Amp primer

The Power Supply

The typical power supply of a tube amp consists of the following components:

Abbreviation :
  • The main power switch -
  • "Mains"
  • The power transformer -
  • "PT"
  • The rectifier -
  • "Rec"
  • The filter capacitors -
  • "FC"
  • The standby switch -
  • "SB"
  • The choke -
  • "choke"
  • The power resistors -
  • "PR"

    Below is the power supply section of a typical 1950's vintage tube guitar amp - The Fender Bandmaster 5E7 :

    The following analysis is an overview of what the power supply circuit does, for a detailed description of the individual components use the left menu at the top of the page. To see the full schematic of this amp : "Tweed" Bandmaster 5E7

    The power enters the amp via the power cord on the bottom left of the circuit. When the main power switch is turned on, thereby completing the circuit, 120 volts AC is sent to the primary side of the power transformer (PT). The secondary section of this PT has 3 different output voltages, also called "taps" or "windings" : 5V AC, 6.3V AC and 680V AC.

  • The 5V tap is the filament supply for the 5U4G tube rectifier. Vacuum tubes need to be heated to work and this is done by the filament or "heater" as it is often called. When the amp is turned on, the heaters come on and stay on until it is turned off. This is what makes the tubes glow and also makes them hot to the touch.
  • The 6.3V tap is the filament supply for the rest of the tubes in the amp. The 6.3V tap on the PT shown above has a "center tap" (CT) that is connected to ground by being connected to the chassis. This is for noise reduction purposes which will be explained further in the main PT section.
  • The symbol for ground in the schematic above is:

  • The 680V tap has a CT as well that is connected to ground. This in effect sends 340V AC to each of the 2 input pins of the rectifier tube. This high voltage winding is sometimes called the "B+" supply.
  • The rectifier tube transforms the Alternating Current (AC) into Direct Current (DC) and sends the positive component of the DC to the Standby switch. The negative component is accessed by the circuit through connection to ground ( the chassis ).

    The standby switch allows you to turn on the main power switch and warm up the tube heaters without sending the high voltage DC to the tubes. This puts less stress on the amp and promotes longer tube life.

    After the standby switch is turned on, the positive DC voltage flows into the rest of the circuit very quickly (the speed of electricity, whatever that is), but the first thing that it hits is the first filter capacitor (FC).

    The symbol for capacitors in the schematic above is:

    The filter capacitors are connected between the power rail and ground and act as reservoirs of positive DC voltage, sort of like a battery. Even though the rectifier changed the AC to DC, the waveform of the DC still has a residual AC component and needs to be "smoothed" out or an unacceptable level of 60HZ hum (50HZ in many parts of the world) will be present in the amplified guitar signal. The FCs smooth this out by constantly charging and discharging , bringing the waveform closer to an ideal straight line. Subsequent FCs in the chain (and the choke) smooth the DC out more and more as it travels on down the power rail. The most important FC in the chain is the first one, and is usually bigger than the rest. In this amp, however, the first 3 FCs are 16 microfarads(U). All of the FCs in this amp are rated at 450V DC; this is the maximum voltage that they are rated to have charge them. 8U is about the smallest rating you will see in tube amp power supplies, about the highest you will see is 100U. With a tube rectifier the highest you can go is about 50 or you get an oscillating "motorboat" effect.

    The choke is the next component in the power supply chain after the first filter cap.

    The symbol for the choke in the schematic above is:

    It also serves to smooth out the DC waveform. Some amps have no choke, instead they have a power resistor in the place that the choke would normally occupy.

    There are 2 power resistors in the schematic above.

    The symbol for resistors in the schematic above is:

    The power resistors main purpose is to lower the B+ voltage as it travels down the power rail towards the begining of the amp circuit. Tube amp circuits are typically designed so that the tubes at the beginning of the circuit have much less voltage on their plates than those at the end of the circuit (the power tubes). The amount of resistance is measured in Ohms. The PRs in the schematic above are rated at 10,000 Ohms (K = 1,000). PRs have to be rated to handle the power of the B+ voltage and current traveling through them and are usually rated at 2 or 3 watts.

    After digesting the above material, you should have a pretty solid understanding of the basics of tube amp power supplies. I will be posting a more detailed analysis of each of the components discussed above soon, probably by the end of March.

    Tube amps contain lethal voltages and should not be worked on by those unfamiliar with the proper safety precautions that should be taken when working on them.