19 Jun
2007
19 Jun
'07
2:34 a.m.
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
net.
porl
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(a)citycable.ch> wrote:
Le Mon, 18 Jun 2007 21:59:25 +0200,
"Giuseppe Zompatori" <siliconjoe(a)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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