What is single ended?

The two main types of output stage are  'Push-Pull'  and  'Single-Ended'. These are two different methods of connecting the output valves to the output transformers. Push-pull designs can be biased in many different ways, including class A, AB and B. Single-ended designs are always biased in class A.

This is a section of an EL34 pentode tube, triode connected output stage. The screen grid is connected to the anode so that 100% of the output signal is used (fed back) to reduce distortion to the level similar to that of a triode valve (single ended amplifiers with low mu triodes have very low distortion). Some circuits use a tapping from the transformer at either 20% or 43% to connect to the screen grid, so that a percentage of the output is connected to the screen grid, this is referred to as 'ultra linear' and has the benefits of both the triodes low distortion and the pentodes higher output power.

Negative feedback can be taken from the speaker winding of the transformer to feed back to the input stage in order to reduce harmonic distortion (particularly 2nd order) which is present in valve amplifiers. Negative feedback in valve amplifiers will increase the low frequency bandwidth and reduce instability, but due to the large phase shifts in output transformers negative feedback must be limited. Many classic valve amplifier designs use no negative feedback at all, preferring the sounds with 'even' harmonic distortion.

A push-pull stage can be biased in class A, where current flows in both valves for the entire input cycle (but in opposite directions in the output transformer, cancelling out 'even' harmonic distortion), or class B, where current flows only half the time in each valve. Class B is rarely used nowadays where cross-over distortion is inherent all of the time even at low output levels. Many modern designs are biased class AB for best efficiency and power output with minimal crossover distortion.