On 19/06/07, Dominique Michel <dominique.michel@citycable.ch> wrote:

Le Mon, 18 Jun 2007 21:59:25 +0200,
"Giuseppe Zompatori" <siliconjoe@gmail.com> a écrit :

> In general there seem to be a lot of problems in simulating the power
> section of push-pull NFB enabled amps in LTSpice... the NFB starts
> oscillating producing really high frequencies even at modest volumes
> eventually inflating the sim duration to infinity... garanted that's
> also what happen in reality but only when the power section is
> severely distorting...
>

The problem here is not the simulator but the valve models. Most of them don't
modelise the grid current or have a very very poor approximation, they have a
poor approximation of Ig2, they don't modelise the variation of the valves
characteristics when Ug2 is changing, and they have a very poor approximation
of the variation of the valves characteristics when Ig2 is changing.

According to the data sheet, some tetrodes can get 2 watts on the command grid
at full output. That implies huge grid current. At the same time, we will get
huge Ig2 variations and Ug2 will drop (even with fixed Ug2 bias) because the
voltage delivered by the power supply will drop at full output (around 100V
drop on some peavey amplifiers!!! The saturation we can heard is for the half
the saturation in the main transformer not in the amplifier or output
transformer!!!).

So, in short, don't expect a good or reliable result of a push-pull class B
guitar amplifier with so limited valves model. And to simulate the whole
amplifier is a must do, but without more reliable models, it doesn't make sens
to try to optimize something with a simulation for that kind of electronics. In
consequence, for me, and as long at we don't have better valves models,
practical experiences are everything.

Dominique
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