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previous: T.B. Hi John, Is it bett... -- 7/17/2000 12:44 PM View Thread

Re: Signal grid resitors values & output tube distortion???

7/19/2000 7:27 PM
Re: Signal grid resitors values & output tube distortion???
I'm not really an expert on these things so I dont want to overstate what I know.  
The job of the signal amplifying stages (the non output stages) is merely to amplify voltage. To get the maximum output of the output stage you want to provide more voltage swing onto the grids of the output tubes.  
In a perfect world the grids would be and infinite impedance to any other circuit and would swing the voltage without drawing any current. In the real world there are leakage currents and interelectrode capacitances which will draw current as a voltage changes on them. Therefore in reality the driver stage needs to supply some current as well as voltage.  
If the valve in the driver stage does not pass alot of current (eg a 12ax7) then the voltage amplification of that stage will sag. Its output is loaded down by the low impedance of the stage after it.  
For high outputs or paralleled output valves it is better to look at valves that pass more current such as 12at7s and 12au7s. These have lower gain but can supply the current needed by high output stages without sagging.  
I hope this is of some help