[sdiy] Tube Saturation/Distortion, Basics?

[Antti Huovilainen]

On Mon, 2 Jun 2003, Michael Zacherl wrote:

> anyone there who can point me to a ressource
> (preferable in the Web) which explains what
> happens to a signal when put through an tube
> amplifier circuit which is saturated?

I have written a paper for school about simulating tube preamp. The url is
http://www.hut.fi/~ajhuovil/tube/. It's a preliminary version still, so
don't save it or spread it.

Tubes themselves are fairly simple waveshapers. However, the input current
rises rapidly (close to exponential) once you approach 0V (tubes have
normally negative voltages on grid). This causes so called blocking
distortion which really means a shift in the DC voltage of input coupling
capacitor. Tube stages also have a so called bypass capacitor connected
between cathode and ground. This increases gain but also causes a bias
shift when tube current goes asymmetric.

What this means is that tube circuits change their bias depending on
signal level, causing a compression like effect. Attack transients get
distorted more (level hasn't shifted and lowered gain yet), retaining the
dynamic feel.

Antti

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