What is Transformer Saturation??

[trobbins]

The only thing I won't get on a recording at home is the speaker vibe.

Or acoustic feedback to the strings, or any circuit/transformer distortion after the MV, or voltage rail sag related compression.

[Helmholtz]

There is yet another OT core related effect. An OT's lower corner frequency depends on its primary inductance, which strongly varies with applied signal voltage. Inductance is low at low signal voltage levels and increases to a maximum at medium to high output, from where it drops caused by beginning saturation.

Consequently OT bass response improves with increasing output until partial saturation sets in.

If this effect actually has audible consequences would depend on a number of factors, especially on the low level inductance of a given OT and the bass responses of amp and speaker. Also global NFB will tend to mask the effect.

BTW, a bigger OT core doesn't mean higher inductance and/or better bass response.

[pdf64]

But at least a big core stops people worrying that it would sound better if it had one (rather than a more modestly sized core)