[LAD] simulating analog audio devices part II

porl sheean porl42 at gmail.com
Tue Jun 19 04:34:54 UTC 2007

thanks guys, i'll look into those models and links when i get a chance. i'm
hoping to be able to learn a lot more about valve amps this way. apart from
a few odds and ends it is hard to find out how they *really* work off the


ps. (offtopic): anyone know the approximate b+ voltage i should using for a
SE 6v6 amp through a 4k primary? i built one from odds and ends (i seem to
like that phrase) and my valves started glowing blue... looked cool though
;) hehe

On 19/06/07, Dominique Michel <dominique.michel at citycable.ch> wrote:
> Le Mon, 18 Jun 2007 21:59:25 +0200,
> "Giuseppe Zompatori" <siliconjoe at 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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