Zin of 14k. This would compel me to use a tube that has an Rp
less than this value. I'd use a tube with Rp = 1/2Zin or 1/3Zin.>>Why use a tube with Rp = 1/2Zin? You can use tubes with a wide variety of Rp values, so long as they are about equal to or less than Zin. My experience has taught me that there will be sonically acceptable results that way and that the "sound" of the individual tube type will contribute most strongly to the sonic impression that results. It does help sonically to move from the LL1660 to the LL1671 when using really low Rp triodes like the 2A3 or 6BX7. The better match obviates some of those out-of-band resonances.
Be careful about getting too hung-up on the oscilloscope trace for the deciding vote in your quest for good sound. It's wise to pay attention to that if you are looking to get a good review in Stereophile. Look at the October issue with the review of the VAC preamp, the line stage of which was designed by me and uses Lundahl transformers. That's where good test results count! Then recognize that the reviewer wasn't disappointed with what he heard, because he doesn't have a 'scope, just ears. In fact, he was enthralled. I shouldn't be so difficult on the subject of testing; I have a bench full of 'scopes and distortion analyzers, but I just don't let them determine what I listen through.
Kevin
