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Re: Cathode Biasing Circuit Design Question


 :
1/11/2001 4:32 PM
Dave H.
Re: Cathode Biasing Circuit Design Question
The cathode end of a common Rk/Ck will only be a "differential ground" if the increase in current through one tube is matched by the decrease in current through the other tube. That will be true for a class A/B amp up to the point where the signal is large enough to cut off the tubes for some part of the cycle. With one tube cut off the increase in current through the other would cause the cathode voltage to change along with the signal so the cathodes are no longer a "differential ground". The cathodes wonât follow the AC waveform when they are not a differential ground because the cathode cap keeps them at AC ground but the average current through the tubes goes up so the DC voltage on the cathodes will go up.  
 
 
 
My old green Mullard book says if you use a separate Rk/Ck for each tube you donât need matched tubes. That was probably OK for the old Mullard tubes but I wouldnât trust any two modern tubes to match up well enough. If you want to use a couple of badly matched tubes either use separate Rk/Ck for each tube and adjust the Rk to match the currents or use a common Rk/Ck and put a few 1N400n diodes in series with the hot tube cathode to balance it with the other tube.  
 
 
 
Dave
 

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