[LAD] (Modular) Synth and Clipping

[John Rigg]

On Sat, Jun 15, 2013 at 11:33:45PM +0200, Dominique Michel wrote:
> > An output transformer will saturate if the frequency is low enough,
> > but the signal level required to saturate it is directly proportional
> > to frequency. In a properly designed guitar or bass amp there will be
> > some transformer distortion at the lowest frequencies but not much
> > above that. If you lowered the frequency enough to fully saturate the
> > transformer it wouldn't sound very good, as you say. (I design guitar
> > amps among other things).
> 
> Me too, and I repair them too. I was talking here about cheap power
> transformers used in some brands of commercial guitar amplifiers, not
> about their output transformers. The main frequency is low enough to
> easily saturate them when they are not properly dimensioned, and this
> saturation will go through everything to the speaker.

Power transformer saturation only occurs if the voltage applied to the
primary is too high. It is not affected directly by the load on the
transformer.

> A typical example are the old Peavey Mace, good transistor preamp and
> driver stage, 6x6L6 for the output, but a too small power transformer to
> drive such a power (160 w RMS), and a bias circuit for the power stage
> that kill the dynamic when it is in saturation. The power transformer is
> definitely too small to drive the tubes at full saturated volume. I
> measured such an amp, the maximum power is the same with a clean sound
> and at full saturation. The sound is very good when the power stage is
> not saturated, but very bad when the power stage is saturated, that
> not only because of the lack of dynamic, but also because of the
> saturation of the power transformer.

The effect you are describing is due to the internal resistance of the
transformer windings and other power supply components, not transformer
saturation. When more current is drawn the supply voltage drops due to
resistive losses. If there's a tube rectifier the effect will be more
pronounced. Some people like that effect but not me. I agree that power
transformers in many commercial designs are undersized.

John