Base and pins – the base, usually plastic, gives the bottom of the tube and pins mechanical strength and reduces the potential that the tube will be damaged during installation and removal. Most small signal tubes(e.g., 12AX7s, 12AU7s, etc.) and some power tubes (e.g., 7868s) don’t have a plastic base. The pins are the electrical connection between the components inside the glass envelope and the tube sockets in the amp, preamp or tuner section of the tube amp. If you want to know what each pin of any tube does, check out the Duncanamps.com searchable database: http://tdsl.duncanamps.com/tubesearch.php Types of vacuum tubes? Tubes like the 12AX7, 12AU7 and 6DJ8, all commonly used in preamp sections of a tube receiver or integrated amp, are all dual triode tubes. This means they are actually two tubes in one glass envelope. Preamp tubes amplify the signal only a little bit in comparison to the amplification achieved with power tubes. There are many other kinds of vacuum tubes. For example, a pentode power tube like the EL84/6BQ5/7189 contains all three components described above (cathode, anode, control grid), but adds two more grids (screen grid and compressor grid). What does the Power Transformer do in a tube amp? Transformers either step up low voltage to higher voltage or step down high voltage to lower voltage. You have a step-down transformer somewhere outside your home that reduces the high-voltage line next to your house from 440 or higher VAC (volts alternating current) to 120 or 240 VAC that is used in your home. AC power is supplier to the power transformer and stepped up to a single higher voltage or various voltages via separate windings in the transformer. The primary winding is the power input side of the transformer, secondary winding is the output side. Why is a rectifier needed? Rectifiers convert AC power to pulsing DC power. The AC output of the power transformer is fed to a rectifier tube or rectifier bridge or diodes that convert the AC (alternating current) power to pulsing DC (direct current) power. The DC power pulses because only 1/2 of the sinusoidal AC power wave is converted into DC power by the rectifier. These DC power pulses are smoothed out by the power supply capacitors (see explanation below). The resultant DC power (B+) is then supplied to the anode and cathode (plates) of the tubes. This AC to DC conversion is required because most parts in a tube amp use DC power. What purpose do the power supply filter capacitors serve? Great link on why old capacitors need to be replaced, and which to replace: http://www.antiqueradio.org/recap.htm Power supply filter caps are polar electrolytics (unlike coupling caps which are non-polar film or paper-in-oil caps) and are wired in parallel (coupling caps are wired in series). The pulsing DC power from the rectifier stage is smoothed out by the power supply filter capacitors to supply consistent DC power to the tubes and other components in the amp. The diagram below shows how the amplitude of the pulses is reduced via the filter caps (and regulator if your amp has one). One analogy is the power supply filter caps act like the springs on your car that absorb the bumps in the road (pulsing DC power from the rectifier)and deliver a smooth ride (constant DC power) to the passengers (tubes). @GordonW further explains how power supply filter caps work in this post. How does a small signal tube work in a tube preamp or power amp? Signal tubes such as 12AX7s, 12AU7s, 6DJ8s are typically used in the preamp section of an integrated amp to amplify the input source signal. You will also see small signal tubes in standalone power amps too, where they are also being used to amplify the preamp signal. The AC signal from from the audio source (e.g., CD player, turntable) is fed to the control grid of the small signal tube and modulates the electron flow from the cathode to anode. In simple terms, the AC signal rides on top of the DC power flowing from the cathode to anode. How does a power tube work in a tube amp? Power tubes consist of a cathode, grid, anode, getter, heater and sometimes other types of grids. The anode from a small signal tube supplies the slightly amplified source signal to the power tube control grid, which controls the audio frequency and volume output of the amp. Maximum # of character reach in this post. Explanation of how a tube amp works is continued here.