| br>For those of you who want to experiment with tube op-amp circuits, this is the schematic of the best-performing tube op-amp ever manufactured (that I know of); the Philbrick SK2-V. Very rare item so you'd probably have to DIY your own. The Nuvistors on the input can be substituted with other triodes having a similar mu and plate resistance. I have no idea how well this circuit performs, but I've done a lot of experimentation with its simpler predecessor, the K2V, and it had some real issues for audio use. This has got to be better.