Stereo power amp

[dunner84]

I am planning a stereo power amp to pair up with a few tube preamps I have built over the years, and I have a few questions.

The amp will be push-pull with el34 on one side and 6l6 on the other.

- I have looked a few stereo pa schematics including Doug's here on this site, and they all seem to have a gain stage before the PI. What is the purpose of this gain stage?
All of my preamps are faithful recreations of popular/classic tube amps right up to the PI input... is there any good reasons I can't skip the additional gain stage?

- Most hifi amps don't seem to share power supply nodes, but most of the stereo guitar PAs do, so I will assume that's OK.

- I am thinking about adding a switch or jack for a mono/bridged mode.. is this as simple as splitting the signal at the input, or should it happen later in the circuit?

[PRR]

What do you mean "PI"? There's many types.

Usually "preamp" brings level up to about 1V. Power tubes need 10V-40V of drive. So a gain stage is wanted.

Often (not always) we take NFB from speaker side to an earlier stage. The power tube input is not early enough. Likewise a unity-gain PI is not enough. We want to get back to where signal is less than 1V.

The long-tail push-pull driver has lower gain than a dedicated gain stage.

And when you build a stand-alone power amp you do not know what people will put in front of it. To cover most cases it is wise to provide a little extra gain and a turn-down pot.