A "choke" is the common name given to an inductor that is used as a power supply filter element. They are typically gapped iron core units, similar in appearance to a small transformer, but with only two leads exiting the housing. The current in an inductor cannot change instantaneously; that is, inductors tend to resist any change in current flow. This property makes them good for use as filter elements, since they tend to "smooth out" the ripples in the rectified voltage waveform.Why use a choke? Why not just a big series resistor?
A choke is used in place of a series resistor because the choke allows better filtering (less residual AC ripple on the supply, which means less hum in the output of the amp) and less voltage drop. An "ideal" inductor would have zero DC resistance. If you just used a larger resistor, you would quickly come to a point where the voltage drop would be too large, and, in addition, the supply "sag" would be too great, because the current difference between full power output and idle can be large, especially in a class AB amplifier.Capacitor input or choke input filter?
There are two common power supply configurations: capacitor input and choke input. The capacitor input filter doesn't necessarily have to have a choke, but it may have one for additional filtering. The choke input supply by definition must have a choke. Capacitor input filters are by far the most commonly used configuration in guitar amplifiers (in fact, I can't think of a production guitar amp that used a choke input filter).How to select a choke:
The capacitor input supply will have a filter capacitor immediately following the rectifier. It may or may not then have a second filter composed of a series resistor or choke followed by another capacitor. The "cap, inductor, cap" network is commonly called a "Pi filter" network. The advantage of the capacitor input filter is higher output voltage, but it has poorer voltage regulation than the choke input filter. The output voltage approaches sqrt(2)*Vrms of the AC voltage.
The choke input supply will have a choke immediately following the rectifier. The main advantage of a choke input supply is better voltage regulation, but at the expense of much lower output voltage. The output voltage approaches (2*sqrt(2)/Pi)*Vrms of the AC voltage. The choke input filter must have a certain minimum current drawn through it to maintain regulation.
The voltage difference between the two filter types can be quite large. For example, assume you have a 300-0-300 tranny and a full-wave rectifier. If you use a capacitor input filter, you'll get a no-load max DC voltage of 424 volts, which will sag down to a voltage dependent on the load current and the resistance of the secondary windings. If you use the same transformer with a choke input filter, the peak output DC voltage will be 270V, and will be much more highly regulated than the capactor input filter (less variations in supply voltage with variations in load current).
If, on the other hand, you are selecting a choke for a capacitor input
supply (such as the typical Marshall or Fender design), then the requirements
are relaxed quite a bit. The purpose of the choke in these type supplies
is not for filtering and voltage regulation, but just for filtering the
DC supply to the screen grids of the output tubes and the preamp section.
The screens typically take around 5-10mA each, and the preamp tubes draw
about 1-2mA or so (for the typical 12AX7; 12AT7's are usually biased for
around ten times that). This means that you can get by with a much smaller
choke, and, in addition, the preamp supply current doesn't vary that much,
so you can get by with a higher DC resistance, which means smaller wire
can be used to wind the choke, which means higher inductance for a given
size core. Just add up the current requirements of the screens and preamp
tubes, and add a bit more for margin. For a 50W amp, a typical value might
I highly suggest going to Duncan Munro's website (http://www.duncanamps.com/) to download his power supply calculator program. It will allow you to experiment with different inductance and capacitance values and see the resulting residual AC ripple and transient response of the supply filter. Both capacitor input and inductor input filters can be simulated. It is a great educational tool.