Amplifier output stages
The two main types of output stages for guitar amp use are single-ended and push-pull. These terms refer to the amplifier output stage circuit "topology", which is the way in which the components in the output stage are connected. The main difference between the two types is in the connection of the tubes to the output transformer, and the type of transformer used.
What does "push-pull" mean?
In a push-pull amplifier the power supply is connected to the center-tap of the transformer and a tube is connected to both the upper and lower end of the center-tapped primary. This allows the tubes to conduct on alternate cycles of the input waveform. A push-pull stage can be biased class A, where current flows in both tubes for the entire input cycle (but in opposite directions in the output transformer, so they sum in-phase in the secondary winding while the DC offset bias current cancels out, to prevent core saturation and allow use of smaller, ungapped cores), or class AB, where current flows alternately in both halves, but less than a full cycle in each, or class B, where current flows only half the time in each tube. Most designs are biased class AB for best efficiency and power output with minimal crossover distortion (but not necessarily best "tone").
A push-pull stage requires at least two tubes to operate, but can have more connected in parallel with each side, resulting in an amp with four, six, or even eight output tubes for higher-power amps. This is called "parallel push-pull" operation. Another advantage of the push-pull circuit is that there is little or no unbalanced DC current in the output transformer if the tubes are matched and the output stage is balanced, since current flows in opposite directions to each tube, allowing a smaller transformer with less iron (translation: cheaper). Also, even order harmonics and distortion products generated in the output stage are canceled out (translation: good for hi-fi buffs, possibly bad for guitar players!) In addition, power supply hum is canceled out, allowing the designer to get by with less filtering of the power supply. This stage generally clips symmetrically, resulting in more odd harmonic distortion.
One possible disadvantage in class AB or class B operation is that the DC supply current changes dramatically between off and full signal, requiring heavier filtering to prevent supply "sag", unless that is what you are looking for, as is sometimes the case in guitar amplification, depending upon the style of the player.
What does "single-ended" mean?
A single-ended guitar amp output stage is generally biased class A, in order to maximize the output power before distortion. The output transformer primary is not center-tapped, having only two connections. One connection goes to the power supply, the other to the plate of the power tube or tubes. Tubes can also be paralleled for more power as in a push-pull stage, resulting in what is called "parallel single-ended".
The single-ended stage is the type of output stage used in the venerable Fender champ guitar amplifier and countless millions of early radios and tv's. It is making a comeback in high-end vacuum tube audio in the form of the single-ended triode stage, which supposedly is the "ultimate" in hi-fi sound reproduction. It is as inefficient as it is good sounding, putting out very low power levels in comparison to push-pull output stages. Another problem with this type of stage is the transformer must handle a continuous DC current. This results in a physically larger and more costly output transformer, which must be gapped to prevent saturation of the core due to this offset DC current.
Disadvantages of single-ended include: no rejection of power supply hum, which mandates heavier filtering to keep the hum to acceptable levels, no rejection of even order harmonics (a great advantage to guitar players!), and generally asymmetrical limiting on overloads which further emphasizes even order harmonics (which are more pleasing to the ear than odd order harmonics). These "disadvantages" give the single-ended output stage a unique tone, compared to the push-pull output stage. Whether it is "better" or not is a matter of taste. Some guitarists prefer single-ended output stages, others prefer push-pull.