16 Jun
2013
16 Jun
'13
9:15 p.m.
Le Sun, 16 Jun 2013 12:13:30 +0100,
John Rigg <ladev6(a)jrigg.co.uk> a écrit :
The Mace have silicon rectifiers. And the supply voltage drop on the
output transformer was at least of 100V, that between the full volume
point (160 w with a clean sinus, I know its hard...), and the fully
saturated point, when at the same time, the output power was almost the
same. At the output, the voltage was dropping with the saturation.
I don't think resistive losses alone can explain such a huge voltage
drop on the supply voltage.
Dominique
On Sat, Jun 15, 2013 at 11:33:45PM +0200, Dominique
Michel wrote:
Power transformer saturation only occurs if the voltage applied to the
primary is too high. It is not affected directly by the load on the
transformer.
This is true for a transformer that is not or normally loaded. But when
it is overloaded, we can get a behaviour similar to a coil: the
saturation is fixed by the current, not by the voltage, due to the
knee of the hysteresis curve. That imply the voltage drop will not only
be a function of the resistive losses, but also of the saturation of
the flux in the iron.
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).
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.
The effect you are describing is due to the internal resistance of the
transformer windings and other power supply components, not
transformer saturation. When more current is drawn the supply voltage
drops due to resistive losses. If there's a tube rectifier the effect
will be more pronounced. Some people like that effect but not me. I
agree that power transformers in many commercial designs are
undersized.
John
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