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In Reply to: Why do some people dislike UL? posted by Ray Moth on March 17, 2007 at 14:12:56:
Any resonance in the transformer from leakage inductance and capacitance is being fedback via the UL taps. UL mode is much more susceptible parasitic oscillation than triode or pentode. (On a parallel push pull EL34 150R screen resistors worked well in triode and pentode mode but I had to change them to 1K for Ultralinear mode to supress parasitics).I've built around 8 or 10 Ultralinear Amps and have always found that the value of the series resistor between the screen and the UL tap makes a big difference to sound
BUT
the addition of zobel networks between anode and screen taps (in addition to the series resistor) makes a huge difference. These zobels are very difficult to dimension properly but even sub-optimal zobels make an incredible difference to detail and clarity.On the one occassion that I tried to do the zobels "scientifically" I sat down with this paper:
http://www.siteswithstyle.com/VoltSecond/Damping_ringing_XFMRS/Damping_ringing_in_xfmrs.html
I ran just the output tubes - no phase splitter or driver and drove one side only while the other was at DC idle. I monitored the anode on that side to draw up a Bode Plot (Phase and Amplitude). Then did the other side. It was quite apparant that one side had a lower resonant frequency than the other and so I calculated a zobel to supress that resonance. (I use what Voltsecond called Rd_opt_Zi values). having supressed the lowest resonance (confirmed by redoing the Bode Plot) I then did the other side to supress the next lowest resonance. Unsurprisingly I ebded up with different zobels on the 2 sides and that makes me suspicious that there maybe something wrong with my method. By the way using Voltseconds method I tried zobels anode to anode, each anode to primary centre tap and across the secondary and confirmed that anode to screen was the right place to put the zobels.A bit of a rave but hopefully there is something there for people to have a think about and experiment with.
Cheers,
Ian
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Follow Ups
- Maybe - because - Gingertube 20:49:25 03/20/07 (0)
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