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previous: Dan Can I add a SAG circuit to a preamp... -- 8/13/2004 4:34 PM View Thread

Re: Sag in preamp

8/13/2004 6:46 PM
SheaRe: Sag in preamp
It can be done, but don't expect much results from a "small value" resistor. The currents passed by the 12AX7s at max signal will be tiny compared to the current passed by one or more power tubes. So, to get an equivalent voltage sag with so much less current, you could need a pretty big resistor. In a guitar amp, B+ voltage sag is caused almost entirely by the output section, and the preamp voltages just track the main B+ supply up & down. To make the preamp itself actually cause that much sag would take a relatively big series resistance in the power supply.  
A more effective way to get that compression effect would be to just tweak how the 12AX7s are biased. The closer you move the bias point to cutoff (i.e., with bigger cathode resitors), the more compression there will be. Lower plate voltage will have that effect too, but it doesn't have to be saggy - just lower. Experiment with unbypassed cathode resistors too.  

Dan Shea,I figuring that would be t... -- 8/13/2004 10:54 PM