How does a Tube Amp work pt.1 This is a series of threads that I hope will help explain the workings of standard Tube Guitar Amps to those who have no idea what's going on. I will try to make it simple and easy to understand but sometimes in an effort to simplify I can complicate things even worse, so hopefully some of the other TECHs and otherwise knowledgeable people on this forum will help out (Are you there Marty?). I'll be using a schematic found on Dr Tube's website (I'll try to put up a link but not sure how to do that) drawn by Joe Piazza. It can be found under the JMP listing entitled : 1987 Plexi http://www.drtube.com/schematics/marshall/1987pljp.gif. First we will look at the Power Supply then we will move on to the input jacks and go through the amp until we reach the speaker jacks. Enjoy the ride but it may be bumpy at times. Power Supply The job of the power supply is to take AC 120 volts from your wall outlet and change it to AC/DC voltages required to operate your amp. Before we start, lets define some terms that will be used in the text. Note: If you are located anywhere other than the United States, your AC system may be 220volts 50Hz Voltage: Electrical pressure or the speed at which electricity flows unit of measure= volts or V Current: The amount of electricity that flows past a certain point unit of measure = amperes or A Wattage: The amount of Voltage multiplied buy the amount of Current. Also known as Power unit of measure = watts or w Resistance: The opposition to the flow of current unit of measure = ohms AC current: Alternating current, This is a current that contains peaks that alternate between positive and negative. Each alternation of positive and negative peaks is called a "cycle". In the standard household system this alteration occurs 60 times per second and is called a 60 cycle system. This is also referred to as 60 Hz.(or the Frequency of the cycle) Your guitar signal is a low voltage AC signal, the strings vibrate at a certain frequency(or Hz). The "A" string vibrates at 110 cycles per second or 110Hz. DC current: Direct current, This is a current that flows in one direction, either positive "or" negative with respect to ground (or zero volts) Okay, now we are ready to start. If you look at the diagram on the bottom right-hand side is the AC input, you should be able to identify the symbol that represents the AC plug. The top and bottom wires are marked ' L' and "N" respectively, the center wire is labeled "E" or earth, another word for ground. This wire is connected to the chassis. The first component we come to is a fuse labeled as 2A, this fuse is a safety device intended to "open" if the current draw on this end of the transformer exceeds 2 amperes. The next component in the line is the "Mains switch". this switch allows current to flow through the transformer and is the main on/off switch for the amp. The next component is the "Mains Voltage Selector". The Power Transformer as several taps on this end which is called the "Primary" these taps allow for a different voltage to be selected and still have the desired voltage present at the "Secondary". The Power Transformer is the heart of the tube amp, it supplies the various voltages needed to work the amp. Without getting too technical a transformer can step up or step down voltage and may have many different sections to supply several voltages at one time. The input side is called the primary and in this case will accept 110 VAC and step this voltage up to around 300-320 VAC and at the same time on another section it will step-down to 6.3 VAC. Some amps will have additional sections to supply voltages for IC circuits and separate Bias circuits. In this case there are only two different sections on the secondary or output of the transformer (not to be confused with an output transformer, more on that later). At the very bottom of the page is the 6.3 VAC supply for the filaments of the tubes. This section of the transformer has a center tap that goes to ground, and a pilot light across the supply lines to indicate that the amp is on. this supply is on when the "Mains" switch is engaged. Each vacuum tube has elements that need heating, the Cathode in particular so that it can give off electrons to make the tube work. These elements must be heated prior to the unleashing of High-voltage on the tube or it can make tube life decrease quickly so this section comes on with the "Mains" switch. Above that on the diagram is a double pole switch labeled "Standby". This section of the transformer supplies the High-voltage or High-tension (HT) to the circuit. After the switch are two .22uf capacitors, these are there to suppress spikes in the line when the standby switch is made. Next in line are two "Diodes" or "rectifiers", these rectifiers serve as a means to convert AC current into DC current. In this case, the high voltage goes into the rectifiers from the top and bottom legs of the secondary windings, ( the center tap is again grounded), the end of the rectifier that the windings are attached to is called the "anode", this rectifier will allow the positive half of the AC voltage pass and reject the negative half. Now this gets a little complicated, but the upper half and lower half of the windings are 180 degrees out of phase with each other, so that when the upper half is positive the lower half is negative and vice versa. Now since AC voltage has positive and negative peaks at the rate of 60 Hz and the rectifiers allow only the positive peaks through, the result is positive going peaks from the rectifier at a rate of 120Hz. This is considered to be DC since it only contains positive peaks but a pure DC has no peaks, it is constant so we need to make our DC more pure. This is the job of the "Filter Caps". Think of a filter cap as a storage device, when the positive peaks are present the capacitors are "charged" like a bucket of water being filled. when the peak goes away leaving a valley, the cap will discharge and the valley will be filled. this is not a perfect action and several caps are used to perform this. Even then the DC is not pure, there is still what is called "ripple" present on the DC line. Ripple is just a small amount of peak and valley left on the DC. This is what creates "hum" in a tube amp. If the caps are bad, the hum is worsened. The filter caps are the parts on the diagram marked "50uf", They are connected from the B+ line to ground. There are two of them followed by a second fuse marked "0.5 A", this is the HT fuse and serves the same purpose as the mains fuse, if the HT current is above .5 amperes the fuse will open. Next comes a part called the "Filter Choke", Without getting technical,this part also helps to smooth out the DC voltages as does the 50uf that follows it. Point "A" dropping down just ahead of thr filter choke is the supply line connected to the center tap of the "Output transformer" to supply V4 and V5 with the plate voltage necessary to operate those two tubes, somewhere around 390 VDC. Point "B" on the other side of the choke will supply about 380 VDC to the "Screen Grids" of those same tubes. The filter choke has some resistance to DC which is why there is a 10 V difference between the input and the output of the choke. The plates of V4 and V5 do not need to be filtered as much as the screens do so the plate voltage is taken off before the choke. After passing through the choke and past the 3rd 50uf cap , the B+ line turns north to pass through 2 10k resistors on their way to the preamp circuits, these resistors drop the voltage further due to the fact that the preamp stages use a lower value of DC voltage than the output tubes. There is one other section that branches off from the upper secondary winding of the transformer, this is the "Bias Circuit", it will be explained later when we come to the Output Tubes in our look at the signal path. Well that is our look at the Power Supply in our 1987, hope you're a little less confused or at least that you are confused on a higher plane. The next installment we will start at the input jacks and make our way through following the signal path until I get burned out again. This ends Part 1 of How does a Tube Amp work.