15 Jun
2013
15 Jun
'13
11:33 p.m.
Le Sat, 15 Jun 2013 21:48:40 +0100,
John Rigg <ladev6(a)jrigg.co.uk> a écrit :
On Sat, Jun 15, 2013 at 08:50:39PM +0200, Dominique
Michel wrote:
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.
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.
Excellent amplifier for jazz music - due to its large power, you can
even use it in small places with a mixer and put a whole gang on it -
but not for rock or blues. Even a small Fender Champ give a much better
sound at full volume. On the other hand, the output transformer of the
Champ is too small, and you will change it as often than the power
transformer of a Mace when playing loud, but the sound will be much
better. It was some years ago, one of my clients was "using" one
output transformer each year with his Champ.
Dominique
As example, when you push guitar amps in clipping
at full volume,
half of the clipping you can ear is, with some brands, not the
clipping of the electronic, but the clipping of the power
transformer. That sounds very bad -:(, and that imply you may have
to change often this transformer when such amps are used to play
blues or rock like styles of music.
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).
John
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