A damaged amplifier is an obvious reason for changing parts out. Another reason is for aesthetics in tone. There’s a lot of iron in a power transformer, so these tend to be the most expensive part in an amp and thus can potentially be the weakest link in the signal chain that contributes to quality and tone…. if manufacturers skimp on the power transformer (which equates to less iron, less cost, less weight, and lower shipping charges.) Another reason to change out a power transformer is to fix damaged that can be caused by gross under loading and impedance mis-matching, among other things. Note that power transformers can carry dangerous levels of electrical power, so careful precaution in replacing one is not only smart, but can prevent serious injury or even death.
The last item in the signal chain of the audio path of a push-pull type guitar amplifier is the output transformer. The signal runs through the output transformer before exiting the speaker. This transformer is a large blocky metal device mounted on the chassis of a guitar amplifier. This transformer converts the low current (and high voltage) signal to a low-voltage (and high current) signal that powers the speakers. The output transformer literally transforms, or transfers, the audio power of the amplifier onto a load (impedance) of the speaker; how efficiently it does this is one measure of quality and the modernity of technology and materials used. The speaker is thus driven, and literally transforms the signal from electrical into vibrational-mechanical signals through the air.
A choke is a toroid, or an inductor, used to filter out unwanted noise and alternating current. In a guitar amplifier, a choke is used to smooth out the ripples from the power supply’s rectified DC power output. It looks something like a transformer, with two leads coming out of the housing. Some guitar amplifiers use a resistor instead of a choke to accomplish similar results and to save money, but a resistor doesn’t eliminate as much AC ripple as a choke. This causes additional residual hum in the amplifier, so a choke is a better choice if quality of sound is a concern.
