A few years ago, I was restoring an ancient Hallicrafters receiver and
noticed the crisp quality of the audio compared with that of a 1960s
transistorised communications receiver. I spoke to one of the experts at BT
Labs, who suggested that the output transformer might have something to do
with it. Also, the characteristics of a bipolar transistor audio output
stage are _not_ identical to those of a pentode or beam tetrode.

73,
John Rabson G3PAI