Distortion in a push-pull amplifier occurs due to the presence of harmonics in the output waveform. Harmonics are additional frequencies that are multiples of the fundamental frequency. The main source of distortion in a push-pull amplifier is the third harmonic.

Explanation:

1. Push-pull amplifier:
A push-pull amplifier is a type of electronic amplifier that uses two active devices (transistors or vacuum tubes) to amplify the input signal. These devices are configured in a push-pull arrangement, where one device amplifies the positive half of the input signal, and the other device amplifies the negative half. This configuration helps in canceling out even-order harmonics, resulting in lower distortion.

2. Harmonics in the output waveform:
When an amplifier is driven by a sinusoidal input signal, it ideally amplifies only the fundamental frequency of the input. However, in practice, the output waveform contains additional frequencies known as harmonics. Harmonics are generated due to nonlinearities in the amplifier's characteristics.

3. Even and odd harmonics:
Harmonics can be classified into two categories: even harmonics and odd harmonics. Even harmonics are frequencies that are multiples of an even number (2nd, 4th, 6th, etc.), while odd harmonics are frequencies that are multiples of an odd number (3rd, 5th, 7th, etc.). In a push-pull amplifier, the even harmonics are canceled out due to the balanced configuration.

4. Third harmonic distortion:
The main source of distortion in a push-pull amplifier is the presence of the third harmonic. The third harmonic is three times the fundamental frequency. It is not canceled out by the push-pull configuration because it is an odd harmonic.

5. Causes of third harmonic distortion:
Third harmonic distortion can occur due to various factors such as mismatched characteristics of the active devices, imbalance in the circuit, nonlinearity in the load impedance, and imperfect cancellation of even harmonics.

6. Effects of third harmonic distortion:
Third harmonic distortion causes the output waveform to have a distorted shape. It introduces additional frequencies at three times the fundamental frequency, which can interfere with the desired signal and degrade the overall quality of the amplified signal.

In conclusion, the main source of distortion in a push-pull amplifier is the third harmonic. While the push-pull configuration cancels out even harmonics, it cannot cancel out odd harmonics such as the third harmonic. This distortion can negatively impact the quality of the amplified signal and needs to be minimized for optimal amplifier performance.