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PI/Driver Theory

5/1/1999 8:25 AM
PI/Driver Theory
Ok I'm still trying to understand WHY things happen. So, What are the sonic differences between the different PI/Driver circuts? Example:how does changing the "tail" or cathode resistor on the long tail pair effect the overall performance? I've seen different circuts using 10K, 15K and 22K for the long part of the long tail pair. Also 470 and 820 ohm cathode resistors in the circut. Most of these circuts keep the same 1meg "pairs". How does changing these values impact the: gain, distortion, frequency/sound of each type of driver? Thanks all --- I'm still learning!! RJH
5/4/1999 9:59 AM
Dave H

I donít know about the sonic differences but I think this is why the component values are different between Vox and Fender PIs.  
The Vox 47k "tail" resistor with more volts dropped across it will be a better approximation to a constant current sink than the 10k Fender "tail". The longer tail reduces the p-p output available but gives a better balance between the two outputs. Vox used equal 100k plate load resistors. Fender had to change one plate load to 82k to balance the outputs. Vox used EL84s which are much more sensitive than 6L6s so they didnít need as much p-p output as Fender.  
The two 1M resistors reference the grids to the bottom of the cathode resistor so this resistor sets the bias current through the tubes in the same way as the 1k5 cathode resistor does on input stages. For AC signals the bottom of the cathode resistor is a virtual ground so the cathode resistor affects the gain in the same way as a non bypassed 1k5 does in an input stage.  
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