Very few phono preamps are push-pull. A better term would be balanced.
Triode- a tube with three elements. The most linear form of amplification known.
Tetrode- a tube with 4 elements. Used for power output. EL34 is an example
Pentode- five elements. Used for high gain and/or power output. 6AU6 is an example of a high gain signal pentoded, 6550 is an example of a pentode power tube.
OTL- Output TransformerLess amplifier. Refers to a tube amplifier that has no output transformer. Most tube amps have a transformer that converts the high voltage/low current of the power tubes to a lower voltage version of the signal that will work with a loudspeaker. OTLs use tubes that don't have this requirement. They can be considerably more transparent because there is no transformer.
Push-pull- refers to any amplifier that uses two output devices to drive the output load. One device is doing the work during the positive part of the signal waveform, the other does the work during the negative half.
SET- single ended triode, meaning that a single triode is amplifying the entire signal (usually refers to the output tube); its not 'push-pull'. BTW SET is a lot less efficient than push-pull so you tend to see lower power levels.
Class A means that the output device is always conducting through the entire waveform right up to full power. It can be in tube or transistor, single-ended or push-pull.
AB means that the output device stops conducting through part of the waveform while its opposite takes over. IOW there are no single-ended embodiments. just push-pull. Can be tube or transistor.
Auto-bias means that the amp takes care of setting the bias on the amp for you. This applies only to tube amps.
Feedback is a method of reducing distortion. Some of the output voltage of the amp is 'fed back' into the input of the amp as a correction to the amp itself. Its not 100% successful as the signal is always changing and the feedback signal is always slightly delayed. This results in some odd-ordered harmonic distortion, which the ear translates as harshness and also volume, as the ear uses odd ordered harmonics to tell how loud a sound is.
This is why some circuits are
Zero feedback- to reduce odd-ordered harmonic generation.
