I can't recall ever seeing this PI in a guitar amp. I found this in a 1958 tech. sheet. Compared to a long tailed pair, it has about twice the gain, and about twice the THD, which is probably the reason it doesn't seem to turn up in HIFI amps, but since when has a bit of distortion ever worried us guitar amp builders. Does anyone have any experience with this circuit, or know where I can find examples of it either used or written up anywhere.