Why does Q2 of a Green Ringer need to be PNP?

[Derringer]

What is unique about a PNP Q in that position as opposed to an NPN Q?

Why couldn't you put an NPN in there with the emiter going to the C3/R6 junction?

http://www.generalguitargadgets.com/pdf/ggg_gro_sc.pdf

Thanks

Bill

[R.G.]

You could do that. The problem is that a PNP makes getting the biggest signal swing easier because of the way the voltages are distributed in Q1.

To get a good split load phase splitter (that's what Q2 is doing) you need the voltage across the transistor with no signal to be half the power supply, and 1/4 of the power supply on each of the emitter and collector resistors, which must be equal values. That means that Q2's base is going to be sitting at 1/4 of the power supply plus one Vbe drop away from the power supply on the emitter side. For 9V, that's going to be 9/4 +0.6, or about 3.1V either above ground (if you use an NPN) or below 9V if you use a PNP. Biasing Q1 to get a good amount of gain and undistorted swing is easier if the collector of Q1 is on the base-emitter side of Q2.

If you switched out Q2 for an NPN, you would need the collector of Q1 to be at 3.1V, not the 5.9V or so it sits at now. 5.9V is more near the center of the 9V power supply, and so you get a bigger undistorted swing on Q1's collector. So it works best if Q1 and Q2 are opposite polarities. But that's not the only way to do it.