Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mojoworker, Jan 29, 2012.
i am new to the site. what does PI stand for in reference to amps/schematics?
Phase Inverter, the preamp tube located closest to the power tubes.
Represented on a schematic in the following ways. The first is a cathodyne PI, and the second is a long tailed pair. Fender used cathodyne in most of their tweed era amps, and switched to the LTP style in the black face era. The Princeton is the only Fender that comes to mind still using a Cathodyne PI. Most Marshalls use a LTP type PI.
Long-tailed pair (LTP) PI
Phase Inverter: its purpose is to take the signal from the preamp and output two signals that are 180 degrees out of phase...which is required to drive a push/pull type power output stage. Push/pull output stages always have an even number of tubes (or transistors). However, single or parallel tube (non-push/pull) output stages don't need a phase inverter stage so those type of amps don't have one...and they can have odd or even number of output tubes (or transistors).