To understand the PTS concept we need to go back in time - to the beginning of the audio in movies, when the first audio amplifiers using 300B tubes were developed. After being forgotten for some decades, these amplifiers were rediscovered by the audio community around the 80's, due to their unique sound quality.
The first AUDIOPAX designs used 300Bs but they were quickly replaced by the LM3 output circuit - an exclusive invention that emulated the same magic of 300B tubes with beam power devices. From this concept, we moved on to a bolder idea: if we can emulate a certain tube, why not a perfect one? A tube that has never existed?

The answer for this challenge was the PTS - "Perfect Triode Simulator", that makes the Model 88 the only single ended amplifier that can be said to use a dream tube that could never be produced - an Ideal Audio Triode.