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DC Coupled Drive for Cathode-Biased Power Tubes

8/15/2000 5:53 PM
Morris
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DC Coupled Drive for Cathode-Biased Power Tubes
I need assistance in the implementation of a DC-coupled drive setup for a single-ended, cathode-biased amp design. The amp currently has the following topology:  
 
input (1/2 12AX7) --> Tone Stack --> Volume Pot --> Gain Stage (1/2 12AX7) --> Gain Stage (1/2 12AU7) --> Output Tube (cathode-biased)  
 
I have the other half of the 12AU7 to play with (or I could use another type of twin triode if necessary). Would I just take a cap off the plate of the 12AU7 gain stage and drive the currently unused 12AU7 section (the follower) with a resistor from the grid of that section to ground (say 220K), and then the plate of the follower section gets B+ and the cathode of that section gets a resistor to ground (or to a negative voltage source?) with the output to the output tube taken from the top of the cathode resistor? What should the value of these resistors be?  
 
If I need a negative voltage source what is the best place in the amp to source it from? If I was getting a custom PT wound, would a tap at some voltage point be a good source for this, and could it also be used for a bias source (in other words, can I use the same voltage source for bias and the follower's cathode voltage)?  
 
Any help in this matter is much appreciated!  
 
Morris

 
Replies:
Brian If you go with the negative supply ... -- 8/15/2000 8:15 PM