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previous: JWK I'm playing around with figures for... -- 1122497816 View Thread

Re: another thought...

7/27/2005 4:02 PM
Re: another thought...
I did some experiments recently on a driver stage for two KT88 in ultralinear mode. Man, do those tubes need a lot of drive. I had a heck of a time getting enough bias voltage. The first plan was two differentially connected 12AT7's direct coupled. The first stage had zener regulated 200V B+ and an active current source to stabilize the operating point and guarantee balance. I really wanted the high impedance input for the feedback. For some reason I couldn't get good balance. I spent several weeks fighting oscillations, bad transient response and distortion.  
Finally I decided to fire up an old Sunn amp for comparison. (Driver is a pentode direct coupled to a split load PI, 6AN8.) It had better transient response and lower distortion. The big surprise was the clipping behavior. Mine clipped symetrically with very little even harmonic content. The Sunn clipped hard on the top of the wave and real soft on the bottom. Lots of even harmonics. I tweeked all the compensation, tried several tubes and tweeked the operating point of the pentode and it made very little difference except at the extremes. Kudos to the designer (David Hafler?) on that one. Dude, you rule.  
I did have to take special precautions with the heater bias because the 12AT7's are only rated at 90V H-K. The 12AZ7 looks very similar to the 12AT7 but has the same H-K rating as a 12AX7. It draws more heater current though.  
Next I'm going to try an EF86 driving a parallel connected 12AT7 split load PI. No way around it, a separate floating heater winding will be required.  
Why is the LTP so popular ? I can't explain it.

Ray Ivers LT,Yep, UL needs al... -- 7/27/2005 4:46 PM