by scott17 » Wed Apr 23, 2014 10:31 am
Hi, thanks. I had not heard an IT amp before this and had not built a fixed bias amp prior to this. I have done much research on fixed bias and IT coupling, and yes I was intrigued by the topology. I had the tubes and most of the parts in stock. I came to discover that paralleling the 6N23P-EV sections and running at this operating point would (should) work well with the Lundahl LL1660S. I had the transformers custom made by Heyboer, as I wanted a 6 ohm seondary to match my speakers. I am very pleased with the way it turned out. Very dynamic. Deep clean bass, clear midrange, sparkling highs. The IT splitter works very well. The stereo imaging is excellent. I measured 10W RMS in triode and 25W RMS in UL, before clipping. I will build more of these, but probably in a conventional chassis and not the framwork system. It's not the easiest thing to work on after it's built, although it can be done.
There is one mistake on the last schematic I posted. I did not tie the cathodes of the 6BY5 rectifiers to the B+. At first I was concerned about possibly exceeding the heater-negative-to-cathode rating of 450V. Since the heater winding is not referenced to ground there's no problem. Also, tieing the cathodes to B+ effectively puts the two rectifiers in parallel, which I did not want.
Scott