An audio amplifier begins with the power supply. The purpose of the supply is to provide the necessary operating voltages for the tubes. DC voltages are required for the plates and screens, and AC for the heaters. In some amplifiers the phono preamp heaters are supplied with DC to minimize hum.
For a verbal description click here.
Refer to the schematic diagram throughout this description. Beginning at the left the line cord plugs into a standard 120 volt AC outlet. Modern plugs are polarized (one prong is wider than the other one). This is for safety and I strongly recommend safety over authenticity. The side of the plug going directly to the transformer primary winding should be the wide prong (neutral wire). Switches and fuses should NEVER be placed in the neutral side of the line. The switch is for turning the amplifier on and off. (OK. So I'm stating the obvious but I have always been told to be thorough.) The fuse protects against overloads. It protects more than just the parts in the amplifier. It is possible, and has likely happened, that an amplifier without a fuse developed a fault, caught on fire and burned someone's house down. So if you're building an amplifier and you don't want to have to build a new house always put in a fuse.
The transformer steps the line voltage up for B+ and down for heaters. This works by turns ratio. Let's say a given transformer has one secondary winding and it has 1/20 the number of turns on the secondary as on the primary. The voltage induced in the secondary will be (120 V) / 20 = 6 volts. If another transformer has 4 times as many turns on it's secondary as there are on it's primary the voltage will be 4 times 120 volts = 480 volts. The voltage a transformer will deliver is determined when it is manufactured and can't be changed.
The secondary at the bottom delivers 6.3 volts and for the particular tube lineup must have a current rating of at least 2.7 amps: 3 for a margin of safety. It could have a rating higher than that but NEVER lower. The practice of drawing the tube heaters separated from the rest of the tubes makes for a simpler diagram (fewer lines that cross). If there is a light to indicate that power is on it will be a 6 volt lamp and be attached to the same secondary as the tube heaters. The hum balance control adjusts the voltages from each side of the heater winding to ground to be equal. Because they are equal in voltage and opposite in phase the magnetic and electric fields will tend to cancel each other out. This will considerably reduce the amount of hum in the amplifier's output.
The secondary at the top is the plate or B+ winding. It supplies a high voltage and the center tap (which is grounded) permits full wave rectification. The ground symbol often indicates nothing more than all of the points are connected together thus saving another line on a complex diagram.
When the top end of the winding goes positive the bottom end goes negative. The upper plate of the 6AX5 goes positive attracting electrons from the cathode. This causes a heavy electron current to flow between cathode and plate and the tube has a relatively low resistance. The lower, which is negative, plate repels the negatively charged electrons and no conduction takes place to this plate. When the next half cycle comes along the top plate will be negative and the lower one positive. The two plates take turns conducting current and conduction takes place over both halves of the cycle. If there were no capacitor the voltage would still fall to zero twice during each cycle as the input voltage passes through zero.
For a much more in depth discussion of rectifier circuits click here.
The 22 ohm resistor is a 1/4 watt unit and serves mainly as a fuse. It is omitted in many amplifiers but if you are building one I recommend its inclusion. A 1/4 watt resistor is only about 10 cents and a new rectifier tube can cost 3, 4, or 5 dollars. If a short develops the resistor will burn out saving the tube. It will also alert you to trouble by the burning smell.
The DC output voltage is given approximately by this equation.
Vdc = 1.4 x (Vac/2) - Rect DropWhere Vdc is the DC output voltage across the first 40 microfarad capacitor, Vac is the voltage of the whole top secondary winding and Rect Drop can be determined by referring to the charts and graphs found in tube manuals. for this particular tube in this application the drop across the tube is approximately 140 volts. If the transformer has a 700 volt center tapped winding the output voltage will be 350 volts which it is in the real amplifier being described here. You may have noted that the output voltage is equal to the secondary voltage/2. This is just a coincidence of the numbers.
For example if the voltage of the upper winding is 500 volts the DC output voltage is
Vdc = 1.4 x (500/2) - 140 = 210 volts.The first capacitor charges up and holds the voltage up when it would otherwise drop to zero. There is some variation in the voltage as the capacitor is charged through the rectifier tube and discharges through the other tubes in the amplifier. The term for this variation is "ripple". The plates of the beam pentodes used in this, and most other amplifiers, are fairly immune to ripple in the plate voltage. Changing the voltage by 10 or 20 volts has a very small effect on the plate current. The voltage on the screen grid has a much larger effect. The 1 k ohm resistor and the second capacitor reduce the ripple voltage to much less than a volt.
For a much more in depth discussion of filtering circuits click here.
The 10 k ohm resistor and the third capacitor reduce the ripple voltage to an even lower level but they perform a much more important function. That is to reduce variations in the DC voltage caused by low frequencies in the audio signal. These variations if fed to the earlier stages of the amplifier can act as signal and start low frequency oscillation sometimes called motor boating. The last resistor and capacitor serve the same function for the magnetic phono preamp. It is very sensitive to variations both line frequency ripple and variations from low frequency audio. These must be reduced to a few millivolts for the amplifier to work properly.
Sideways; A Technical Discussion of Power Supplies.
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This site begun March 14, 2001
This page last updated August 1, 2002