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previous: RJH Ok I'm still trying to understand W... -- 5/1/1999 8:25 AM View Thread

Re: PI/Driver Theory

5/4/1999 9:59 AM
Dave H
Re: PI/Driver Theory
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.  
 
Dave