why multiple stages?

[milothicus]

I'm looking at this schematic from hoffman for a tube preamp pedal...



it's got 2 12AX7s, running the 4 triodes one after another. is there any reason for the four stages? could this be built with just one 12AX7? i also would expect to see a triode stage after the tone stack before the master volume, but i don't know why i expect that...

I guess i don't quite understand what each stage does to the signal. each stange is just an amplifier, right? meaning it increases the amplitude of the signal?

[Shea]

The last triode is a cathode follower.

The first stage probably doesn't break up at all. The next two stages do the clipping. Having more than one clipping stage generally gives you a thicker and smoother distortion than just one.

FWIW, this is a modified version of the Marshall 2204 / 2203 preamp.

[Bruce / Mission Amps]

The last triode is a cathode follower.

The first stage probably doesn't break up at all. The next two stages do the clipping. Having more than one clipping stage generally gives you a thicker and smoother distortion than just one.

FWIW, this is a modified version of the Marshall 2204 / 2203 preamp.

And, as drawn, the third stage has a huge amount of localized negative feedback from 15Hz on up, which would greatly impact that stage's gain.

[milothicus]

so from my understanding, the first stage just 'normalizes the signal'. the cathode bypass cap gives more gain, to overdrive the input of the next stage...

the second triode provides less gain, and is followed by two resistors that act as a divider, to reduce the signal.

what are the series capacitor and resistor between the V2-A plate and grid for? EDIT: feedback, as mentioned by Bruce

then the fourth stage is a cathode follower which reduces the impedence, but i'm not sure what the goal of that is...

and then the simple master volume and output are pretty clear.

[Shea]

so from my understanding, the first stage just 'normalizes the signal'. the cathode bypass cap gives more gain, to overdrive the input of the next stage...

the second triode provides less gain, and is followed by two resistors that act as a divider, to reduce the signal.

what are the series capacitor and resistor between the V2-A plate and grid for? EDIT: feedback, as mentioned by Bruce

then the fourth stage is a cathode follower which reduces the impedence, but i'm not sure what the goal of that is...

and then the simple master volume and output are pretty clear.

I've read that the direct-coupled cathode follower in Marshall preamps adds some distortion of its own. The combination of gain stage and cathode follower is reported to give a smooth distortion that's more symmetrical than you'd get from a gain stage by itself.

I don't know whether all that negative feedback might change that, though.