Sag - a small question

[pqt_bach]

Hello,
i know that when sag occures the tubes don't get enough juice. Does this happen in the entire amp or only at the power stage?

thanks.

[12afael]

it come from the power supply so it affect to all tubes.

[brett]

Hi
sag is traditionally associated with valve recitifiers.  They often have significant output impedance, which shows up when there's a large current draw by the amp (Ohm's law: voltage loss = current x impedance). There's also internal resistance in the secondaries of the power transformer.

I'm no expert, but for the voltage drop we are talking about (say 400 ohms x 100mA = 40V) the effect on any one stage alone would seem likely to be small (a voltage loss of 10% or thereabouts).  However, knocking 10% off the voltage gain of 3 stages results in a loss of 27% (1-(0.9*0.9*0.9)). In power terms, this is a loss of 54% (0.27^2).

The sag effect can, to some extent, be simulated by placing a resistor in-line with the power supply (before the filter capacitors).  Once the filter caps have been "drained", the B+ line "sees" all of the resistance in the line, and voltage falls.

In a typical solid-state amp like the Ruby, two to four ohms will provide enough resistance to limit the amount of sustainable power into an eight ohm load.  You'll have to work out an approriate filter cap size by trial and error. Start at 220uF per amp of supply current.
cheers
 

[pqt_bach]

Thanks guys