Compression drivers of all types, from what I remember, had a phase plug. The Tangerine came later, (mid 70's) first used in the Altec Model 19. The phase plug was needed to make the compression driver work properly. I don't know the mechanical or electrical reasons why. I think Altec came out with the Tangerine because one of the chief complaints about these drivers was a lack of HF extension. I remember my home pair of A-7's having more of a "whoooshhh" sound on cymbals rather than the nice extended "crash" sound a cymbal has when struck with a stick or mallet. This was much improved with the Model 19. IMHO