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previous: Chris Harden Sounds like a 12AU7 would be OK. W... -- 6/5/2000 7:27 PM View Thread

Re: Building a 'clean' PI

6/5/2000 11:04 PM
Ray Ivers
Re: Building a 'clean' PI
I'm replying to both your posts here. You really can't use equal values for the potential divider, Chris; you have to have some signal present at the second stage's grid, and this signal is present only if the two resistors are slightly unequal. Does the scope show identical output at each plate?  
The 12AU7 will be much better than a 12AX7 at driving the output tubes into the grid current range; unfortunately, small-value capacitors generally don't excel at passing AC current, so your gains will be limited compared to transformer or direct-coupled cathode follower grid drive. The 12AU7 is pulling an acceptable amount of current, I think - more than 20 percent of max, which is what TubeCAD considers the minimum for good performance.  

Chris Harden I've finally moved the scope and si... -- 6/6/2000 5:37 PM