Almost all of the different output stage configurations were devised in an effort to reduce the adverse effect of the output transformer, so it is not surprising to find that there have been some designs that dispense with the output transformer. These are often known as Futterman3 amplifiers (who patented the notion), or OTLs.

Driving low impedance loads directly is not natural for a valve, so radical approaches are needed. Special valves have to be used, which invariably were not designed for audio, and they therefore have extremely questionable linearity and consistency. Examples are the 6080/6AS7G double triode, series regulator valve, and television line scan output valves such as the PL504 and PL519 pentodes. Efficiency is generally on the low side of appalling. Output stages invariably use paralleled White cathode followers with plenty of global feedback to reduce the output resistance. See Fig. 6.12.

Fig. 6.12: OTL output stage (paralleled White cathode followers)

These amplifiers are quirky in the extreme, yet some designers think that the problems of output transformers are so severe that they persist in making successful OTL amplifiers.