Multi-Stage Preamp

[bluefoxicy]

What is the point of a multi-stage preamp? Why would I want two 12AX7 tubes instead of one? Won't a full line-level signal into a 12AX7, or something from a tube with a mu of 60 or so, overdrive the 12AX7?

Is this only relevant in a push-pull design? I only know how to build class A single-ended so far... want to build a 2-4 6V6 push-pull one day but :/

[Paul P]

When a preamp tube clips it only clips one side of the signal. The signal coming
out of the first tube is inverted (if it's not wired as a cathode follower) so you use
a second tube to clip the other side of the signal.

Paul P

[bluefoxicy]

Yes, but I mean having 4 gain stages i.e. a 12AT7 into a 12AU7 or a 12AU7 into a 12AX7 or 3 12AX7 or whatever.

[Paul P]

The more stages you have the more you can play with the 'shape' of your
signal. And some stages are just to recover losses caused by circuits like
tone stacks, effects loops and reverb circuits.

Paul P

[bluefoxicy]

Ok, so it's not useful to add more 12AX7 tubes to drive a 6V6 harder (maybe I should raise the grid leak on the 6V6 from 470K)

[iammike.music]

It's like Paul P said, it gives you more of an oportunity to shape your sound; a distortion that is shaped over several stages often sounds much richer than just having one stage do all the clipping (in my experience anyways).

But you are correct, if you just want to get the signal up to line level to feed a power amp you don't necessarily need several stages. All you would need for a P-P setup is input > PI > power

-Mike

[bluefoxicy]

single-stage PA distortion sounds like shit, grinds too much. Bad frequency response or something.

[hasserl]

When a preamp tube clips it only clips one side of the signal. The signal coming
out of the first tube is inverted (if it's not wired as a cathode follower) so you use
a second tube to clip the other side of the signal.

Paul P

Say What?!? That is not correct. A preamp tube can have asymmetrical clipping, but it can also have symmetrical clipping of the signal as well. It depends on the way the tube is biased. If it is biased to operate in the middle of the linear region it will clip symmetrically.

Whether the signal is inverted or not has nothing to do with it. You certainly do not need two stages to clip the signal symmetrically, that is simply not true.

[Paul P]

I originally wrote "it (usually?) only clips one side of the signal.." because
I figured you could probably do both at once, though I never have. You
can clip one side by running out of B+ and clip the other by running out
of bias, but are they done together in guitar preamps ? From what I've
read I got the impression that what's usually done is to clip one side at
a time.

Paul P