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RC Network trick

Back when I was twiddling with the Amity, one thing I tried to get prettier square-wave response was terminate the IT with a series RC network in parallel with the secondary. I picked the R to match the Rp of the driving tubes (about 4K plate-to-plate for 5687/7044/7119 family) and trimmed the C for nicest square-wave response ... somewhere around 60pF as I recall.

The square-waves got nicer, but I didn't care for the sound ... more opaque and dynamically constricted. The drivers did not appreciate the extra load one bit. I figured I could live with about 15% overshoot on the square-waves, especially since they were load-insensitive and had nothing to do with feedback networks losing phase margin. (Which they certainly resembled.)

Once again, I really appreciate your insight on how the IT wants to have a certain current flow for a given core material ... makes me wonder what the IT would sound like with the Lundahl amorphous cores, which they do offer for line-level transformers. Hmm ...


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